Re: libudev, a question about the enumerate API.

On Mon, Mar 7, 2011 at 17:01, Alan Ott [off-list ref] wrote:

On 03/07/2011 10:27 AM, Kay Sievers wrote:

quoted

The usual enumeration in udev never uses the device tree in
/sys/devices/, only the subsystem lists /sys/{class,bus}/. We could
have something to enumerate all child devices of a given device. It's
not simple, but could be made working.

The documentation in Documentation/sysfs-rules.txt seems to warn against
using /sys/class and sys/bus:

- devices are only "devices"
 There is no such thing like class-, bus-, physical devices,
 interfaces, and such that you can rely on in userspace. Everything is
 just simply a "device". Class-, bus-, physical, ... types are just
 kernel implementation details which should not be expected by
 applications that look for devices in sysfs.

...
     - all elements of a devpath must be real directories. Symlinks
       pointing to /sys/devices must always be resolved to their real
       target and the target path must be used to access the device.
       That way the devpath to the device matches the devpath of the
       kernel used at event time.
     - using or exposing symlink values as elements in a devpath string
       is a bug in the application

...

     - accessing attributes reached by a symlink pointing to another device,
       like the "device"-link, is a bug in the application

...

- Hierarchy in a single device tree
 There is only one valid place in sysfs where hierarchy can be examined
 and this is below: /sys/devices.

and more...

I figured that this document was probably out of date, since even libudev
uses /sys/class. What's your take on this document?

I wrote that document. :) It's partly out-of-date, but this is still true.

Udev, since quite some years, even the old and dead HAL supports
/sys/subsystem/ already, which is not in the kernel today, but might
be soon.

If that directory is available, /sys/{class,bus}/ will not be looked at:
  http://git.kernel.org/?p=linux/hotplug/udev.git;a=blob;f=libudev/libudev-enumerate.c;h=128813c7d618717690f79d0eca82776a6c4fa011;hb=HEAD#l776

Kay
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