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devsk
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Joined: 24 Oct 2003
Posts: 3003
Location: Bay Area, CA
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 Posted: Sun Jun 22, 2008 4:27 am Post subject: Need little help with UDEV (SOLVED) |
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I want to assign /dev/video0 to the webcam (because that's the only way skype works with it) and assign /dev/video1 to the pvrusb2 device. I wrote this simple (but crazy) rule:
| Code: | KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", ATTRS{name}=="UVC Camera (046d:0991)", SYMLINK+="v4l/webcam", PROGRAM="/bin/sh -c '/bin/ln -sf /dev/v4l/webcam /dev/video0;/bin/ln -sf /dev/v4l/pvrusb2 /dev/video1'"
KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", ATTRS{name}=="", SYMLINK+="v4l/pvrusb2", PROGRAM="/bin/sh -c '/bin/ln -sf /dev/v4l/webcam /dev/video0;/bin/ln -sf /dev/v4l/pvrusb2 /dev/video1'" |
And I get:
| Code: | # l video[0-9]
lrwxrwxrwx 1 root root 15 Jun 21 21:12 video0 -> /dev/v4l/webcam
lrwxrwxrwx 1 root root 10 Jun 21 21:12 video1 -> v4l/video1
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What I had expected was:
| Code: | # l video[0-9]
lrwxrwxrwx 1 root root 15 Jun 21 21:13 video0 -> /dev/v4l/webcam
lrwxrwxrwx 1 root root 16 Jun 21 21:13 video1 -> /dev/v4l/pvrusb2 |
As you can see, its inherently racy because once udev finishes executing PROGRAM, the links are fine but when pvrusb2 finishes loading it replaces /dev/video1 link with the v4l/video1. It happens to be correct in this case but it can easily create a mess.
The basic problem is that I want a proper symlink for /dev/video0 which UDEV allows but the symlink gets replaced by the driver itself to point to the v4l/video* location. So, depending upon who (pvrusb2/uvcvideo or UDEV) wins the race, I get right or wrong symlinks.
One solution can be that I have a cron job running which can rectify the symlinks every 5 seconds with: | Code: | | /bin/ln -sf /dev/v4l/webcam /dev/video0;/bin/ln -sf /dev/v4l/pvrusb2 /dev/video1 |
but that sounds like crazy hack and may interfere with both driver loading and udev doing its thing.
I also tried putting sleep in PROGRAM="/bin/sh -c 'sleep 10;/bin/ln -sf...'" but that didn't help.
Anybody got better ideas?
Last edited by devsk on Tue Jun 24, 2008 7:57 pm; edited 1 time in total |
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devsk
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Joined: 24 Oct 2003
Posts: 3003
Location: Bay Area, CA
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| Posted: Sun Jun 22, 2008 6:28 am Post subject: |
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I know people around here (except me of course...  ) have the smarts. So, where are the ideas? c'mon people! |
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devsk
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Joined: 24 Oct 2003
Posts: 3003
Location: Bay Area, CA
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| Posted: Mon Jun 23, 2008 10:00 pm Post subject: |
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| anyone? what happened to the UDEV experts around here? |
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PaulBredbury
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Joined: 14 Jul 2005
Posts: 7310
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| Posted: Tue Jun 24, 2008 8:28 am Post subject: |
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Sounds like you want:
| Code: | KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", ATTRS{name}=="UVC Camera (046d:0991)", SYMLINK+="video0"
KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", ATTRS{name}=="", SYMLINK+="video1" |
| Quote: | | but the symlink gets replaced by the driver itself |
What? What driver? Surely you mean "another udev rule"? In which case, there is OPTIONS="last_rule" |
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devsk
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Joined: 24 Oct 2003
Posts: 3003
Location: Bay Area, CA
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| Posted: Tue Jun 24, 2008 5:04 pm Post subject: |
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| PaulBredbury wrote: | Sounds like you want:
| Code: | KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", ATTRS{name}=="UVC Camera (046d:0991)", SYMLINK+="video0"
KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", ATTRS{name}=="", SYMLINK+="video1" |
| Quote: | | but the symlink gets replaced by the driver itself |
What? What driver? Surely you mean "another udev rule"? In which case, there is OPTIONS="last_rule" |
No, I meant the pvrusb2 driver. Here is what I get with the above rules (which is BTW the first thing I tried) after modprobe pvrusb2:
| Code: | # l /dev/video[01]
lrwxrwxrwx 1 root root 10 Jun 24 09:55 /dev/video0 -> v4l/video0
lrwxrwxrwx 1 root root 10 Jun 24 09:55 /dev/video1 -> v4l/video0
# l /dev/v4l/
total 0
drwxr-xr-x 2 root root 80 Jun 24 09:55 ./
drwxr-xr-x 17 root root 15860 Jun 24 09:55 ../
crw-rw---- 1 root video 81, 64 Jun 24 09:55 radio0
crw-rw---- 1 root video 81, 0 Jun 24 09:55 video0
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Here is what I get with "last_rule":
| Code: | # l /dev/video[01]
crw-rw---- 1 root root 81, 0 Jun 24 09:56 /dev/video0
lrwxrwxrwx 1 root root 6 Jun 24 09:56 /dev/video1 -> video0
# l /dev/v4l/
total 0
drwxr-xr-x 2 root root 60 Jun 24 09:56 ./
drwxr-xr-x 17 root root 15840 Jun 24 09:56 ../
crw-rw---- 1 root video 81, 64 Jun 24 09:56 radio0
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In both cases video0 is taken by the pvrusb2, exactly opposite of what I wanted. |
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PaulBredbury
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Joined: 14 Jul 2005
Posts: 7310
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| Posted: Tue Jun 24, 2008 5:11 pm Post subject: |
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Sounds like both rules are matching for the same device. There's a difference between ATTR and ATTRS - man udev explains.
Get debugging:
| Code: | | udevcontrol log_priority=debug |
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devsk
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Joined: 24 Oct 2003
Posts: 3003
Location: Bay Area, CA
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| Posted: Tue Jun 24, 2008 6:20 pm Post subject: |
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| PaulBredbury wrote: | Sounds like both rules are matching for the same device. There's a difference between ATTR and ATTRS - man udev explains.
Get debugging:
| Code: | | udevcontrol log_priority=debug |
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OK, changed to ATTR but the confusion continues. I will post the debug later.
After modprobe pvrusb2:
| Code: | # l /dev/video[01]
lrwxrwxrwx 1 root root 10 Jun 24 11:14 /dev/video0 -> v4l/video0
lrwxrwxrwx 1 root root 10 Jun 24 11:14 /dev/video1 -> v4l/video0
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After modprobe uvcvideo:
| Code: | # l /dev/video[01]
lrwxrwxrwx 1 root root 10 Jun 24 11:15 /dev/video0 -> v4l/video1
lrwxrwxrwx 1 root root 10 Jun 24 11:15 /dev/video1 -> v4l/video1
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devsk
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Joined: 24 Oct 2003
Posts: 3003
Location: Bay Area, CA
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| Posted: Tue Jun 24, 2008 7:46 pm Post subject: |
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Ok, I think we are getting closer. I removed the SYMLINK from the following line from file 40-video.rules and everything works like I need it:
| Code: |
# KERNEL=="video[0-9]*", NAME="v4l/video%n", SYMLINK+="%k", GROUP="video"
KERNEL=="video[0-9]*", NAME="v4l/video%n", GROUP="video"
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What was happening was that it creates its own symlink from this file and it is opposite of the symlink that I want, and hence one module overwrites the symlink of the other.
Now, how do I make this change permanent? |
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PaulBredbury
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Joined: 14 Jul 2005
Posts: 7310
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| Posted: Tue Jun 24, 2008 7:52 pm Post subject: |
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Overrule 40-video.rules, by using OPTIONS="last_rule"
Alternatively, you could name your file 91-local.rules and use SYMLINK= instead of SYMLINK+=
Note the removal of the plus sign. |
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devsk
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Joined: 24 Oct 2003
Posts: 3003
Location: Bay Area, CA
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| Posted: Tue Jun 24, 2008 7:55 pm Post subject: |
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Ok got it! This is what worked without changing 40-video.rules.
| Code: | KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", ATTR{name}=="UVC Camera (046d:0991)", NAME="v4l/video%n", GROUP="video", SYMLINK="video0", OPTIONS="last_rule"
KERNEL=="video[0-9]*", SUBSYSTEM=="video4linux", ATTR{name}=="", NAME="v4l/video%n", GROUP="video", SYMLINK="video1", OPTIONS="last_rule" |
The key was SYMLINK= instead of +=, adding NAME and OPTIONS="last_rule".
Thanks Paul for the help! Appreciate it! |
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