Re: work-around for video4linux sysfs

[Kees Cook <kees@outflux.net>]

Hi Kay,

On Thu, Aug 02, 2007 at 11:24:08AM +0200, Kay Sievers wrote:
> We will not create any persistent links which contain simple
> enumeration or driver names. That just doesn't make sense. Driver
> names are kernel implementation details that are free to change any
> time, you can match on them with rules, but not put them into
> persistent link names.

This doesn't seem consistent with things I saw in /dev/disk/by-path,
but I'm totally open to suggestions for /dev/v4l.  e.g.:

pci-0000:00:0e.0-scsi-0:0:0:0-part1
                  ^driver
                          ^simple enumeration

Things like the hda/sda swap for SATA changed the driver name here,
so I don't think it's unreasonable to use a driver name as part of
the symlink.  The simple enumeration in this case is based on physical
characteristics of the device, which I can see as being different from
the per-driver v4l enumeration.

Note, however, that my proposed code does not use the per-driver
enumeration because there was no way to access it reliably.  The
proposed code produces a tree like so:

/dev/v4l
`-- by-path
    |-- pci-0000:01:06.0-cx8800 -> ../../video0
    `-- pci-0000:01:07.0-ivtv -> ../../video1

> Also the enumeration number appended to the end of the name creates
> more problem than is solves, because it again depends on discovery
> order of the device, which can also change at any time.

AFAIK, the v4l per-driver enumeration has a fixed order (i.e. ivtv0,
ivtv1).  (The general v4l device enumeration is not -- video0, video1).

Again, I just want a unique identifier for the v4l devices, and was
offering a working code example.  Without the driver-name-fetching, the
path-id output had this ugly trailing "-", which seemed to be calling
for more stuff to be put after it.  (e.g. "pci-0000:01:06.0-")

I have no problem changing it to be more in line with what you'd like
to see; I just want to solve the problem.  :)  Can you make some
suggestions on what a clean solution would look like?

-- 
Kees Cook                                            @outflux.net

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