Re: Class device namespaces

[Jean Delvare]

Hi Kay,

On Sun, 29 Mar 2009 18:36:46 +0200, Kay Sievers wrote:
> On Sun, Mar 29, 2009 at 17:48, Jean Delvare <khali@xxxxxxxxxxxx> wrote:
> > I am a little confused by the directories created when one registers a
> > class device. When a class device is registered as the children of a
> > real device, a subdirectory by the class name is created, and the class
> > device is created there, effectively granting each class a separate
> > namespace. Example:
> >
> > /sys/devices/pci0000:00/0000:00:1f.3/i2c-adapter/i2c-0
> >
> > where 0000:00:1f.3 is the physical device, i2c-adapter the class name
> > and i2c-0 the class device.
> >
> > OTOH, if I create a class device as the children of another class
> > device, the class device is created directly, without a directory
> > between the parent and the child. Example:
> >
> > /sys/class/i2c-adapter/i2c-0/i2c-0
> >
> > where the first i2c-0 is an i2c-adapter class device, and the second
> > i2c-0 is an i2c-dev class device. I would have expected:
> >
> > /sys/class/i2c-adapter/i2c-0/i2c-dev/i2c-0
> >
> > The current behavior seems inconsistent to me. Is it done so on purpose,
> > or is this accidental? If on purpose, what's the reason?
> 
> It's on purpose. The "glue" directory exists only between class
> devices and bus devices. There is no need for class devices to have
> such a "glue".

The example I have is one where the need exists.

>                When we moved the class devs as childs of the bus devs,
> people complained, that they could no longer rename their netif to
> "irq", because the name was already taken by a pci dev atrribute. :)

That's what I had guessed, although this only moves the problem: if a
class has the same name as an attribute, a collision will happen.
Probably that's just less likely to happen than collisions between
class device names and device attributes.

> > I am asking because this is causing trouble in practice. We have both
> > i2c-dev and firmware_class which try to create class devices by the
> > same name and this of course collides. While I would blame
> > firmware_class for coming up with an horrible naming scheme (or
> > actually, for not coming up with any naming scheme) it might still be a
> > good idea to prevent such collisions at the driver core level.
> 
> You have multiple instances, which request a firmware file for the
> same parent device at the same time? Can't they request the firmware
> for the actual child instead of using the same parent?

No, there's a single firmware request which collides with an i2c-dev
class device name. The firmware request happens to use the parent
device's name as its class device name, and it happens that both the
i2c-adapter class and the i2c-dev class name their class devices the
same way (which makes sense as they refer to the same thing,
i2c-adapter for the kernel-space access and i2c-dev for the user-space
access), and the latter is a child of the former. So when an
i2c-adapter class device is used as the parent for a firmware request,
and the i2c-dev driver is loaded, you get a collision.

It could be argued that i2c-adapter and i2c-dev should be the same
class, but changing this now would break several user-space tools so I
doubt it's going to happen. The fact that i2c-dev is optional can also
be seen as a desirable feature.

> If the same parent needs to work, the firmware class needs to be fixed
> to allow that. Maybe it should use the requested firmware filename
> with the '/' translated to '!' as the name in sysfs, instead of the
> easy-to-have-a-clash device name of the parent?

Changing the name of the firmware class devices was already attempted
once, but was quickly reverted because it broke some user-space tools:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=109f0e93b6b728f03c1eb4af02bc25d71b646c59
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=7d640c4a5b36c4733460065db1554da924044511

Unless the user-space tool in question (Dell BIOS updater) has been
modified to no longer depend on this since then, changing the firmware
class device names isn't possible.

Now that we have a use case which would justify that a class device
child of another class device gets its own namespace, is there a chance
to see it happen?

Thanks,
-- 
Jean Delvare
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