bus notifiers for the generic device model

There are certain bus types (notably MCA and parisc internal ones) that like to do generic houskeeping operations (claim slots, claim uniform resources etc.) when drivers are attached to devices.

I've added a fairly generic notifier callback to the bus_type so that these busses can intercept and process the events they're interested in.

At the moment, the only events are attachment and detachment of drivers but in principle, more could be added.

James

You can import this changeset into BK by piping this whole message to:
'| bk receive [path to repository]' or apply the patch as usual.

===================================================================

[email protected], 2002-12-04 10:21:28-06:00, [email protected]
make bus notify events more general in generic device model

[email protected], 2002-12-03 14:16:53-06:00, [email protected]
add device model bus notifier

This adds a bus notifier (per bus type) which is called after a device driver successfully attaches to a device. The idea behind this is that there are certain operations that need to be performed globally after driver attachment (bus resource claiming etc.). This notifier is set by the bus so that it can do these.

 drivers/base/bus.c     |    4 ++++
 include/linux/device.h |    9 +++++++++
 2 files changed, 13 insertions(+)

diff -Nru a/drivers/base/bus.c b/drivers/base/bus.c
@@ -145,6 +145,8 @@
 		 dev->bus_id,dev->driver->name);
 	list_add_tail(&dev->driver_list,&dev->driver->devices);
 	sysfs_create_link(&dev->driver->kobj,&dev->kobj,dev->kobj.name);
+	if(dev->bus && dev->bus->notify)
+		dev->bus->notify(dev, BUS_NOTIFIER_DRIVER_ATTACHED);
 }
 
 

@@ -260,6 +262,8 @@
 		if (drv->remove)
 			drv->remove(dev);
 		dev->driver = NULL;
+		if(dev->bus && dev->bus->notify)
+			dev->bus->notify(dev, BUS_NOTIFIER_DRIVER_DETACHED);
 	}
 }
 

diff -Nru a/include/linux/device.h b/include/linux/device.h
@@ -58,6 +58,13 @@
 	DEVICE_GONE		= 3,
 };
 
+/* bus notifier events: events that the bus type may choose to be notified
+ * about */
+enum bus_notifier {
+	BUS_NOTIFIER_DRIVER_ATTACHED,
+	BUS_NOTIFIER_DRIVER_DETACHED,
+};
+
 struct device;
 struct device_driver;
 struct device_class;

@@ -75,6 +82,8 @@
 	struct device * (*add)	(struct device * parent, char * bus_id);
 	int		(*hotplug) (struct device *dev, char **envp, 
 				    int num_envp, char *buffer, int buffer_size);
+	/* notify the bus that something happened */
+	void		(*notify)(struct device *dev, enum bus_notifier event);
 };
 
 

===================================================================

This BitKeeper patch contains the following changesets:
1.927..1.928
## Wrapped with gzip_uu ##

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`
end

But doesn't the bus specific core know when drivers are attached, as it was told to register or unregister a specific driver? So I don't see why this is really needed.

thanks,

greg k-h

The problem is that the bus specific core registration no-longer knows if the probes succeeded or failed (and if they did, what devices were attached), since probing is controlled by the base core.

What the bus needs to know is when a driver attaches to a specific device (and what device it has attached to).

Unless you have a better way of getting the attachment information out of the bus after the base probes have executed, a notifier seemed to be the simplest thing.

James

Not quite. Well, I guess you can modify all of your drivers to do this, but see below for an easier way.

Why not have a call in the driver that notifies the bus specific core of this? Or just check the status of the return value of your "probe" function that the bus provides. See usb_device_probe() and pci_device_probe() for examples of this.

thanks,

greg k-h

Well, I did do it this way for parisc. However, I assumed from reading the driver model docs that we were transitioning to using the generic driver probe rather than doing probe interceptions like this.

Doing it like this just seems rather clumsy. wouldn't it be better to deprecate bus specific registrations in favour of the generic one?

There is another advantage to notifiers: they can be chained. Certain bus architectures need to do additional setup for things like pci devices. They would be able to do this by attaching a notifier.

James

I don't know. Then for every bus specific driver, they would have to do the "dereference the structure" logic before they can start to determine if they should bind to this specific device. That's all the PCI and USB code really does, strip out the proper structures that are relevant to the specific bus type, and then call the driver.

So in the end you would probably have a lot of duplicated code that would be better off being in one place, like it is today :)

That seems a bit overkill. The bus specific code can add those architecture setup hooks right now if it's needed, right?

thanks,

greg k-h

I tend to agree with Greg, though this exposes one of the great tradeoffs of the driver model. We can consolidate data structures and operations on them into common ones, but sooner or later, we have to call down to the bus (or class)-specific objects, necessitating a conversion to the relevant object.

The current infrastructure was designed the way it is to provide seamless transition while features of the generic model evolved. This is precisely why the bus drivers provide intermediate calls for the generic struct device_driver. The thought was that one day we could hopefully convert everything to the generic way and everything would be fantastic.

Along they way, we've found several things stand in the way of making this a smooth transition, so the plan is to stick with the bus intermediaries until the infrastructure matures enough to tackle those problems more easily.

[ Replying to James... ]

I don't see how your patch adresses chaining; there is only one notifier per bus type.

On the other hand, I've considered implementing something like before (Most recently in the last couple of days). Essentially, one could have components register with a subsystem (e.g. bus type), that are called when a device is added or removed.

This would make conditional features and extensions easy to call. For example, drivers/pci/proc.c could register with the PCI bus to be called each time a device was added. This would eliminate uglies like:

int pci_proc_attach_device(struct pci_dev *dev);

#ifdef CONFIG_PROC_FS
        pci_proc_attach_device(dev);

#endif

Ditto for usbfs.

It would also provide a means to eliminate the power management information in struct device, as PM could be an extension of the top-level device subsystem, only called if CONFIG_PM is set.

However, there are a few details that are kinda nasty (like associating data between the extension and the device), and I've shelved the idea again..

-pat

Well, you can do it by storing the pointer privately and hijacking it. However, I think we'd use the existing kernel notifier chain stuff for that if it is valuable.

I'm happy with keeping probe and remove in the bus specific driver template and having the <bustype>_add_driver install generic device probe and remove routines to handle these cases. My point was that the docs implied I should use the generic driver probe and remove routines, which I can't without some type of functionality like this.

If you envisage us never eliminating the driver specific probe and remove routines, I'm happy. I'm less happy if there will come a day when I have to revisit all the converted drivers to do the elimination.

James

Likewise... I think with very minor changes, existing PCI-API-compliant drivers should stay pretty much the same as they are now.

Just to follow up..

I don't see us eliminating the bus-specific probe() and remove() methods. At least not for a very long time.

-pat

I believe that the stuff you're allowing the bus to do once the driver has been attached, is traditionally triggered by the driver in their probe() method once they've found a valid device. Which, in most cases, is a perfectly valid place to do it -- drivers have always been considered masters of their own destiny, and the things that this patch allows the bus to do seems like they should be triggered by the driver, with the bus supplying helpers.

The fact that the probe() method is overloaded to setup the device as well as verify that it should attach to it is a potential opportunity for improvement. There's been talk in the past about splitting it up into probe() and start() (or perhaps something better). The first would only verify that the device was a valid one to attach to, and the latter would actually do the dirty work of setting it up. It would be called in drivers/base/bus.c::attach(), like this:

===== drivers/base/bus.c 1.26 vs edited =====

@@ -228,6 +228,10 @@
 {
 	pr_debug("bound device '%s' to driver '%s'\n",
 		 dev->bus_id,dev->driver->name);
+
+	if (dev->driver->start)
+		dev->driver->start(dev);
+
 	list_add_tail(&dev->driver_list,&dev->driver->devices);
 	sysfs_create_link(&dev->driver->kobj,&dev->kobj,dev->kobj.name);
 }

I don't recall why the change was never done. Perhaps because of other distractions, or it seemed like it would be too much of a PITA to convert drivers to a two-step init sequence (though I think it could be done in a compatible manner).

-pat

Possibly because of the "do it in open(2)" rule?

Ignoring the device model entirely, if a driver does a lot of talking-to-the-hardware in its probe phase, I consider it buggy, in 2.4 or 2.5.

The network driver and chardev ones typically follow this rule quite well... probe is simple, just registering interfaces with the kernel. dev->open is where the driver should (and usually does) power-up the hardware, [re-]initialize it, etc.

So each time you come upon a driver that wants dev->driver->start(), look closely at the code and wonder why it can't perform the dev->driver->start() code in its interface's dev->open member.

Jeff

Oh, right. Sorry, I should have said 'init()' or 'setup()' rather than 'start()'.

In many drivers there are two discrete operations that happen in the probe() method: verifying the device can be claimed, and registering the interfaces. By breaking it into two calls, you can conceptually separate the actions. It would also give the bus a chance to intercept the call and perform whatever housekeeping it needed to do at that point, giving James what he wanted.

However, I don't see a great benefit of doing this, besides making it _blatantly_ obvious what each call is supposed to do. The driver can call the bus to perform housekeeping duties during the setup (probe()) period, and everything should be happy.

-pat

The change is when a device is bound to a driver (i.e. when attach / detach is called bus.c ).

-andmike
--
Michael Anderson
[email protected]

Whis is called after probe / remove is called for the driver, which can point to the bus specific functions, like PCI and USB cores do.

thanks,

greg k-h