IIO comments

I've started looking at the current code base in staging, this is basically a log of what I'm finding there that may get improved:

* Your bus_type is not really a bus in the sense we normally use it, because it does not have any driver/device matching. You only register devices from the same files that drive the hardware. This is normally done using a class, not a bus.

* Since you specify the sysfs attributes globally in the documentation, it would be nice if drivers did not have to implement them as attribute show function, but as a function that simply returns a value that is then printed by common code. You can do this using a structure of function pointers that each driver fills with the relevant ones, like file_operations, replacing the attribute groups.

* iio_allocate_device() could get a size argument and allocate the dev_data together with the iio_dev in a single allocation, like netdev_alloc does. In addition, you can pass a structure with all constant data, such as THIS_MODULE, and the operations and/or attribute groups, instead of having to set them individually for each dev.

* It's not clear to me why you need one additional device and one chardev for each interrupt line of a device, my feeling is that this could be simplified a lot if you find a way to represent an iio_dev with a single chardev to user space. This may have been an attempt to avoid ioctl() calls, but I think in this case an ioctl interface would be cleaner than a complex hierarchy of devices. Another alternative would be for the device driver to register multiple iio_devs in case it needs multiple interfaces.

* Neither of your two file operations supports poll(), only read. Having poll() support is essential if you want to wait for multiple devices in user space. Maybe it's possible to convert the code to use eventfd instead of rolling your own event interface?

* The name rip_lots doesn't really mean anything to me as a non-native speaker. Maybe think of a better word for this.

Also, there is only one driver providing such a function, so this is probably a good candidate for deferring to a later stage, along with all the the entire ring file code that seems rather pointless otherwise.

* The exported symbols should probably become EXPORT_SYMBOL_GPL

* iio-trig-sysfs should not be a platform driver

That's probably enough for today, to start some discussion about the core. Overall, I'm pleasantly surprised by the quality of the code, it's much better than I was expecting after having looked at drivers from hardware vendors a lot recently.

Hi Arnd,

Quoted 3 lines

I've started looking at the current code base in staging, this is basically a log of what I'm finding there that may get improved:

Thanks for taking a look!

Quoted 6 lines

* Your bus_type is not really a bus in the sense we normally use it, because it does not have any driver/device matching. You only register devices from the same files that drive the hardware. This is normally done using a class, not a bus.

That's what I originally thought and indeed how it was initially done. The responses from Greg KH and Kay Sievers to one of our ABI discussions convinced me otherwise.

http://lkml.org/lkml/2010/1/20/233 + the previous email in that thread.

Quoted 8 lines

* Since you specify the sysfs attributes globally in the documentation, it would be nice if drivers did not have to implement them as attribute show function, but as a function that simply returns a value that is then printed by common code. You can do this using a structure of function pointers that each driver fills with the relevant ones, like file_operations, replacing the attribute groups.

This may indeed work. However it would be a fair bit more fiddly than that a simple function pointer structure as the sets of attributes supplied by different devices vary considerably.

So to confirm I understand you correctly we either have something like:
/* access ops */

int (*get_accel_x)(struct iio_dev *devinfo)
char *(*get_accel_offset)(struct iio_dev *devinfo); /* note these can't return a value as they may well be floating point */
char *(*get_accel_gain)(struct iio_dev *devinfo)
int (*get_accel_y)(struct iio_dev *devinfo)

Sticky cases that will make this fiddly.

* Unknown maximum number of some types of attributes.
- Could use a

	int (*get_in)(struct iio_dev dev_info, int channel);
	int num_in;

* Whatever you pick also immediately becomes restrictive as the main class has to be updated to handle any new elements.

* Note we'll have function points for all the scan element related stuff as well. Basically the set that would be needed is huge.

At the end of the day I'm unconvinced this makes sense from the point of view of simplifying the driver code. The gain is that it enforces the abi and may allow in kernel uses.
Ultimately we lifted this approach from hwmon where it works well under similar circumstances.

Alternative is a small set such as:

/* access ops */
int (*get_channel_value)(struct iio_dev *info, enum CHANNEL); char (*get_channel_offset)(struct iio_dev *info, enum CHANNEL);

and a list of which channels exist. Then the driver side becomes a massive switch statement rather than the attribute tables. Again, I'm not convinced that is actually cleaner or easier to understand. Also this approach pretty much means you end up with large numbers of equal attributes as you can't trivially group them. e.g we have accel_scale to cover case where it's the same for all accelerometer axes.

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* It's not clear to me why you need one additional device and one chardev for each interrupt line of a device, my feeling is that this could be simplified a lot if you find a way to represent an iio_dev with a single chardev to user space. This may have been an attempt to avoid ioctl() calls, but I think in this case an ioctl interface would be cleaner than a complex hierarchy of devices. Another alternative would be for the device driver to register multiple iio_devs in case it needs multiple interfaces.

In a sense this isn't quite as bad as it seems. The only cases we have so far (and it's more or less all that's out there) are those where the device has one interrupt line for 'events' - e.g threshold interrupts of one type or another, and one for one for data ready signals. The data ready signals are handled as triggers and hence don't have a chardev (directly).

The other uses of chrdevs are for accessing buffers. Each buffer may have two of them. The first is a dirty read chrdev. The second is for out of band flow information to userspace. Basically you control what events will come out of the flow control chrdev. Programs block on that and read from the data flow chrdev according to what comes down the line. The messy events stuff is to handle event escalation. Dealing with getting a 50% full event and then a 75% full event with no inherent knowledge of if you read anything in between is a pain. We could specify only one level event exists, which would simplify the code somewhat. Perhaps wise for an initial merge though I'm loath to not support functionality that is fairly common in hardware buffers.

Some buffer implementations don't provide flow control (kfifo_buf) so for them you just have a read chrdev (actually this isn't true right now - I need to fix this). This chrdev is separate from the 'event' chrdevs purely because for speed reasons. You don't want to have what is coming down this line change except due to software interaction (and to do that you normally have to stop the buffer fill).

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* Neither of your two file operations supports poll(), only read. Having poll() support is essential if you want to wait for multiple devices in user space. Maybe it's possible to convert the code to use eventfd instead of rolling your own event interface?

I'll look into this one. Curiously I don't think it has ever been suggested before. (maybe I'm just forgetful)

We have had the suggestion of using inputs event interface and I am still considering that option. The issue
there is that it is really far too heavyweight. So far we have no use cases for multiple readers and we never want to aggregate across devices. Also input views events as carrying data. All actual readings in our case are going through the faster buffer route.

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Also, there is only one driver providing such a function, so this is probably a good candidate for deferring to a later stage, along with all the the entire ring file code that seems rather pointless otherwise.

Lots of drivers do it. They are simply using one of the software provided buffers. ring_sw or kfifo_buf (and yes they do have useless names as well so we'll clean that up for which ever ones we keep!)

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* iio-trig-sysfs should not be a platform driver

This one got discussed before merging it and I agree. We let it go then because there was a clear use case, a working driver and no one had the time to do a better job. Michael eventually talked Greg round to letting it in.

http://marc.info/?t=129665398600002&r=1&w=2

Quoted 11 lines · Jonathan Cameron

Hi Arnd,

Quoted 3 lines

I've started looking at the current code base in staging, this is basically a log of what I'm finding there that may get improved:

Thanks for taking a look!

Quoted 6 lines

* Your bus_type is not really a bus in the sense we normally use it, because it does not have any driver/device matching. You only register devices from the same files that drive the hardware. This is normally done using a class, not a bus.

Quoted 6 lines

That's what I originally thought and indeed how it was initially done. The responses from Greg KH and Kay Sievers to one of our ABI discussions convinced me otherwise.

http://lkml.org/lkml/2010/1/20/233 + the previous email in that thread.

(adding Kay to Cc here)

I see. However, I'd still disagree with Kay here. Given that the common use of existing subsystems is to have "bus" for things that are physically connected, and "class" for things that are logical abstractions, I think we should continue that way for new subsystems, even if there is little difference technically.

There is little point discussing how things should have been done in the past when we're stuck with them. The best we can do now is make the code as consistent as possible.

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* Since you specify the sysfs attributes globally in the documentation, it would be nice if drivers did not have to implement them as attribute show function, but as a function that simply returns a value that is then printed by common code. You can do this using a structure of function pointers that each driver fills with the relevant ones, like file_operations, replacing the attribute groups.

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This may indeed work. However it would be a fair bit more fiddly than that a simple function pointer structure as the sets of attributes supplied by different devices vary considerably.

It's certainly more work for the subsystem, but the intention is to simplify the individual drivers, to make things scale better as many more drivers get added.

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So to confirm I understand you correctly we either have something like:
/* access ops */

int (*get_accel_x)(struct iio_dev *devinfo)
char *(*get_accel_offset)(struct iio_dev *devinfo); /* note these can't return a value as they may well be floating point */
char *(*get_accel_gain)(struct iio_dev *devinfo)
int (*get_accel_y)(struct iio_dev *devinfo)

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Sticky cases that will make this fiddly.

You can't really return a char* because of the memory allocation, but aside from that, the idea is right, yes.

Note that this forces all drivers to use the same unit for data they return in the attributes, but I consider this a good thing.

For attributes that really want to return a floating point number, I think we should either define a fixed-point form with a constant exponent, or some other representation that is allowed in the kernel.

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* Unknown maximum number of some types of attributes.
- Could use a

	int (*get_in)(struct iio_dev dev_info, int channel);
	int num_in;

Good point. So a driver may have multiple attributes of the same type, which need to get enumerated, but should all behave the same way, right?

Your suggestion seems good, but I'd still recommend making it so that the core chooses the attribute names, not the device driver. One possible way to lay this out is something like:

/* global defines */

enum iio_channel_type {
	IIO_CHAN_TEMPERATURE,
	IIO_CHAN_ADC,
	IIO_CHAN_FOO,
	...
};

static const char *channel_names[] = {
	[IIO_CHAN_TEMPERATURE]	= "temperature%d",
	[IIO_CHAN_ADC]		= "adc%d",
	[IIO_CHAN_FOO]		= "foo%d",
	...
};

typedef unsigned long long iio_result_t;

struct iio_channel_defs {
	iio_result_t (*get)(struct iio_dev *dev_info, int channel);
	
	enum iio_channel_type channels[0];
};

/* example driver */

enum {
	FOO_CHAN_TEMP0,
	FOO_CHAN_TEMP1,
	FOO_CHAN_ADC,
};

struct iio_channel_defs foo_driver_channels = {
	.get = &foo_get_sensor,

	.channels = {
		/* this device has two temperature and one adc input */
		[FOO_CHAN_TEMP0] = IIO_CHAN_TEMPERATURE,
		[FOO_CHAN_TEMP1] = IIO_CHAN_TEMPERATURE,
		[FOO_CHAN_ADC]   = IIO_CHAN_ADC,
	},
};

iio_result_t foo_getsensor(struct iio_dev *dev_info, int channel)

{
	switch (channel) {
	case FOO_CHAN_TEMP0:
		return read_temp0(dev_info);
	case FOO_CHAN_TEMP1:
		return read_temp1(dev_info);
	case FOO_CHAN_ADC:
		return read_adc(dev_info);
	}
	return -EINVAL;
}

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* Whatever you pick also immediately becomes restrictive as the main class has to be updated to handle any new elements.

I believe that's a good thing, we want to limit the number of different interfaces that people come up with. If two drivers are similar enough, they should probably use the same one.

It also makes it easier to verify that all attributes are documented properly, because the documentation only has to be matched with a single file, not with every single driver.

It also forces out-of-tree modules to use the same interfaces that other drivers are using. If someone has hardware for something new, it will only have an interface to talk to after it has been discussed and documented in mainline.

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* Note we'll have function points for all the scan element related stuff as well. Basically the set that would be needed is huge.

At the end of the day I'm unconvinced this makes sense from the point of view of simplifying the driver code. The gain is that it enforces the abi and may allow in kernel uses.
Ultimately we lifted this approach from hwmon where it works well under similar circumstances.

Alternative is a small set such as:

/* access ops */
int (*get_channel_value)(struct iio_dev *info, enum CHANNEL); char (*get_channel_offset)(struct iio_dev *info, enum CHANNEL);

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and a list of which channels exist. Then the driver side becomes a massive switch statement rather than the attribute tables. Again, I'm not convinced that is actually cleaner or easier to understand. Also this approach pretty much means you end up with large numbers of equal attributes as you can't trivially group them. e.g we have accel_scale to cover case where it's the same for all accelerometer axes.

The whole point would be to avoid duplication, so maybe something a little smarter is needed, but the current code duplicates the definition of attributes to every driver, and I'm sure we can find a solution that requires less code.

In many cases, you should be able to replace a long switch statement by a table driven approach inside of the driver.

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* It's not clear to me why you need one additional device and one chardev for each interrupt line of a device, my feeling is that this could be simplified a lot if you find a way to represent an iio_dev with a single chardev to user space. This may have been an attempt to avoid ioctl() calls, but I think in this case an ioctl interface would be cleaner than a complex hierarchy of devices. Another alternative would be for the device driver to register multiple iio_devs in case it needs multiple interfaces.

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In a sense this isn't quite as bad as it seems. The only cases we have so far (and it's more or less all that's out there) are those where the device has one interrupt line for 'events' - e.g threshold interrupts of one type or another, and one for one for data ready signals. The data ready signals are handled as triggers and hence don't have a chardev (directly).

If you only have the two, you might be able to handle them both in the poll function, where the regular case means POLLIN or POLLRDNORM, while the threshold interrupt gets turned into POLLPRI or POLLRDBAND.

I'm not sure if that covers all cases you are interested in, but it sounds like a useful simplification. It also makes it possible for user programs to only wait for one type of event.

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The other uses of chrdevs are for accessing buffers. Each buffer may have two of them. The first is a dirty read chrdev. The second is for out of band flow information to userspace. Basically you control what events will come out of the flow control chrdev. Programs block on that and read from the data flow chrdev according to what comes down the line. The messy events stuff is to handle event escalation. Dealing with getting a 50% full event and then a 75% full event with no inherent knowledge of if you read anything in between is a pain. We could specify only one level event exists, which would simplify the code somewhat. Perhaps wise for an initial merge though I'm loath to not support functionality that is fairly common in hardware buffers.

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Some buffer implementations don't provide flow control (kfifo_buf) so for them you just have a read chrdev (actually this isn't true right now - I need to fix this). This chrdev is separate from the 'event' chrdevs purely because for speed reasons. You don't want to have what is coming down this line change except due to software interaction (and to do that you normally have to stop the buffer fill).

Doesn't epoll handle the event escalation with its edge triggered mode? Basically you can get notified every time that there is more to be read.

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* Neither of your two file operations supports poll(), only read. Having poll() support is essential if you want to wait for multiple devices in user space. Maybe it's possible to convert the code to use eventfd instead of rolling your own event interface?

I'll look into this one. Curiously I don't think it has ever been suggested before. (maybe I'm just forgetful)

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We have had the suggestion of using inputs event interface and I am still considering that option. The issue
there is that it is really far too heavyweight. So far we have no use cases for multiple readers and we never want to aggregate across devices. Also input views events as carrying data. All actual readings in our case are going through the faster buffer route.

I'm slightly confused, mainly because I have no idea how the buffer and event interfaces are actually being used. I would have expected an interface as simple as:

* One iio device per sensor that can provide data, possibly many per hardware device

* One chardev for each iio device

* Use epoll to wait for data and/or out-of-band messages
* Use chrdev read to get events from the buffer

* Use sysfs to read from sensors that are not event based

What you have is obviously more complex than this, and you probably have good reasons that I still need to understand.

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Also, there is only one driver providing such a function, so this is probably a good candidate for deferring to a later stage, along with all the the entire ring file code that seems rather pointless otherwise.

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Lots of drivers do it. They are simply using one of the software provided buffers. ring_sw or kfifo_buf (and yes they do have useless names as well so we'll clean that up for which ever ones we keep!)

Ok, I see. I did not realize that iio_rip_sw_rb was getting assigned to ->rip_lots.

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* iio-trig-sysfs should not be a platform driver

This one got discussed before merging it and I agree. We let it go then because there was a clear use case, a working driver and no one had the time to do a better job. Michael eventually talked Greg round to letting it in.

http://marc.info/?t=129665398600002&r=1&w=2

Ok. I still think that it is a really bad idea because the driver is essentially a software feature that could be use on any machine, but using a platform device ties it directly to a specific machine that registers the device. It would make much more sense to remove all the driver registration from there and just create the iio device at module load time, or to have some interface that can be used to dynamically create these triggers if the module is loaded.

Arnd

Quoted 28 lines · Arnd Bergmann

Quoted 11 lines · Jonathan Cameron

Hi Arnd,

Quoted 3 lines

I've started looking at the current code base in staging, this is basically a log of what I'm finding there that may get improved:

Thanks for taking a look!

Quoted 6 lines

* Your bus_type is not really a bus in the sense we normally use it, because it does not have any driver/device matching. You only register devices from the same files that drive the hardware. This is normally done using a class, not a bus.

Quoted 6 lines

That's what I originally thought and indeed how it was initially done. The responses from Greg KH and Kay Sievers to one of our ABI discussions convinced me otherwise.

http://lkml.org/lkml/2010/1/20/233 + the previous email in that thread.

(adding Kay to Cc here)

I see. However, I'd still disagree with Kay here. Given that the common use of existing subsystems is to have "bus" for things that are physically connected, and "class" for things that are logical abstractions, I think we should continue that way for new subsystems, even if there is little difference technically.

Quoted 3 lines

There is little point discussing how things should have been done in the past when we're stuck with them. The best we can do now is make the code as consistent as possible.

Leaving this for Kay.

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* Since you specify the sysfs attributes globally in the documentation, it would be nice if drivers did not have to implement them as attribute show function, but as a function that simply returns a value that is then printed by common code. You can do this using a structure of function pointers that each driver fills with the relevant ones, like file_operations, replacing the attribute groups.

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This may indeed work. However it would be a fair bit more fiddly than that a simple function pointer structure as the sets of attributes supplied by different devices vary considerably.

I've cc'd Jean and Guenter as they may be able to offer so insight from what they see in hwmon (sorry for dropping you in half way through a discussion!)

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It's certainly more work for the subsystem, but the intention is to simplify the individual drivers, to make things scale better as many more drivers get added.

I'm yet to be convinced this simplifies things. We basically move from one form of big table in the drivers to another and add complexity to the core. I'd be fine with that complexity if it actually simplified the drivers, but I'm not sure it does. Perhaps the only way to really answer that is to actually try doing it with a few drivers and see where we end up.

Quoted 10 lines

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So to confirm I understand you correctly we either have something like:
/* access ops */

int (*get_accel_x)(struct iio_dev *devinfo)
char *(*get_accel_offset)(struct iio_dev *devinfo); /* note these can't return a value as they may well be floating point */
char *(*get_accel_gain)(struct iio_dev *devinfo)
int (*get_accel_y)(struct iio_dev *devinfo)

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Sticky cases that will make this fiddly.

You can't really return a char* because of the memory allocation, but aside from that, the idea is right, yes.

sure. It would need a bit of management code around that or some suitable encapsulation.

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Note that this forces all drivers to use the same unit for data they return in the attributes, but I consider this a good thing.

True, though the other approach to enforcing this is what hwmon does. Thorough review against the spec for all new drivers.

Quoted 74 lines

For attributes that really want to return a floating point number, I think we should either define a fixed-point form with a constant exponent, or some other representation that is allowed in the kernel.

Quoted 4 lines

* Unknown maximum number of some types of attributes.
- Could use a

     int (*get_in)(struct iio_dev dev_info, int channel);
     int num_in;

Good point. So a driver may have multiple attributes of the same type, which need to get enumerated, but should all behave the same way, right?

Your suggestion seems good, but I'd still recommend making it so that the core chooses the attribute names, not the device driver. One possible way to lay this out is something like:

/* global defines */

enum iio_channel_type {
      IIO_CHAN_TEMPERATURE,
      IIO_CHAN_ADC,
      IIO_CHAN_FOO,
      ...
};

static const char *channel_names[] = {
      [IIO_CHAN_TEMPERATURE]  = "temperature%d",
      [IIO_CHAN_ADC]          = "adc%d",
      [IIO_CHAN_FOO]          = "foo%d",
      ...
};

typedef unsigned long long iio_result_t;

struct iio_channel_defs {
      iio_result_t (*get)(struct iio_dev *dev_info, int channel);

      enum iio_channel_type channels[0];
};

/* example driver */

enum {
      FOO_CHAN_TEMP0,
      FOO_CHAN_TEMP1,
      FOO_CHAN_ADC,
};

struct iio_channel_defs foo_driver_channels = {
      .get = &foo_get_sensor,

      .channels = {
              /* this device has two temperature and one adc input */
              [FOO_CHAN_TEMP0] = IIO_CHAN_TEMPERATURE,
              [FOO_CHAN_TEMP1] = IIO_CHAN_TEMPERATURE,
              [FOO_CHAN_ADC]   = IIO_CHAN_ADC,
      },
};

iio_result_t foo_getsensor(struct iio_dev *dev_info, int channel)

{
      switch (channel) {
      case FOO_CHAN_TEMP0:
              return read_temp0(dev_info);
      case FOO_CHAN_TEMP1:
              return read_temp1(dev_info);
      case FOO_CHAN_ADC:
              return read_adc(dev_info);
      }
      return -EINVAL;
}

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* Whatever you pick also immediately becomes restrictive as the main class has to be updated to handle any new elements.

I believe that's a good thing, we want to limit the number of different interfaces that people come up with. If two drivers are similar enough, they should probably use the same one.

Limiting them is fine, but there are still an awful lot of them - so we still end up with big and ugly tables. Again currently enforcement of interface is done by review.

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It also makes it easier to verify that all attributes are documented properly, because the documentation only has to be matched with a single file, not with every single driver.

That would be a gain. However, the real thing you need to check with use of an attribute is not it's naming (which is a trivial match against the docs), but that the units of whatever is returned are correct. Thus you end up looking closely at the docs alongside every driver anyway and trawling through datasheets.

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It also forces out-of-tree modules to use the same interfaces that other drivers are using. If someone has hardware for something new, it will only have an interface to talk to after it has been discussed and documented in mainline.

The question this brings up is how to do we then handle the one
of a kind interface elements? However general our spec
there are always things we don't really want in the core as they will only ever be used by one specific driver.

Right now we operate a policy which says all elements must be documented, but those that are not 'general' shouldn't go in the main doc files. Things can move to general the moment we find a second device using them. (sometimes we are wrong in thinking something is so obscure no one else will ever want it). Note these obscure elements don't get supported by any remotely standard userspace code. Take the TOAS 258x driver from Jobn Brenner for example. That has one nasty calibration attribute. Try as we might we really haven't managed to blugeon that one into a 'standard' form (it's the coefficients of a magic transfer function which takes into account the exact glass window over the sensor). Normally we'd argue this should be platform data, but Jon has real users where devices that are otherwise identical have different glass - hence it must be runtime controlled.

The moment we put a route in for registering additional attributes the out-of-tree argument is weakened.

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* Note we'll have function points for all the scan element related stuff as well. Basically the set that would be needed is huge.

At the end of the day I'm unconvinced this makes sense from the point of view of simplifying the driver code. The gain is that it enforces the abi and may allow in kernel uses.
Ultimately we lifted this approach from hwmon where it works well under similar circumstances.

Alternative is a small set such as:

/* access ops */
int (*get_channel_value)(struct iio_dev *info, enum CHANNEL); char (*get_channel_offset)(struct iio_dev *info, enum CHANNEL);

Quoted 6 lines

and a list of which channels exist. Then the driver side becomes a massive switch statement rather than the attribute tables. Again, I'm not convinced that is actually cleaner or easier to understand. Also this approach pretty much means you end up with large numbers of equal attributes as you can't trivially group them. e.g we have accel_scale to cover case where it's the same for all accelerometer axes.

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The whole point would be to avoid duplication, so maybe something a little smarter is needed, but the current code duplicates the definition of attributes to every driver, and I'm sure we can find a solution that requires less code.

A little less perhaps, but it's still going to be a big table, just of subtly different things.

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In many cases, you should be able to replace a long switch statement by a table driven approach inside of the driver.

I think the only way to answer this is to do a quick implementation of this and see how well it works.

I'll have a crack at this sometime soon with a representative set of drivers and see how it goes.

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* It's not clear to me why you need one additional device and one chardev for each interrupt line of a device, my feeling is that this could be simplified a lot if you find a way to represent an iio_dev with a single chardev to user space. This may have been an attempt to avoid ioctl() calls, but I think in this case an ioctl interface would be cleaner than a complex hierarchy of devices. Another alternative would be for the device driver to register multiple iio_devs in case it needs multiple interfaces.

Quoted 6 lines

In a sense this isn't quite as bad as it seems. The only cases we have so far (and it's more or less all that's out there) are those where the device has one interrupt line for 'events' - e.g threshold interrupts of one type or another, and one for one for data ready signals. The data ready signals are handled as triggers and hence don't have a chardev (directly).

If you only have the two, you might be able to handle them both in the poll function, where the regular case means POLLIN or POLLRDNORM, while the threshold interrupt gets turned into POLLPRI or POLLRDBAND.

Quoted 4 lines

I'm not sure if that covers all cases you are interested in, but it sounds like a useful simplification. It also makes it possible for user programs to only wait for one type of event.

I will need to have a play and get back to you on this.

I don't suppose there is anything similar to this in kernel I could take a look at? Its not an area I'm even remotely familiar with. Right now I don't see exactly how this helps us with the 'out of band' messages not colliding with the main data flow.

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The other uses of chrdevs are for accessing buffers. Each buffer may have two of them. The first is a dirty read chrdev. The second is for out of band flow information to userspace. Basically you control what events will come out of the flow control chrdev. Programs block on that and read from the data flow chrdev according to what comes down the line. The messy events stuff is to handle event escalation. Dealing with getting a 50% full event and then a 75% full event with no inherent knowledge of if you read anything in between is a pain. We could specify only one level event exists, which would simplify the code somewhat. Perhaps wise for an initial merge though I'm loath to not support functionality that is fairly common in hardware buffers.

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Some buffer implementations don't provide flow control (kfifo_buf) so for them you just have a read chrdev (actually this isn't true right now - I need to fix this). This chrdev is separate from the 'event' chrdevs purely because for speed reasons. You don't want to have what is coming down this line change except due to software interaction (and to do that you normally have to stop the buffer fill).

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Doesn't epoll handle the event escalation with its edge triggered mode? Basically you can get notified every time that there is more to be read.

We also need to know how much more this is to be read. So at the moment, if a 50% full turns up and isn't picked up by userspace before a 75% full event, the 50% is escalated to become a 75% event which userspace then receives. To be honest I'm tempted, for now at least to drop this functionality. It has few users and makes the events system considerably more complex. We can think about how / whether to introduce it later.

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* Neither of your two file operations supports poll(), only read. Having poll() support is essential if you want to wait for multiple devices in user space. Maybe it's possible to convert the code to use eventfd instead of rolling your own event interface?

I'll look into this one. Curiously I don't think it has ever been suggested before. (maybe I'm just forgetful)

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We have had the suggestion of using inputs event interface and I am still considering that option. The issue
there is that it is really far too heavyweight. So far we have no use cases for multiple readers and we never want to aggregate across devices. Also input views events as carrying data. All actual readings in our case are going through the faster buffer route.

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I'm slightly confused, mainly because I have no idea how the buffer and event interfaces are actually being used. I would have expected an interface as simple as:

* One iio device per sensor that can provide data, possibly many per hardware device

One per device.

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* One chardev for each iio device

currently 1-3. (event line, buffer access, buffer event)

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* Use epoll to wait for data and/or out-of-band messages
* Use chrdev read to get events from the buffer

and data?

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* Use sysfs to read from sensors that are not event based

What you have is obviously more complex than this, and you probably have good reasons that I still need to understand.

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Also, there is only one driver providing such a function, so this is probably a good candidate for deferring to a later stage, along with all the the entire ring file code that seems rather pointless otherwise.

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Lots of drivers do it. They are simply using one of the software provided buffers. ring_sw or kfifo_buf (and yes they do have useless names as well so we'll clean that up for which ever ones we keep!)

Ok, I see. I did not realize that iio_rip_sw_rb was getting assigned to ->rip_lots.

renamed to read_first_n which is a somewhat clearer I hope.

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* iio-trig-sysfs should not be a platform driver

This one got discussed before merging it and I agree. We let it go then because there was a clear use case, a working driver and no one had the time to do a better job. Michael eventually talked Greg round to letting it in.

http://marc.info/?t=129665398600002&r=1&w=2

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Ok. I still think that it is a really bad idea because the driver is essentially a software feature that could be use on any machine, but using a platform device ties it directly to a specific machine that registers the device. It would make much more sense to remove all the driver registration from there and just create the iio device at module load time, or to have some interface that can be used to dynamically create these triggers if the module is loaded.

Yup, the dynamic route to creation is what I favour. As you say this needs fixing up before going out of staging. We definitely need these for a bridge to input to implement polled input devices.

I'll put a todo in place for that element in staging so we don't forget.

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Arnd

Quoted 5 lines · Arnd Bergmann

On Wednesday 16 March 2011, Jonathan Cameron wrote:

It's certainly more work for the subsystem, but the intention is to simplify the individual drivers, to make things scale better as many more drivers get added.

I'm yet to be convinced this simplifies things. We basically move from one form of big table in the drivers to another and add complexity to the core. I'd be fine with that complexity if it actually simplified the drivers, but I'm not sure it does. Perhaps the only way to really answer that is to actually try doing it with a few drivers and see where we end up.

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* Whatever you pick also immediately becomes restrictive as the main class has to be updated to handle any new elements.

I believe that's a good thing, we want to limit the number of different interfaces that people come up with. If two drivers are similar enough, they should probably use the same one.

Limiting them is fine, but there are still an awful lot of them - so we still end up with big and ugly tables.

Again currently enforcement of interface is done by review.

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It also makes it easier to verify that all attributes are documented properly, because the documentation only has to be matched with a single file, not with every single driver.

That would be a gain. However, the real thing you need to check with use of an attribute is not it's naming (which is a trivial match against the docs), but that the units of whatever is returned are correct. Thus you end up looking closely at the docs alongside every driver anyway and trawling through datasheets.

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It also forces out-of-tree modules to use the same interfaces that other drivers are using. If someone has hardware for something new, it will only have an interface to talk to after it has been discussed and documented in mainline.

The question this brings up is how to do we then handle the one
of a kind interface elements? However general our spec
there are always things we don't really want in the core as they will only ever be used by one specific driver.

Right now we operate a policy which says all elements must be documented, but those that are not 'general' shouldn't go in the main doc files. Things can move to general the moment we find a second device using them. (sometimes we are wrong in thinking something is so obscure no one else will ever want it). Note these obscure elements don't get supported by any remotely standard userspace code. Take the TOAS 258x driver from Jobn Brenner for example. That has one nasty calibration attribute. Try as we might we really haven't managed to blugeon that one into a 'standard' form (it's the coefficients of a magic transfer function which takes into account the exact glass window over the sensor). Normally we'd argue this should be platform data, but Jon has real users where devices that are otherwise identical have different glass - hence it must be runtime controlled.

The moment we put a route in for registering additional attributes the out-of-tree argument is weakened.

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The whole point would be to avoid duplication, so maybe something a little smarter is needed, but the current code duplicates the definition of attributes to every driver, and I'm sure we can find a solution that requires less code.

A little less perhaps, but it's still going to be a big table, just of subtly different things.

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In many cases, you should be able to replace a long switch statement by a table driven approach inside of the driver.

I think the only way to answer this is to do a quick implementation of this and see how well it works.

I'll have a crack at this sometime soon with a representative set of drivers and see how it goes.

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If you only have the two, you might be able to handle them both in the poll function, where the regular case means POLLIN or POLLRDNORM, while the threshold interrupt gets turned into POLLPRI or POLLRDBAND.

I'm not sure if that covers all cases you are interested in, but it sounds like a useful simplification. It also makes it possible for user programs to only wait for one type of event.

I will need to have a play and get back to you on this.

I don't suppose there is anything similar to this in kernel I could take a look at? Its not an area I'm even remotely familiar with. Right now I don't see exactly how this helps us with the 'out of band' messages not colliding with the main data flow.

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Doesn't epoll handle the event escalation with its edge triggered mode? Basically you can get notified every time that there is more to be read.

We also need to know how much more this is to be read. So at the moment, if a 50% full turns up and isn't picked up by userspace before a 75% full event, the 50% is escalated to become a 75% event which userspace then receives. To be honest I'm tempted, for now at least to drop this functionality. It has few users and makes the events system considerably more complex. We can think about how / whether to introduce it later.

Yup, the dynamic route to creation is what I favour. As you say this needs fixing up before going out of staging. We definitely need these for a bridge to input to implement polled input devices.

I'll put a todo in place for that element in staging so we don't forget.

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Quoted 5 lines · Arnd Bergmann

On Wednesday 16 March 2011, Jonathan Cameron wrote:

It's certainly more work for the subsystem, but the intention is to simplify the individual drivers, to make things scale better as many more drivers get added.

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I'm yet to be convinced this simplifies things. We basically move from one form of big table in the drivers to another and add complexity to the core. I'd be fine with that complexity if it actually simplified the drivers, but I'm not sure it does. Perhaps the only way to really answer that is to actually try doing it with a few drivers and see where we end up.

Yes. I'm pretty sure it's possible to do better than the current interface, but don't have a way to prove it.

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* Whatever you pick also immediately becomes restrictive as the main class has to be updated to handle any new elements.

I believe that's a good thing, we want to limit the number of different interfaces that people come up with. If two drivers are similar enough, they should probably use the same one.

Limiting them is fine, but there are still an awful lot of them - so we still end up with big and ugly tables.

So we should find a way to make it a nice and small table instead ;-)

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Again currently enforcement of interface is done by review.

I believe enforcing the rules using the compiler is always better than by review, if all other factors are the same.

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It also makes it easier to verify that all attributes are documented properly, because the documentation only has to be matched with a single file, not with every single driver.

That would be a gain. However, the real thing you need to check with use of an attribute is not it's naming (which is a trivial match against the docs), but that the units of whatever is returned are correct. Thus you end up looking closely at the docs alongside every driver anyway and trawling through datasheets.

Are you currently printing the unit as part of the attribute output? If that gets handled by the core, any driver that tries to use something else would simply be considered a bug.

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It also forces out-of-tree modules to use the same interfaces that other drivers are using. If someone has hardware for something new, it will only have an interface to talk to after it has been discussed and documented in mainline.

The question this brings up is how to do we then handle the one
of a kind interface elements? However general our spec
there are always things we don't really want in the core as they will only ever be used by one specific driver.

Right now we operate a policy which says all elements must be documented, but those that are not 'general' shouldn't go in the main doc files. Things can move to general the moment we find a second device using them. (sometimes we are wrong in thinking something is so obscure no one else will ever want it). Note these obscure elements don't get supported by any remotely standard userspace code. Take the TOAS 258x driver from Jobn Brenner for example. That has one nasty calibration attribute. Try as we might we really haven't managed to blugeon that one into a 'standard' form (it's the coefficients of a magic transfer function which takes into account the exact glass window over the sensor). Normally we'd argue this should be platform data, but Jon has real users where devices that are otherwise identical have different glass - hence it must be runtime controlled.

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The moment we put a route in for registering additional attributes the out-of-tree argument is weakened.

Would it help to have multiple types of interfaces (I suppose you already do), each with their own table?

This way you could enforce that all drivers of one type use exactly the same specification, but also make it possible to have new types, even those that have only one driver by design.

In the case of the TOAS 258x driver (I can't find that in the tree), I fear that would still have to become a standard attribute of some sort.

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The whole point would be to avoid duplication, so maybe something a little smarter is needed, but the current code duplicates the definition of attributes to every driver, and I'm sure we can find a solution that requires less code.

A little less perhaps, but it's still going to be a big table, just of subtly different things.

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In many cases, you should be able to replace a long switch statement by a table driven approach inside of the driver.

I think the only way to answer this is to do a quick implementation of this and see how well it works.

I'll have a crack at this sometime soon with a representative set of drivers and see how it goes.

Ok, cool! Just send out the drivers as you get there, and maybe someone has ideas to simplify them further if you're not happy with the result.

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If you only have the two, you might be able to handle them both in the poll function, where the regular case means POLLIN or POLLRDNORM, while the threshold interrupt gets turned into POLLPRI or POLLRDBAND.

I'm not sure if that covers all cases you are interested in, but it sounds like a useful simplification. It also makes it possible for user programs to only wait for one type of event.

I will need to have a play and get back to you on this.

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I don't suppose there is anything similar to this in kernel I could take a look at? Its not an area I'm even remotely familiar with. Right now I don't see exactly how this helps us with the 'out of band' messages not colliding with the main data flow.

One way to deal with out of band data (if it's more than just a flag)

is to have an ioctl function that returns structured data, while the regular read function returns the buffered input stream.

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Doesn't epoll handle the event escalation with its edge triggered mode? Basically you can get notified every time that there is more to be read.

We also need to know how much more this is to be read. So at the moment, if a 50% full turns up and isn't picked up by userspace before a 75% full event, the 50% is escalated to become a 75% event which userspace then receives. To be honest I'm tempted, for now at least to drop this functionality. It has few users and makes the events system considerably more complex. We can think about how / whether to introduce it later.

Yes, good point. I also don't get the use case of this. Why would user space not just always read all data that is available?

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Yup, the dynamic route to creation is what I favour. As you say this needs fixing up before going out of staging. We definitely need these for a bridge to input to implement polled input devices.

I'll put a todo in place for that element in staging so we don't forget.

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Are you currently printing the unit as part of the attribute output? If that gets handled by the core, any driver that tries to use something else would simply be considered a bug.

Nope. It's done set by the Docs. This we need to keep for compatibility (where relevant) with hwmon. It's a bug now if it doesn't meet the spec but you'd be amazed how often these are wrong. Sometimes it is a nightmare figuring out from the datasheet what they actually are!

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The moment we put a route in for registering additional attributes the out-of-tree argument is weakened.

Would it help to have multiple types of interfaces (I suppose you already do), each with their own table?

This way you could enforce that all drivers of one type use exactly the same specification, but also make it possible to have new types, even those that have only one driver by design.

Sadly there are no clean definitions of types. You'd be amazed what downright weird sets end up in a given device.
Might be some way of breaking it down into a set of tables though in a vaguely coherent way. Each device would then have one or more of these for which it would define one or more of the functions.

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is to have an ioctl function that returns structured data, while the regular read function returns the buffered input stream.

That would work, but it is it really worth it to kill off the chrdev?

Note we still need most of the underlying structure just to keep the various attributes well categorized.

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Doesn't epoll handle the event escalation with its edge triggered mode? Basically you can get notified every time that there is more to be read.

We also need to know how much more this is to be read. So at the moment, if a 50% full turns up and isn't picked up by userspace before a 75% full event, the 50% is escalated to become a 75% event which userspace then receives. To be honest I'm tempted, for now at least to drop this functionality. It has few users and makes the events system considerably more complex. We can think about how / whether to introduce it later.

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Yes, good point. I also don't get the use case of this. Why would user space not just always read all data that is available?

Good point. We could just move the handling of these events into the actual driver (this is only ever relevant for hardware buffers). Thus it could keep track of what it knows is there. Note these devices don't always provide any other way of knowing and yes you can read more data from them than is actually there. Whether you can tell this has happened is very much device dependent.

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Yup, the dynamic route to creation is what I favour. As you say this needs fixing up before going out of staging. We definitely need these for a bridge to input to implement polled input devices.

I'll put a todo in place for that element in staging so we don't forget.

Note I have quite a few changes currently queued up so might be a little while before I get to trying out the more complex stuff you have suggested.

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The moment we put a route in for registering additional attributes the out-of-tree argument is weakened.

Would it help to have multiple types of interfaces (I suppose you already do), each with their own table?

This way you could enforce that all drivers of one type use exactly the same specification, but also make it possible to have new types, even those that have only one driver by design.

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Sadly there are no clean definitions of types. You'd be amazed what downright weird sets end up in a given device.
Might be some way of breaking it down into a set of tables though in a vaguely coherent way. Each device would then have one or more of these for which it would define one or more of the functions.

Right, or you could mandate that each device has exactly one of these, but that you could have multiple iio devices in a single hardware device.

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is to have an ioctl function that returns structured data, while the regular read function returns the buffered input stream.

That would work, but it is it really worth it to kill off the chrdev?

I don't know.

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Note we still need most of the underlying structure just to keep the various attributes well categorized.

The hardest part of interface design is always to find the simplest possible way that can cover all the corner cases. I'm trying to come up with possible simplifications, but I don't know all the requirements that get in the way.

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Doesn't epoll handle the event escalation with its edge triggered mode? Basically you can get notified every time that there is more to be read.

We also need to know how much more this is to be read. So at the moment, if a 50% full turns up and isn't picked up by userspace before a 75% full event, the 50% is escalated to become a 75% event which userspace then receives. To be honest I'm tempted, for now at least to drop this functionality. It has few users and makes the events system considerably more complex. We can think about how / whether to introduce it later.

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Yes, good point. I also don't get the use case of this. Why would user space not just always read all data that is available?

Good point. We could just move the handling of these events into the actual driver (this is only ever relevant for hardware buffers). Thus it could keep track of what it knows is there. Note these devices don't always provide any other way of knowing and yes you can read more data from them than is actually there. Whether you can tell this has happened is very much device dependent.

Can you give an example? I don't understand what you mean with "can read mode data [...] than is actually there".

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Yup, the dynamic route to creation is what I favour. As you say this needs fixing up before going out of staging. We definitely need these for a bridge to input to implement polled input devices.

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I'll put a todo in place for that element in staging so we don't forget.

Note I have quite a few changes currently queued up so might be a little while before I get to trying out the more complex stuff you have suggested.

Ok, no problem. Just take your time.

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Right, or you could mandate that each device has exactly one of these, but that you could have multiple iio devices in a single hardware device.

That might lead to some very ugly largely false divides. I guess trying it is the only way to find out. Some things will be really fiddly if we make this divide, particularly buffering. The hardware tends to spit out 'scans' of a set of channels taken at more or less the same time. If we split into multiple iio devices we'll need to split this data into streams for each one. The interactions between the available channels can also be extremely complex, so we need to make it absolutely clear which physical device they are on. Note that you can only tell what 'scan' elements are configured by trying to set up what you want, then reading back to see what you got. In some cases you even end up with more than you asked for.

Probably the only way to do work out how manageable this is will be to list what would be in this table, with vaguely sane restrictions. Some awkward corner cases include differential channels where we at least in theory have a combinatorial explosion. (inX-inY_raw) We also have to double to cover _raw and _input cases though these could easily be a pair of tables or use some metadata for each channel.

Taking just the stuff in docs for the raw channel readings (and restricting based on what I have to hand). Input devices only.

in0, in1, in2, in3, in4, in5, in6, in7, in8, in9, in10, in11,
in0-in1, in0-in1, in2-in3, in3-in2, in5-in4, in4-in5, in7-in6,
in8-in9, in9-in8, in10-in11, in11-in10
accel_x, accel_y, accel_z, accel_x_peak, accel_y_peak, accel_z_peak, accel_xyz_squared_peak, temp, temp_x, temp_y, temp_z, gyro_x, gyro_y, gyro_z,
magn_x, magn_y, magn_z, incli_x, inci_y, incli_z,
illuminance0, proximity, intensity_infrared.

So a starting set of 46. Also note that the really large tables will be handling the event controls. The only consistent way we have come up with for that so far is to have the option of a separate value and enable attribute for every channel and every type of event. This would
definitely require more complex metadata to do. I'd suggest a bitmap
specifying which are available.

The compound versions (accel_xyz_squared_peak etc) are relatively unusual.

We haven't covered energy meters or resolvers here as they don't have documented interfaces yet. It'll be about another 10 for them.

So all in all a flat table structure is going to be huge. Given the core won't operate on these things terribly often (basically setup and tear down) rather than a table of things that may or may not be set, we could have an association table which says what is present.

It would be interesting to work out what the minumum structure required to generate everything associated with a given channel actually looks like...

struct CHAN {
       enum CHAN_TYPE type;
       int index;  (x = 0, y = 1 etc).

       long sharedmask;                   //which of the functions are shared
       	    				  //so can we do single accel_bias
					  //rather than accel_x_bias etc.. 
       long eventmask;                    //what events exist;
       bool scannable;
       etc.
}

Then set of callbacks to be registered:

read_val(type, index);
read_calibbais(type index);

etc.

This would keep the table reasonably short at the cost of a fair number of function pointers. Using the masks, the core can figure out what attributes should exist and wire them up.

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Good point. We could just move the handling of these events into the actual driver (this is only ever relevant for hardware buffers). Thus it could keep track of what it knows is there. Note these devices don't always provide any other way of knowing and yes you can read more data from them than is actually there. Whether you can tell this has happened is very much device dependent.

Can you give an example? I don't understand what you mean with "can read mode data [...] than is actually there".

I might have the details of this wrong as I haven't checked everything against the datasheets.
Lets take the sca3000 VTI accelerometers.

They have 64 scan (read of x, y, z set) long hardware ring buffers. There are two watershed events for how full the buffer is, 50% and 75%. These result in a pin being raised if enabled. This is routed through a gpio on the host as an interrupt. These thresholds are edge triggered so will not retrigger unless the contents falls below the level then rises past it again.

Currently we pass that event straight up to userspace. Userspace can then know there is at least that much data in the buffer and hence request that much data back.

If user space attempts to read less data from the buffer than is there it is all fine and assuming user space always reads at least 50% then
we will get a new event. If only one of the 50% full or 75% full
events is enabled, then we don't need to read which one occurred. Thus we cut out one transaction on the bus (given the bus is slow this really can matter). The delays around a transaction can be a significant part of the time taken to do a transaction (so this isn't swamped by the time taken to read 32*3*2 bytes).

There is a separate register for the number of samples in the buffer. Right now we don't use it. On other parts this may not be present (as it needn't ever be used. It was this case I was referring two.
I'd forgotten it existed till I checked just now.

So options to clean up this case and avoid event escalation.

1) Allow only one watershead interrupt to be configured on the device at a time. (issue is if we ever hit a device where we can't turn them off).

2) Burn a transaction reading the buffer count then make sure we don't read more than is there. The events then just become a use of poll - perhaps different flags for 50% full or 75% full if they are both enabled.

3) Handle the two watershead events in the driver, basically holding state. Things may get fiddly if an event shows up during a read.

For simplicity of review I'm tempted to go with 1 and make the a requirement of all drivers unless someone comes up with a very good reason why we need this functionality.

Quoted 4 lines · On Thursday 17 March 2011 17:47:52 Jonathan Cameron wrote:

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Right, or you could mandate that each device has exactly one of these, but that you could have multiple iio devices in a single hardware device.

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That might lead to some very ugly largely false divides. I guess trying it is the only way to find out. Some things will be really fiddly if we make this divide, particularly buffering. The hardware tends to spit out 'scans' of a set of channels taken at more or less the same time. If we split into multiple iio devices we'll need to split this data into streams for each one. The interactions between the available channels can also be extremely complex, so we need to make it absolutely clear which physical device they are on. Note that you can only tell what 'scan' elements are configured by trying to set up what you want, then reading back to see what you got. In some cases you even end up with more than you asked for.

Ok, that makes sense. As you say, it's still possible to split the hardware streams into multiple streams on character devices, but it's less clear that this would simplify anything.

However, when you have devices that have separate hardware buffers for their channels, I think there is a much stronger incentive to model them as separate character devices.

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Probably the only way to do work out how manageable this is will be to list what would be in this table, with vaguely sane restrictions. Some awkward corner cases include differential channels where we at least in theory have a combinatorial explosion. (inX-inY_raw) We also have to double to cover _raw and _input cases though these could easily be a pair of tables or use some metadata for each channel.

Taking just the stuff in docs for the raw channel readings (and restricting based on what I have to hand). Input devices only.

in0, in1, in2, in3, in4, in5, in6, in7, in8, in9, in10, in11,
in0-in1, in0-in1, in2-in3, in3-in2, in5-in4, in4-in5, in7-in6,
in8-in9, in9-in8, in10-in11, in11-in10
accel_x, accel_y, accel_z, accel_x_peak, accel_y_peak, accel_z_peak, accel_xyz_squared_peak, temp, temp_x, temp_y, temp_z, gyro_x, gyro_y, gyro_z,
magn_x, magn_y, magn_z, incli_x, inci_y, incli_z,
illuminance0, proximity, intensity_infrared.

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So a starting set of 46. Also note that the really large tables will be handling the event controls. The only consistent way we have come up with for that so far is to have the option of a separate value and enable attribute for every channel and every type of event. This would
definitely require more complex metadata to do. I'd suggest a bitmap
specifying which are available.

I don't completely understand the notation. Regarding the various {in0, in1, in2, ...} inputs, is there a fundamental difference between them? In the code example I gave, a driver would simply list a set of inputs of the same type (IIO_CHAN_IN) and let the core enumerate them. What does "in0-in1" mean?

Also, the only table that would have to list all of them would be in the core, to list the names, and possibly a function point for pretty-printing the data, but many of these function pointers would point to the same few functions.

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The compound versions (accel_xyz_squared_peak etc) are relatively unusual.

We haven't covered energy meters or resolvers here as they don't have documented interfaces yet. It'll be about another 10 for them.

Fair enough.

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So all in all a flat table structure is going to be huge. Given the core won't operate on these things terribly often (basically setup and tear down) rather than a table of things that may or may not be set, we could have an association table which says what is present.

Well, a driver only needs to list what it has, and that could be written in a rather compact way.

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It would be interesting to work out what the minumum structure required to generate everything associated with a given channel actually looks like...

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struct CHAN {
       enum CHAN_TYPE type;
       int index;  (x = 0, y = 1 etc).

Do you have drivers that have sparse indices? The core could simply enumerate them when it encounters channels of the same type for one device.

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       long sharedmask;                   //which of the functions are shared
       	    				  //so can we do single accel_bias
					  //rather than accel_x_bias etc.. 
       long eventmask;                    //what events exist;
       bool scannable;
       etc.
}

Ok, I didn't think of other fields, but these seem to be needed, even if I don't understand them.

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Then set of callbacks to be registered:

read_val(type, index);
read_calibbais(type index);

etc.

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This would keep the table reasonably short at the cost of a fair number of function pointers. Using the masks, the core can figure out what attributes should exist and wire them up.

I don't think you need many function pointers. Having a function pointer in struct chan is may be a good idea, but if you have ten inputs that are all alike, they can all point to the same function, right?

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Good point. We could just move the handling of these events into the actual driver (this is only ever relevant for hardware buffers). Thus it could keep track of what it knows is there. Note these devices don't always provide any other way of knowing and yes you can read more data from them than is actually there. Whether you can tell this has happened is very much device dependent.

Can you give an example? I don't understand what you mean with "can read mode data [...] than is actually there".

I might have the details of this wrong as I haven't checked everything against the datasheets.
Lets take the sca3000 VTI accelerometers.

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They have 64 scan (read of x, y, z set) long hardware ring buffers. There are two watershed events for how full the buffer is, 50% and 75%. These result in a pin being raised if enabled. This is routed through a gpio on the host as an interrupt. These thresholds are edge triggered so will not retrigger unless the contents falls below the level then rises past it again.

Ok, I see. So the hardware provides no way at all to find out how many events you are allowed to read, other than being allowed to read half the events if you got the 50% interrupt, right?

I wonder what the hardware people were thinking, there should at least be a way to find out whether the buffer is completely empty!

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Currently we pass that event straight up to userspace. Userspace can then know there is at least that much data in the buffer and hence request that much data back.

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If user space attempts to read less data from the buffer than is there it is all fine and assuming user space always reads at least 50% then
we will get a new event. If only one of the 50% full or 75% full
events is enabled, then we don't need to read which one occurred. Thus we cut out one transaction on the bus (given the bus is slow this really can matter). The delays around a transaction can be a significant part of the time taken to do a transaction (so this isn't swamped by the time taken to read 32*3*2 bytes).

To be honest, this sounds like a horrible user interface. The kernel should always try to shield the user from details of the hardware buffer and provide an interface that can be used like a pipe: If there is any data to read, let the user get it, otherwise block the reading process or return -EAGAIN (in case of O_NONBLOCK) and wait for the user to call poll() for notification.

The kernel probably can spare much more RAM for input data than the device, so the natural implementation in the kernel would register a handler for the interrupt that reads all available data into a kernel fifo buffer, which gets emptied on the next read from user space.

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There is a separate register for the number of samples in the buffer. Right now we don't use it. On other parts this may not be present (as it needn't ever be used. It was this case I was referring two.
I'd forgotten it existed till I checked just now.

Ok. I truely hope that most hardware has something like this, but we can probably work around it as explained above if not.

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So options to clean up this case and avoid event escalation.

1) Allow only one watershead interrupt to be configured on the device at a time. (issue is if we ever hit a device where we can't turn them off).

2) Burn a transaction reading the buffer count then make sure we don't read more than is there. The events then just become a use of poll - perhaps different flags for 50% full or 75% full if they are both enabled.

3) Handle the two watershead events in the driver, basically holding state. Things may get fiddly if an event shows up during a read.

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For simplicity of review I'm tempted to go with 1 and make the a requirement of all drivers unless someone comes up with a very good reason why we need this functionality.

I would argue for a combination of 1 & 2. Configuring which of the two interrupts you want would be determined by the real-time and/or power management requirements, but should not be visible to the application reading the data, only when setting up the device.

Arnd
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Quoted 5 lines · Arnd Bergmann

I don't completely understand the notation. Regarding the various {in0, in1, in2, ...} inputs, is there a fundamental difference between them? In the code example I gave, a driver would simply list a set of inputs of the same type (IIO_CHAN_IN) and let the core enumerate them. What does "in0-in1" mean?

in0-in1 is a differential adc channel. Literally outputs value on physical pin 1 subtracted from physical pin 2.

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It would be interesting to work out what the minumum structure required to generate everything associated with a given channel actually looks like...

struct CHAN {
       enum CHAN_TYPE type;
       int index;  (x = 0, y = 1 etc).

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Do you have drivers that have sparse indices? The core could simply enumerate them when it encounters channels of the same type for one device.

Sadly yes we do. Some IMUs have 3D accelerometer and 2D gyros.

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I don't think you need many function pointers. Having a function pointer in struct chan is may be a good idea, but if you have ten inputs that are all alike, they can all point to the same function, right?

Agreed. I had them in there originally but decided it was getting rather clunky. In a sense this will look a little like taking the current huge attribute tables and breaking them up into bits associated with each channel. We may want a certain amount of 'private_data' space in the channel array as well to allow for things like addresses. Not sure on that yet though.

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Ok. I truely hope that most hardware has something like this, but we can probably work around it as explained above if not.

Yes. Though do beware. spi and i2c buses for some of these things can be 'very' slow and often congested on the actual boards. Hence we sometimes spend a lot of effort to avoid transactions.

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For simplicity of review I'm tempted to go with 1 and make the a requirement of all drivers unless someone comes up with a very good reason why we need this functionality.

I would argue for a combination of 1 & 2. Configuring which of the two interrupts you want would be determined by the real-time and/or power management requirements, but should not be visible to the application reading the data, only when setting up the device.

I'd prefer to allow some direct control. There are use cases where for filtering purposes you are only interested in a particular length block of data. Still, that control may be the exception rather than rule. Lets just turn on the 50% by default then vast majority of users won't ever touch it!

Quoted 8 lines · Jonathan Cameron

Quoted 5 lines · Arnd Bergmann

I don't completely understand the notation. Regarding the various {in0, in1, in2, ...} inputs, is there a fundamental difference between them? In the code example I gave, a driver would simply list a set of inputs of the same type (IIO_CHAN_IN) and let the core enumerate them. What does "in0-in1" mean?

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in0-in1 is a differential adc channel. Literally outputs value on physical pin 1 subtracted from physical pin 2.

Ok, I see. So these would be fairly hard to enumerate, right? Would it be possible to have one attribute with named "diff%d" and another attribute associated with it that describes which channels are compared?

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It would be interesting to work out what the minumum structure required to generate everything associated with a given channel actually looks like...

struct CHAN {
       enum CHAN_TYPE type;
       int index;  (x = 0, y = 1 etc).

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Do you have drivers that have sparse indices? The core could simply enumerate them when it encounters channels of the same type for one device.

Sadly yes we do. Some IMUs have 3D accelerometer and 2D gyros.

Ok, I see. So you might have {x0,y0} for one sensor but {x1,y1,z1} for the other one, right?

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I don't think you need many function pointers. Having a function pointer in struct chan is may be a good idea, but if you have ten inputs that are all alike, they can all point to the same function, right?

Agreed. I had them in there originally but decided it was getting rather clunky. In a sense this will look a little like taking the current huge attribute tables and breaking them up into bits associated with each channel. We may want a certain amount of 'private_data' space in the channel array as well to allow for things like addresses. Not sure on that yet though.

Makes sense. So you either need a private-data pointer for each element and point that to another static data structure, or you need two arrays of different structures but using the same indices.

I think both ways would work, but it would be nice to come up with a cleaner solution.

Maybe it could be an anonymous union of an unsigned long and a pointer, so you can initialize either of the two members, depending on how complex the driver needs it.

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Ok. I truely hope that most hardware has something like this, but we can probably work around it as explained above if not.

Yes. Though do beware. spi and i2c buses for some of these things can be 'very' slow and often congested on the actual boards. Hence we sometimes spend a lot of effort to avoid transactions.

Do the transactions require spinning on the CPU, or do they always work in the background when they are slow?

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For simplicity of review I'm tempted to go with 1 and make the a requirement of all drivers unless someone comes up with a very good reason why we need this functionality.

I would argue for a combination of 1 & 2. Configuring which of the two interrupts you want would be determined by the real-time and/or power management requirements, but should not be visible to the application reading the data, only when setting up the device.

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I'd prefer to allow some direct control. There are use cases where for filtering purposes you are only interested in a particular length block of data. Still, that control may be the exception rather than rule. Lets just turn on the 50% by default then vast majority of users won't ever touch it!

Ok.

Arnd

Quoted 8 lines · Arnd Bergmann

Quoted 7 lines · Jonathan Cameron

Quoted 5 lines · Arnd Bergmann

I don't completely understand the notation. Regarding the various {in0, in1, in2, ...} inputs, is there a fundamental difference between them? In the code example I gave, a driver would simply list a set of inputs of the same type (IIO_CHAN_IN) and let the core enumerate them. What does "in0-in1" mean?

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in0-in1 is a differential adc channel. Literally outputs value on physical pin 1 subtracted from physical pin 2.

Ok, I see. So these would be fairly hard to enumerate, right? Would it be possible to have one attribute with named "diff%d" and another attribute associated with it that describes which channels are compared?

Could do, but what would it gain us? If the information is available to the core, then it can use it give us the explicit naming? As shown below we have to handle holes in the enumeration anyway so this isn't to much of a problem.

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Ok. I truely hope that most hardware has something like this, but we can probably work around it as explained above if not.

Yes. Though do beware. spi and i2c buses for some of these things can be 'very' slow and often congested on the actual boards. Hence we sometimes spend a lot of effort to avoid transactions.

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Do the transactions require spinning on the CPU, or do they always work in the background when they are slow?

If you have a proper controller it's often DMA based. So the issue is more congestion on the bus limiting possible sampling rates than cpu load. To a certain extent we can just ignore this issue as long as we are open to changes to a driver to minimise bus usage if someone has a use case that actually requires it for a given device.

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Quoted 5 lines · Arnd Bergmann

I don't completely understand the notation. Regarding the various {in0, in1, in2, ...} inputs, is there a fundamental difference between them? In the code example I gave, a driver would simply list a set of inputs of the same type (IIO_CHAN_IN) and let the core enumerate them. What does "in0-in1" mean?

in0-in1 is a differential adc channel. Literally outputs value on physical pin 1 subtracted from physical pin 2.

Ok, I see. So these would be fairly hard to enumerate, right? Would it be possible to have one attribute with named "diff%d" and another attribute associated with it that describes which channels are compared?

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Could do, but what would it gain us? If the information is available to the core, then it can use it give us the explicit naming? As shown below we have to handle holes in the enumeration anyway so this isn't to much of a problem.

Maybe I was seeing problems that don't exist here. Wouldn't you need to numeric identifiers to describe a differential channel?

I guess if it's always in${i}-in${i+1}, it's still not too hard.

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I guess if it's always in${i}-in${i+1}, it's still not too hard.

I think they have been so far, but doubt this is universal. How about having a diff type and just having a pair of indices in the channel structure? Actually may need a third for x^2+y^2+z^2 devices. (iirc there are parts that do x^2+y^2 despite also having a z channel)
...

Quoted 6 lines · Jonathan Cameron

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I guess if it's always in${i}-in${i+1}, it's still not too hard.

I think they have been so far, but doubt this is universal. How about having a diff type and just having a pair of indices in the channel structure? Actually may need a third for x^2+y^2+z^2 devices. (iirc there are parts that do x^2+y^2 despite also having a z channel)
...

If two identifiers are common, that would probably be fine.

If you have a x^2+y^2+z^2 device, it might be easier to call that a different type with a fixed name, as long as there is a small number of combinations.

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I'm slightly confused, mainly because I have no idea how the buffer and event interfaces are actually being used. I would have expected an interface as simple as:

* One iio device per sensor that can provide data, possibly many per hardware device

One per device.

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* One chardev for each iio device

currently 1-3. (event line, buffer access, buffer event)

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* Use epoll to wait for data and/or out-of-band messages
* Use chrdev read to get events from the buffer

and data?

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* Use sysfs to read from sensors that are not event based

What you have is obviously more complex than this, and you probably have good reasons that I still need to understand.

I just went back to the older email and noticed that I missed some of your important replies.

Quoted 4 lines · Jonathan Cameron

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I'm slightly confused, mainly because I have no idea how the buffer and event interfaces are actually being used. I would have expected an interface as simple as:

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* One iio device per sensor that can provide data, possibly many per hardware device

One per device.

I'm not sure what that means. One iio device per hardware device?

What about hardware devices that have multiple unrelated streams of buffered input data?

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* One chardev for each iio device

currently 1-3. (event line, buffer access, buffer event)

It would be really nice to unify this, as I said. What are the reasons why you think it cannot or should not be done?

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* Use epoll to wait for data and/or out-of-band messages
* Use chrdev read to get events from the buffer

and data?

I mean get the data associated with the event. The event itself as you said does not have any data, so we would not need to read it, just to use poll()/epoll() in order to wait for it.

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* Use sysfs to read from sensors that are not event based

What you have is obviously more complex than this, and you probably have good reasons that I still need to understand.

Arnd
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Quoted 2 lines · Arnd Bergmann

What about hardware devices that have multiple unrelated streams of buffered input data?

Certainly plausible, but so far the only ones I've seen that actually do this are really just two bits of silicon in the same plastic package. They tend to use different i2c addresses or spi chip selects anyway so as far as the kernel is concerned are completely separate. You are correct that any device which truly has different streams of data would indeed need more than one device.

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* One chardev for each iio device

currently 1-3. (event line, buffer access, buffer event)

It would be really nice to unify this, as I said. What are the reasons why you think it cannot or should not be done?

Simplicity perhaps, but I'll definitely give your suggestions a go and see where we end up.

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* Use epoll to wait for data and/or out-of-band messages
* Use chrdev read to get events from the buffer

and data?

I mean get the data associated with the event. The event itself as you said does not have any data, so we would not need to read it, just to use poll()/epoll() in order to wait for it.

Sure. But devices can do a heck of a lot of different events. (certainly 10's or maybe more). I'm not immediately clear on how to handle this via poll etc. This is probably just because I've never tried though!

Quoted 11 lines · Jonathan Cameron

Quoted 2 lines · Arnd Bergmann

What about hardware devices that have multiple unrelated streams of buffered input data?

Certainly plausible, but so far the only ones I've seen that actually do this are really just two bits of silicon in the same plastic package. They tend to use different i2c addresses or spi chip selects anyway so as far as the kernel is concerned are completely separate. You are correct that any device which truly has different streams of data would indeed need more than one device.

Ok.

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* One chardev for each iio device

currently 1-3. (event line, buffer access, buffer event)

It would be really nice to unify this, as I said. What are the reasons why you think it cannot or should not be done?

Simplicity perhaps, but I'll definitely give your suggestions a go and see where we end up.

Since I haven't fully understood the distinction between the three chardevs, it may of course turn out a bad idea, but I think it would simplify the core code if you could assume that every iio device has exactly one chardev interface, so you could give them the same unique number and manage the life time together.

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* Use epoll to wait for data and/or out-of-band messages
* Use chrdev read to get events from the buffer

and data?

I mean get the data associated with the event. The event itself as you said does not have any data, so we would not need to read it, just to use poll()/epoll() in order to wait for it.

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Sure. But devices can do a heck of a lot of different events. (certainly 10's or maybe more). I'm not immediately clear on how to handle this via poll etc. This is probably just because I've never tried though!

(e)poll can generally distinguish between very few types of
activity: data for reading available, space for writing available, out-of-band events (to be read with e.g. ioctl) and errors.

If you want to wait for multiple equal types of events for one hardware device, it would be logical to have multiple character devices for them, so a user could open and wait for some of them independent of the others.

Intuitively, I would also expect these to be separate iio devices for the same hardware (each with one chardev), but there may be good reasons why that is not possible.

Quoted 26 lines · Arnd Bergmann

Quoted 11 lines · Jonathan Cameron

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What about hardware devices that have multiple unrelated streams of buffered input data?

Certainly plausible, but so far the only ones I've seen that actually do this are really just two bits of silicon in the same plastic package. They tend to use different i2c addresses or spi chip selects anyway so as far as the kernel is concerned are completely separate. You are correct that any device which truly has different streams of data would indeed need more than one device.

Ok.

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* One chardev for each iio device

currently 1-3. (event line, buffer access, buffer event)

It would be really nice to unify this, as I said. What are the reasons why you think it cannot or should not be done?

Simplicity perhaps, but I'll definitely give your suggestions a go and see where we end up.

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Since I haven't fully understood the distinction between the three chardevs, it may of course turn out a bad idea, but I think it would simplify the core code if you could assume that every iio device has exactly one chardev interface, so you could give them the same unique number and manage the life time together.

It simplifies that corner, but I'm a little worried that it will add a lot of interlinks between the currently fairly disconnected elements that go through a character device.

If we can keep those links to a minimum (which I think we can, but haven't tried yet!) it will be a sensible move.

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* Use epoll to wait for data and/or out-of-band messages
* Use chrdev read to get events from the buffer

and data?

I mean get the data associated with the event. The event itself as you said does not have any data, so we would not need to read it, just to use poll()/epoll() in order to wait for it.

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Sure. But devices can do a heck of a lot of different events. (certainly 10's or maybe more). I'm not immediately clear on how to handle this via poll etc. This is probably just because I've never tried though!

(e)poll can generally distinguish between very few types of
activity: data for reading available, space for writing available, out-of-band events (to be read with e.g. ioctl) and errors.

If you want to wait for multiple equal types of events for one hardware device, it would be logical to have multiple character devices for them, so a user could open and wait for some of them independent of the others.

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Intuitively, I would also expect these to be separate iio devices for the same hardware (each with one chardev), but there may be good reasons why that is not possible.

For reasons above,

there can only be one iio device per
physical hardware. We could define some other intermediate representation similar to the bus structure we currently have, but then I'm not sure where we gain.

As we only care about single reader cases here,
the reader can simply configure which events it is interested in to be the only ones produced. A good chunk of the sysfs interface is concerned with doing this.

The ioctl approach you suggest can then be used to query what actually occurred.

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Quoted 7 lines

Since I haven't fully understood the distinction between the three chardevs, it may of course turn out a bad idea, but I think it would simplify the core code if you could assume that every iio device has exactly one chardev interface, so you could give them the same unique number and manage the life time together.

It simplifies that corner, but I'm a little worried that it will add a lot of interlinks between the currently fairly disconnected elements that go through a character device.

If we can keep those links to a minimum (which I think we can, but haven't tried yet!) it will be a sensible move.

Hi Arnd,

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I've started looking at the current code base in staging, this is basically a log of what I'm finding there that may get improved:

Thanks for taking a look!

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* Your bus_type is not really a bus in the sense we normally use it, because it does not have any driver/device matching. You only register devices from the same files that drive the hardware. This is normally done using a class, not a bus.

That's what I originally thought and indeed how it was initially done. The responses from Greg KH and Kay Sievers to one of our ABI discussions convinced me otherwise.

http://lkml.org/lkml/2010/1/20/233 + the previous email in that thread.

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* Since you specify the sysfs attributes globally in the documentation, it would be nice if drivers did not have to implement them as attribute show function, but as a function that simply returns a value that is then printed by common code. You can do this using a structure of function pointers that each driver fills with the relevant ones, like file_operations, replacing the attribute groups.

This may indeed work. However it would be a fair bit more fiddly than that a simple function pointer structure as the sets of attributes supplied by different devices vary considerably.

So to confirm I understand you correctly we either have something like:
/* access ops */

int (*get_accel_x)(struct iio_dev *devinfo)
char *(*get_accel_offset)(struct iio_dev *devinfo); /* note these can't return a value as they may well be floating point */
char *(*get_accel_gain)(struct iio_dev *devinfo)
int (*get_accel_y)(struct iio_dev *devinfo)

Sticky cases that will make this fiddly.

* Unknown maximum number of some types of attributes.
- Could use a

	int (*get_in)(struct iio_dev dev_info, int channel);
	int num_in;

* Whatever you pick also immediately becomes restrictive as the main class has to be updated to handle any new elements.
* Note we'll have function points for all the scan element related stuff as well. Basically the set that would be needed is huge.

At the end of the day I'm unconvinced this makes sense from the point of view of simplifying the driver code. The gain is that it enforces the abi and may allow in kernel uses.
Ultimately we lifted this approach from hwmon where it works well under similar circumstances.

Alternative is a small set such as:

/* access ops */
int (*get_channel_value)(struct iio_dev *info, enum CHANNEL); char (*get_channel_offset)(struct iio_dev *info, enum CHANNEL);

and a list of which channels exist. Then the driver side becomes a massive switch statement rather than the attribute tables. Again, I'm not convinced that is actually cleaner or easier to understand. Also this approach pretty much means you end up with large numbers of equal attributes as you can't trivially group them. e.g we have accel_scale to cover case where it's the same for all accelerometer axes.

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* iio_allocate_device() could get a size argument and allocate the dev_data together with the iio_dev in a single allocation, like netdev_alloc does. In addition, you can pass a structure with all constant data, such as THIS_MODULE, and the operations and/or attribute groups, instead of having to set them individually for each dev.

Good idea.

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* iio_allocate_device() could get a size argument and allocate the dev_data together with the iio_dev in a single allocation, like netdev_alloc does. In addition, you can pass a structure with all constant data, such as THIS_MODULE, and the operations and/or attribute groups, instead of having to set them individually for each dev.

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Good idea.

There is an issue here I'd forgotten. Some of our structures have a elements which are cacheline aligned. For first version of this I'll set our alignment requirements as L1_CACHE_BYTES. Would be nice to be able to relax that down the line though as it means we use at least two cachelines even if we don't care about that alignment requirement.