IIO device indexing issue

Hi,

When we were using kernel 3.2 and with that board files we just got IIO devices with static order so that we knew exactly what iio:device0 is.

Now with device tree that order is not so static anymore especially when using device overlays (have not yet tried that thou). If there would be deferred probe for the device then if we have multiple iio:device's then those could in theory be in any order.

In example libiio uses iio device index to open the IIO device. So the problem is to know what iio:device is what.

If we look files under /sys/bus/iio/iio:deviceX. They do not reveal what device this is (especially if there are multiple device of same type). There does not seem to be a way to give custom name for the iio device from device tree that could have been used as a cue.

Device tree aliases kinda sounded a good idea to try. Eg. define adc0 alias and then link it to actual device node in tree.

Aliases could be found under /proc/device-tree/aliases. Looking at it shows what device tree node it is. However there does not seem to be actual link from any /proc/device-tree entries to kernel drivers exposed under sysfs. And even the path components in device tree are not in same format as in sysfs. So there is no 1:1 mapping possible here.

IIO devices in /sys/bus/iio/iio:deviceX seem to be symlinks to actual devices under /sys/devices and from there is physical path to the device and under that the IIO device with the same name as is under /sys/bus/iio.

So in theory we could make a mapping configuration file that specifies logical name for iio device and then physical parent path for the device and then find under it iio:device* files to determine what is the iio device index for this physical device. Then open IIO device with that index in example with libiio. Sounds a bit complex?

So did we miss something on how this should be defined/accessed or is there a slight issue here on how to identify iio devices?

Don't know how device tree overlays affect this if we first load device tree overlay with some configuration and then unload it and load another one with different configuration.

Quoted 37 lines · Vesa Jääskeläinen

Hi,

When we were using kernel 3.2 and with that board files we just got IIO devices with static order so that we knew exactly what iio:device0 is.

Now with device tree that order is not so static anymore especially when using device overlays (have not yet tried that thou). If there would be deferred probe for the device then if we have multiple iio:device's then those could in theory be in any order.

In example libiio uses iio device index to open the IIO device. So the problem is to know what iio:device is what.

If we look files under /sys/bus/iio/iio:deviceX. They do not reveal what device this is (especially if there are multiple device of same type). There does not seem to be a way to give custom name for the iio device from device tree that could have been used as a cue.

Device tree aliases kinda sounded a good idea to try. Eg. define adc0 alias and then link it to actual device node in tree.

Aliases could be found under /proc/device-tree/aliases. Looking at it shows what device tree node it is. However there does not seem to be actual link from any /proc/device-tree entries to kernel drivers exposed under sysfs. And even the path components in device tree are not in same format as in sysfs. So there is no 1:1 mapping possible here.

IIO devices in /sys/bus/iio/iio:deviceX seem to be symlinks to actual devices under /sys/devices and from there is physical path to the device and under that the IIO device with the same name as is under /sys/bus/iio.

So in theory we could make a mapping configuration file that specifies logical name for iio device and then physical parent path for the device and then find under it iio:device* files to determine what is the iio device index for this physical device. Then open IIO device with that index in example with libiio. Sounds a bit complex?

Quoted 6 lines

So did we miss something on how this should be defined/accessed or is there a slight issue here on how to identify iio devices?

Don't know how device tree overlays affect this if we first load device tree overlay with some configuration and then unload it and load another one with different configuration.

Hi,

Yes, excellent analysis. This is a real issue. As you said there is no guarantee that the IIO device numbers are stable, they are assigned in the order the devices are registered. If the device are registered in a different order for whatever reason the numbers will change. You can use readlink on the IIO device in /sys/bus/iio/devices to get its position in the global device topology and be able to tell what the parent device is, but this path might not be completely stable either though. E.g. if your device is on a I2C bus and the number of the I2C bus is dynamically assigned it might change when the probe order changes.

Alias seem to be one way to solve this issue. The big question remains is how to communicate the alias to userspace. For other device classes the alias index is used as the device index e.g. a device with alias i2c0 ends up being the i2c adapter with index 0. But with IIO where we support a wide range of different devices that does not really work. E.g. what to do if you have adc0 and dac0 as aliases for different devices. So you'd need a different way to expose the alias.

Some bindings also use the "label" property to assign a unique name to node. But again the same issue how to expose that information to applications.

Quoted 64 lines · Lars-Peter Clausen

Quoted 37 lines · Vesa Jääskeläinen

Hi,

When we were using kernel 3.2 and with that board files we just got IIO devices with static order so that we knew exactly what iio:device0 is.

Now with device tree that order is not so static anymore especially when using device overlays (have not yet tried that thou). If there would be deferred probe for the device then if we have multiple iio:device's then those could in theory be in any order.

In example libiio uses iio device index to open the IIO device. So the problem is to know what iio:device is what.

If we look files under /sys/bus/iio/iio:deviceX. They do not reveal what device this is (especially if there are multiple device of same type). There does not seem to be a way to give custom name for the iio device from device tree that could have been used as a cue.

Device tree aliases kinda sounded a good idea to try. Eg. define adc0 alias and then link it to actual device node in tree.

Aliases could be found under /proc/device-tree/aliases. Looking at it shows what device tree node it is. However there does not seem to be actual link from any /proc/device-tree entries to kernel drivers exposed under sysfs. And even the path components in device tree are not in same format as in sysfs. So there is no 1:1 mapping possible here.

IIO devices in /sys/bus/iio/iio:deviceX seem to be symlinks to actual devices under /sys/devices and from there is physical path to the device and under that the IIO device with the same name as is under /sys/bus/iio.

So in theory we could make a mapping configuration file that specifies logical name for iio device and then physical parent path for the device and then find under it iio:device* files to determine what is the iio device index for this physical device. Then open IIO device with that index in example with libiio. Sounds a bit complex?

Quoted 6 lines

So did we miss something on how this should be defined/accessed or is there a slight issue here on how to identify iio devices?

Don't know how device tree overlays affect this if we first load device tree overlay with some configuration and then unload it and load another one with different configuration.

Hi,

Yes, excellent analysis. This is a real issue. As you said there is no guarantee that the IIO device numbers are stable, they are assigned in the order the devices are registered. If the device are registered in a different order for whatever reason the numbers will change. You can use readlink on the IIO device in /sys/bus/iio/devices to get its position in the global device topology and be able to tell what the parent device is, but this path might not be completely stable either though. E.g. if your device is on a I2C bus and the number of the I2C bus is dynamically assigned it might change when the probe order changes.

Alias seem to be one way to solve this issue. The big question remains is how to communicate the alias to userspace. For other device classes the alias index is used as the device index e.g. a device with alias i2c0 ends up being the i2c adapter with index 0. But with IIO where we support a wide range of different devices that does not really work. E.g. what to do if you have adc0 and dac0 as aliases for different devices. So you'd need a different way to expose the alias.

Quoted 3 lines

Some bindings also use the "label" property to assign a unique name to node. But again the same issue how to expose that information to applications.

To continue from this "label" property idea I was wondering if we would add it as new optional(?) file node for IIO devices.

One could then specify it like:

tscadc: tscadc@44e0d000 {
    compatible = "ti,am3359-tscadc";
...
    am335x_adc: adc {
        compatible = "ti,am3359-adc";
...
        label = "Port A";
    };
};

And this would generate file /sys/bus/iio/iio:deviceX/label with contents of "Port A".

Then during the application startup it would just need to scan all devices under /sys/bus/iio and determine what labelled device it wants to use.

It would be up to device's developer to determine what labels to use in their designs. This would not break ABI and would be just an extension for it.

One could also auto-assign label "am335x_adc" in this case too. But if you include existing arch device tree then changing label in top level is kinda a bit annoying as you would then need to duplicate all properties with another label and disable arch device tree's settings. Could cause also conflict if there are references elsewhere to existing arch nodes.

Having following in device's device tree file would allow one to override label or just only specify that.

#include "am33xx.dtsi"

&tscadc {
    status = "okay";

    adc {
        ti,adc-channels = <4 5 6 7>;

        label = "Port A";
    };
};

I think this "label" model would be simple to understand.

Whether this needs to be implemented as per device driver feature or could be implemented as generic IIO functionality I do not know.

Thanks,
Vesa Jääskeläinen

Quoted 104 lines · Vesa Jääskeläinen

Quoted 64 lines · Lars-Peter Clausen

Quoted 37 lines · Vesa Jääskeläinen

Hi,

When we were using kernel 3.2 and with that board files we just got IIO devices with static order so that we knew exactly what iio:device0 is.

Now with device tree that order is not so static anymore especially when using device overlays (have not yet tried that thou). If there would be deferred probe for the device then if we have multiple iio:device's then those could in theory be in any order.

In example libiio uses iio device index to open the IIO device. So the problem is to know what iio:device is what.

If we look files under /sys/bus/iio/iio:deviceX. They do not reveal what device this is (especially if there are multiple device of same type). There does not seem to be a way to give custom name for the iio device from device tree that could have been used as a cue.

Device tree aliases kinda sounded a good idea to try. Eg. define adc0 alias and then link it to actual device node in tree.

Aliases could be found under /proc/device-tree/aliases. Looking at it shows what device tree node it is. However there does not seem to be actual link from any /proc/device-tree entries to kernel drivers exposed under sysfs. And even the path components in device tree are not in same format as in sysfs. So there is no 1:1 mapping possible here.

IIO devices in /sys/bus/iio/iio:deviceX seem to be symlinks to actual devices under /sys/devices and from there is physical path to the device and under that the IIO device with the same name as is under /sys/bus/iio.

So in theory we could make a mapping configuration file that specifies logical name for iio device and then physical parent path for the device and then find under it iio:device* files to determine what is the iio device index for this physical device. Then open IIO device with that index in example with libiio. Sounds a bit complex?

Quoted 6 lines

So did we miss something on how this should be defined/accessed or is there a slight issue here on how to identify iio devices?

Don't know how device tree overlays affect this if we first load device tree overlay with some configuration and then unload it and load another one with different configuration.

Hi,

Yes, excellent analysis. This is a real issue. As you said there is no guarantee that the IIO device numbers are stable, they are assigned in the order the devices are registered. If the device are registered in a different order for whatever reason the numbers will change. You can use readlink on the IIO device in /sys/bus/iio/devices to get its position in the global device topology and be able to tell what the parent device is, but this path might not be completely stable either though. E.g. if your device is on a I2C bus and the number of the I2C bus is dynamically assigned it might change when the probe order changes.

Alias seem to be one way to solve this issue. The big question remains is how to communicate the alias to userspace. For other device classes the alias index is used as the device index e.g. a device with alias i2c0 ends up being the i2c adapter with index 0. But with IIO where we support a wide range of different devices that does not really work. E.g. what to do if you have adc0 and dac0 as aliases for different devices. So you'd need a different way to expose the alias.

Quoted 3 lines

Some bindings also use the "label" property to assign a unique name to node. But again the same issue how to expose that information to applications.

To continue from this "label" property idea I was wondering if we would add it as new optional(?) file node for IIO devices.

One could then specify it like:

tscadc: tscadc@44e0d000 {
    compatible = "ti,am3359-tscadc";
...
    am335x_adc: adc {
        compatible = "ti,am3359-adc";
...
        label = "Port A";
    };
};

And this would generate file /sys/bus/iio/iio:deviceX/label with contents of "Port A".

Then during the application startup it would just need to scan all devices under /sys/bus/iio and determine what labelled device it wants to use.

It would be up to device's developer to determine what labels to use in their designs. This would not break ABI and would be just an extension for it.

One could also auto-assign label "am335x_adc" in this case too. But if you include existing arch device tree then changing label in top level is kinda a bit annoying as you would then need to duplicate all properties with another label and disable arch device tree's settings. Could cause also conflict if there are references elsewhere to existing arch nodes.

Having following in device's device tree file would allow one to override label or just only specify that.

#include "am33xx.dtsi"

&tscadc {
    status = "okay";

    adc {
        ti,adc-channels = <4 5 6 7>;

        label = "Port A";
    };
};

Quoted 6 lines

I think this "label" model would be simple to understand.

Whether this needs to be implemented as per device driver feature or could be implemented as generic IIO functionality I do not know.

Thanks,
Vesa Jääskeläinen

This would be a very useful feature as I'm encountering this problem at the moment. The label concept is simple and easy to use.

Quoted 64 lines · Lars-Peter Clausen

Quoted 37 lines · Vesa Jääskeläinen

Hi,

When we were using kernel 3.2 and with that board files we just got IIO devices with static order so that we knew exactly what iio:device0 is.

Now with device tree that order is not so static anymore especially when using device overlays (have not yet tried that thou). If there would be deferred probe for the device then if we have multiple iio:device's then those could in theory be in any order.

In example libiio uses iio device index to open the IIO device. So the problem is to know what iio:device is what.

If we look files under /sys/bus/iio/iio:deviceX. They do not reveal what device this is (especially if there are multiple device of same type). There does not seem to be a way to give custom name for the iio device from device tree that could have been used as a cue.

Device tree aliases kinda sounded a good idea to try. Eg. define adc0 alias and then link it to actual device node in tree.

Aliases could be found under /proc/device-tree/aliases. Looking at it shows what device tree node it is. However there does not seem to be actual link from any /proc/device-tree entries to kernel drivers exposed under sysfs. And even the path components in device tree are not in same format as in sysfs. So there is no 1:1 mapping possible here.

IIO devices in /sys/bus/iio/iio:deviceX seem to be symlinks to actual devices under /sys/devices and from there is physical path to the device and under that the IIO device with the same name as is under /sys/bus/iio.

So in theory we could make a mapping configuration file that specifies logical name for iio device and then physical parent path for the device and then find under it iio:device* files to determine what is the iio device index for this physical device. Then open IIO device with that index in example with libiio. Sounds a bit complex?

Quoted 6 lines

So did we miss something on how this should be defined/accessed or is there a slight issue here on how to identify iio devices?

Don't know how device tree overlays affect this if we first load device tree overlay with some configuration and then unload it and load another one with different configuration.

Hi,

Yes, excellent analysis. This is a real issue. As you said there is no guarantee that the IIO device numbers are stable, they are assigned in the order the devices are registered. If the device are registered in a different order for whatever reason the numbers will change. You can use readlink on the IIO device in /sys/bus/iio/devices to get its position in the global device topology and be able to tell what the parent device is, but this path might not be completely stable either though. E.g. if your device is on a I2C bus and the number of the I2C bus is dynamically assigned it might change when the probe order changes.

Alias seem to be one way to solve this issue. The big question remains is how to communicate the alias to userspace. For other device classes the alias index is used as the device index e.g. a device with alias i2c0 ends up being the i2c adapter with index 0. But with IIO where we support a wide range of different devices that does not really work. E.g. what to do if you have adc0 and dac0 as aliases for different devices. So you'd need a different way to expose the alias.

Quoted 3 lines

Some bindings also use the "label" property to assign a unique name to node. But again the same issue how to expose that information to applications.

To continue from this "label" property idea I was wondering if we would add it as new optional(?) file node for IIO devices.> One could then specify it like:

tscadc: tscadc@44e0d000 {
    compatible = "ti,am3359-tscadc";
...
    am335x_adc: adc {
        compatible = "ti,am3359-adc";
...
        label = "Port A";
    };
};

And this would generate file /sys/bus/iio/iio:deviceX/label with contents of "Port A".

Then during the application startup it would just need to scan all devices under /sys/bus/iio and determine what labelled device it wants to use.

It would be up to device's developer to determine what labels to use in their designs. This would not break ABI and would be just an extension for it.

One could also auto-assign label "am335x_adc" in this case too. But if you include existing arch device tree then changing label in top level is kinda a bit annoying as you would then need to duplicate all properties with another label and disable arch device tree's settings. Could cause also conflict if there are references elsewhere to existing arch nodes.

Having following in device's device tree file would allow one to override label or just only specify that.

#include "am33xx.dtsi"

&tscadc {
    status = "okay";

    adc {
        ti,adc-channels = <4 5 6 7>;

        label = "Port A";
    };
};

I think this "label" model would be simple to understand.

Whether this needs to be implemented as per device driver feature or could be implemented as generic IIO functionality I do not know.

Thanks,
Vesa Jääskeläinen

Signature · 3 lines

To unsubscribe from this list: send the line "unsubscribe linux-iio" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Quoted 47 lines · Vesa Jääskeläinen

To continue from this "label" property idea I was wondering if we would add it as new optional(?) file node for IIO devices.> One could then specify it like:

tscadc: tscadc@44e0d000 {
    compatible = "ti,am3359-tscadc";
...
    am335x_adc: adc {
        compatible = "ti,am3359-adc";
...
        label = "Port A";
    };
};

And this would generate file /sys/bus/iio/iio:deviceX/label with contents of "Port A".

Then during the application startup it would just need to scan all devices under /sys/bus/iio and determine what labelled device it wants to use.

It would be up to device's developer to determine what labels to use in their designs. This would not break ABI and would be just an extension for it.

One could also auto-assign label "am335x_adc" in this case too. But if you include existing arch device tree then changing label in top level is kinda a bit annoying as you would then need to duplicate all properties with another label and disable arch device tree's settings. Could cause also conflict if there are references elsewhere to existing arch nodes.

Having following in device's device tree file would allow one to override label or just only specify that.

#include "am33xx.dtsi"

&tscadc {
    status = "okay";

    adc {
        ti,adc-channels = <4 5 6 7>;

        label = "Port A";
    };
};

Quoted 4 lines

I think this "label" model would be simple to understand.

Whether this needs to be implemented as per device driver feature or could be implemented as generic IIO functionality I do not know.

The principal looks good to me. It's not however only an IIO issue so perhaps we should expand the discussion to include other subsystems likely to have similar issues (though perhaps to a lesser degree) such as hwmon and input? Any others?

We could also autobuild the label from other sources such as ACPI to uniquely describe the instance of the device (afterall all we really care about is that it always has the same name on the same hardware, being able to assign meaningful simple names would just be the icing on the cake!)

Jonathan

Quoted 3 lines

Thanks,
Vesa Jääskeläinen

--
To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html

Hi,

Quoted 57 lines · Jonathan Cameron

Quoted 47 lines · Vesa Jääskeläinen

To continue from this "label" property idea I was wondering if we would add it as new optional(?) file node for IIO devices.> One could then specify it like:

tscadc: tscadc@44e0d000 {
     compatible = "ti,am3359-tscadc";
...
     am335x_adc: adc {
         compatible = "ti,am3359-adc";
...
         label = "Port A";
     };
};

And this would generate file /sys/bus/iio/iio:deviceX/label with contents of "Port A".

Then during the application startup it would just need to scan all devices under /sys/bus/iio and determine what labelled device it wants to use.

It would be up to device's developer to determine what labels to use in their designs. This would not break ABI and would be just an extension for it.

One could also auto-assign label "am335x_adc" in this case too. But if you include existing arch device tree then changing label in top level is kinda a bit annoying as you would then need to duplicate all properties with another label and disable arch device tree's settings. Could cause also conflict if there are references elsewhere to existing arch nodes.

Having following in device's device tree file would allow one to override label or just only specify that.

#include "am33xx.dtsi"

&tscadc {
     status = "okay";

     adc {
         ti,adc-channels = <4 5 6 7>;

         label = "Port A";
     };
};

Quoted 4 lines

I think this "label" model would be simple to understand.

Whether this needs to be implemented as per device driver feature or could be implemented as generic IIO functionality I do not know.

The principal looks good to me. It's not however only an IIO issue so perhaps we should expand the discussion to include other subsystems likely to have similar issues (though perhaps to a lesser degree) such as hwmon and input? Any others?

Quoted 5 lines

We could also autobuild the label from other sources such as ACPI to uniquely describe the instance of the device (afterall all we really care about is that it always has the same name on the same hardware, being able to assign meaningful simple names would just be the icing on the cake!)

There is a similar discussion for the new character device GPIO interface.[1]

Simple labels have the problem that they could be the same for two devicetree overlays or chips that are connected via SPI.

Quoted 67 lines · Markus Pargmann

Hi,

Quoted 57 lines · Jonathan Cameron

Quoted 47 lines · Vesa Jääskeläinen

To continue from this "label" property idea I was wondering if we would add it as new optional(?) file node for IIO devices.> One could then specify it like:

tscadc: tscadc@44e0d000 {
     compatible = "ti,am3359-tscadc";
...
     am335x_adc: adc {
         compatible = "ti,am3359-adc";
...
         label = "Port A";
     };
};

And this would generate file /sys/bus/iio/iio:deviceX/label with contents of "Port A".

Then during the application startup it would just need to scan all devices under /sys/bus/iio and determine what labelled device it wants to use.

It would be up to device's developer to determine what labels to use in their designs. This would not break ABI and would be just an extension for it.

One could also auto-assign label "am335x_adc" in this case too. But if you include existing arch device tree then changing label in top level is kinda a bit annoying as you would then need to duplicate all properties with another label and disable arch device tree's settings. Could cause also conflict if there are references elsewhere to existing arch nodes.

Having following in device's device tree file would allow one to override label or just only specify that.

#include "am33xx.dtsi"

&tscadc {
     status = "okay";

     adc {
         ti,adc-channels = <4 5 6 7>;

         label = "Port A";
     };
};

Quoted 4 lines

I think this "label" model would be simple to understand.

Whether this needs to be implemented as per device driver feature or could be implemented as generic IIO functionality I do not know.

The principal looks good to me. It's not however only an IIO issue so perhaps we should expand the discussion to include other subsystems likely to have similar issues (though perhaps to a lesser degree) such as hwmon and input? Any others?

Quoted 5 lines

We could also autobuild the label from other sources such as ACPI to uniquely describe the instance of the device (afterall all we really care about is that it always has the same name on the same hardware, being able to assign meaningful simple names would just be the icing on the cake!)

There is a similar discussion for the new character device GPIO interface.[1]

Quoted 3 lines

Simple labels have the problem that they could be the same for two devicetree overlays or chips that are connected via SPI.

As far as I understand the overlays, you always specify a hardware hierarchy where you want to add additional information to device tree. This means that while adding device with overlays you have full control what device it is and then can configure what label property device should have.

If you would add two SPI devices those would be unique items in the device tree this means that overlay adding those would be unique and then you can assign unique label for the devices.

When you have full control for device tree and device tree overlays added to your device then this should not be problem?

Problem not addressed by device tree would be dynamically configured buses like USB which could have user connected devices. But then again you probably do not configure this with device tree either? In this case if "label" would be read&write property then udev or such could then fill it with details based on your configuration.

As commented on the thread you pointed out was question should the label property have physical path components in there. There was comment that symlink (in IIO case this would be symlinks under /sys/bus/iio/) should have the details available if this is needed for decision.

Basically application developer could have following tools to perform his decision:

Scan /sys/bus/iio/ for iio:deviceN's.

- Get "name" property (driver identification).
- Do realpath for symlink to get physical address
- Get "label" property to have cue what this device is for

On other thread one idea was to have UUID for devices as a label (fstab
+ partition UUID's was the idea behind it). How to make it stick for every boot is the problem here however. You could use UUID in hash mode (eg. version 5) and then just determine what details affect it. For USB devices which could be moved to other slots this is still an issue as on some cases it would be irrelevant on what slot it is and in some cases it could be relevant. There are also other properties that are device specific like should the UUID hash include serial number of the device or not? This same problem matches to other automatic label cases. So I suppose there is no one solution that fits for all cases.

To setup label automatically you seem to need to have local configuration.

If the process would be something like:

- Setup device tree with optional label property.
- If label is not defined in device tree leave it as empty
- When user space starts up it can have local configuration that adjusts labels
- If there is a daemon in system like udev and new device is inserted then it could have rules for configuring labels.

I think this would allow one to setup whatever identification one wants to use for label.