linux-kernel - Class device namespaces

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Date:	Sun, 29 Mar 2009 17:48:36 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Greg Kroah-Hartman <gregkh@...e.de>
Cc:	linux-kernel@...r.kernel.org, Kay Sievers <kay.sievers@...y.org>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	Michael E Brown <Michael_E_Brown@...l.com>
Subject: Class device namespaces

Hi Greg,

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?

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.

Thanks,
-- 
Jean Delvare
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