Re: Udev. Problems with ordering hardware using /dev/video

[Florian Kulzer <florian.kulzer+debian@icfo.es>]

On Wed, Aug 01, 2007 at 00:45:54 +0200, Nigel Henry wrote:
> > On Tue, Jul 31, 2007 at 19:47:51 +0200, Nigel Henry wrote:

[...]

> > > My TV card was set as /dev/video0, and some time later the webcam was set
> > > as /dev/video1. this works fine with the pre 2.6.17 kernels (no Udev),
> > > and the TV card consistently is /dev/video0, and the webcam /dev/video1.
> > > Booting with the 2.6.17 kernel is a different scenario. Very hit and
> > > miss. Sometimes I boot up and starting Xawtv, I get the TV /dev/video0.
> > > Othertimes I bootup and start Xawtv, and get my webcam /dev/video1.

[...]

> Output below. Bear in mind that /dev/video0 should be the TV card, /dev/video1 
> the webcam, at least that is how they are loaded with pre Udev kernels.

[...]

> djmons@debian:~$ udevinfo -a -p $(udevinfo -q path -n /dev/video0)

[...]

>   looking at device '/class/video4linux/video0':
>     KERNEL=="video0"
>     SUBSYSTEM=="video4linux"
>     DRIVER==""
>     ATTR{exposure}=="0"
>     ATTR{hue}=="128"
>     ATTR{contrast}=="54"
>     ATTR{saturation}=="110"
>     ATTR{brightness}=="124"
>     ATTR{sensor}=="OV7620"
>     ATTR{bridge}=="OV511+"
>     ATTR{model}=="Generic Camera _no ID_"
>     ATTR{custom_id}=="0"
>     ATTR{name}=="OV511 USB Camera"
>     ATTR{dev}=="81:0"

[...]

> djmons@debian:~$ udevinfo -a -p $(udevinfo -q path -n /dev/video1)

[...]

>   looking at device '/class/video4linux/video1':
>     KERNEL=="video1"
>     SUBSYSTEM=="video4linux"
>     DRIVER==""
>     ATTR{card}=="10"
>     ATTR{name}=="BT878 video _Hauppauge _bt878__"
>     ATTR{dev}=="81:1"

[...]

> I saw that someone with network device problems, and Udev, was able to specify 
> the devices by name on 2 separate lines, so that Udev always recognised them 
> by name. I don't know if something similar is possible with my situation with 
> the TV card, and the webcam.

For network cards see: /etc/udev/rules.d/z25_persistent-net.rules.

/usr/share/doc/udev/writing_udev_rules/index.html describes in detail
how udev rules work. Based on the output that you posted this should
get you started:

### START ###

SUBSYSTEM=="video4linux", \
  ATTR{name}=="BT878 video _Hauppauge _bt878__", \
  NAME="video0"

SUBSYSTEM=="video4linux", \
  ATTR{name}=="OV511 USB Camera", \ 
  NAME="video1"

### END ###

Put these rules into a file "local-tvwebcam.rules" in /etc/udev/ with
ownership and permissions like the other *.rules files in this
directory. Then create a symlink ("ln -s ...") in /etc/udev/rules.d/
which points to your rules file. The names in rules.d determine the
order in which the rules are tested. You might have to experiment a bit;
I would try to start with something like 010_... .

My experience with network cards was that the NAME assignments sometimes
did not work reliably if I tried to use them to swap two existing
(kernel) names (i.e. eth0 and eth1). If you have similar problems then
it is probably best to replace the two NAME assignments in the rules by
two SYMLINK directives:

### START ###

SUBSYSTEM=="video4linux", \
  ATTR{name}=="BT878 video _Hauppauge _bt878__", \
  SYMLINK+="mytvcard"

SUBSYSTEM=="video4linux", \
  ATTR{name}=="OV511 USB Camera", \ 
  SYMLINK+="mywebcam"

### END ###

This will give you /dev/mytvcard and /dev/mywebcam as symlinks which
should always point to the correct /dev/video? device. Then you just
have to reconfigure your applications once to use the device symlinks
instead of the video? nodes.

-- 
Regards,            | http://users.icfo.es/Florian.Kulzer
          Florian   |