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Predictable video assignment for HD-PVR using udev

Version 1 (christian, 08/06/2010 06:52 pm)

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h1. Predictable video assignment for HD-PVR using udev
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If you have more than one HD-PVR on your system then it is possible that they will occasionally swap positions on video0 and video1 after a reboot. The result of course is that then your channel changer will change the channel on one set top box (STB) and you will end up recording on another.






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In order to prevent this, you can use *udev* rules to force assignment of specific HD_PVRs to specific video devices.






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First, execute the following:






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<pre>udevadm info -a -p $(udevadm info -q path -n /dev/video0)</pre>






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and look for something like:






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<pre>    ATTRS{product}=="Hauppauge HD PVR"






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    ATTRS{serial}=="xxxxxxxx"</pre>






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where *xxxxxxxx* is the short form serial number of the HD PVR






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Alternatively this can be found using:






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<pre>udevadm info --query=all --name=/dev/video0</pre>






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and look for






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<pre>E: ID_SERIAL_SHORT=xxxxxxxx</pre>






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Next, you need to create a file for udev to execute when an HD-PVR is found. The file should be */etc/udev/rules.d/* and you can call it something like *78-hdpvr.rules*. This file will have the following content with one line for each HD-PVR you have:






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<pre>KERNEL=="video[0-9]*" , ATTRS{product}=="Hauppauge HD PVR" , ATTRS{serial}=="xxxxxxxx" , NAME="v4l/video0", SYMLINK+="video0"






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KERNEL=="video[0-9]*" , ATTRS{product}=="Hauppauge HD PVR" , ATTRS{serial}=="yyyyyyyy" , NAME="v4l/video1", SYMLINK+="video1"</pre>






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where *xxxxxxxx* and *yyyyyyyy* are the values obtained with udevadm.






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To clarify, this will assign the HD-PVR with the matching serial number to /dev/v4l/videoX and create a symlink in /dev/videoX. The "+=" means preserve any other links created to these devices so be default on our system /dev/video will be the same symlink as /dev/video0.






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If you want to verify that udev id doing what you want then you can use the following command:






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<pre>udevadm test /class/video4linux/video0</pre>






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substituting the appropriate video identifies (ie. video0 or video1). Read through the resulting output and verify that it executed your command and created the needed links.