Re: work-around for video4linux sysfs

[Kees Cook <kees@outflux.net>]

On Wed, Aug 01, 2007 at 02:58:35PM -0700, Greg KH wrote:
> On Wed, Aug 01, 2007 at 02:31:30PM -0700, Kees Cook wrote:
> > Is the idea of /dev/v4l/by-path/... sane?  Once v4l sysfs was fixed,
> > it seems that to support this symlink, the path_id tool would still need
> > at least the second patch hunk.  Is this correct?
> 
> Probably, and yes, the v4l by-path stuff is a good idea, I'm not trying
> to keep that from happening.  But are there ids on these devices you can
> use too, to get a "by-id" path as well?  Do the USB or PCI devices have
> a serial number associated with them?

If there are serial numbers, the v4l sub-drivers aren't fetching them,
or I'm too dense to find them.  This is basically why I opted for the
by-path symlinks, since nothing else looked like a reasonable unique
string.  I had originally hoped to build one using something like:

$driver_name-$device_type_name-$driver_init_order

For example, if I had 2 ivtv devices, and one bttv device, I'd expect to
see something like:

ivtv-pvr250-0
ivtv-pvr500-1
bttv-thingy-0

But the v4l drivers don't have a common string for the "device_type_name"
in my example.  (They have a long description string that is usually
truncated, but whose layout isn't common between drivers.)  While ivtv
internally knows about ivtv0 and ivtv1, there doesn't seem to be a way
to query a v4l driver about which sub-instance it is.  This might be
another nice thing to plumb into sysfs, but I imagine it would be needed
for each v4l driver.

In the 3 example devices above, ivtv0 always inits before ivtv1 in the
same order, but bttv may come before ivtv, swapping the video0, video1,
video2 devices all over the place.

> > By my estimation, the v4l sysfs is missing a "driver" linkage?  I'm not
> > entirely sure where to start looking.
> 
> What does the v4l device sysfs directory tree look like?  Is the
> "driver" link the only thing you are missing to do this in an easier
> manner in your script?

I think if that was in there, it would at least solve the current gross
need to cd into the directory to pull a PWD.  :)  There may still be a
need to append the driver name to the path-id output, but I suspect that
would be simple to do in the already-needed second hunk of the patch.

-- 
Kees Cook                                            @outflux.net

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