[mythtv-users] Understanding /dev/videoN requirements

[Robert Tsai]

On Fri, Feb 10, 2006 at 07:32:59AM -0500, Kirk Anderson wrote:
> I have an HD-3000 and PVR-350 card that have been running together
> fine for many months. Recently I attemted to add a USB webcam onto
> my Myth box. All hell broke loose. I spent several days playing with
> udev rules trying to massage these three /dev/videoN devices back to
> their pre-webcam configuration.  But I could never get Myth to work
> as long as that webcam was in the system.
> 
> Wondering if anyone else has similar experiences.
> 
> Here's what was there pre-webcam:
>     /dev/video0  --> HD-3000
>     /dev/video1  --> HD-3000
>     /dev/video3  --> PVR-350
> 
> "mythtv-setup" successfully detects these video devices and is happy
> to configure them properly.
> 
> Now, I add in the webcam.  By default (without modifying udev), the
> webcam gets assigned as /dev/video1. This apparently steals the
> device from the HD-3000 and also somehow breaks the PVR-350.
> 
> So then I got the idea to modify the udev rules files tomove the
> webcam to /dev/webcam0. This works ok. But even when I force
> (through udev rules) the HD-3000 and PVR-350 to be back to their
> original /dev/video positions,  "mythtv-setup" doesn't seem to
> recognize them as legitimate devices.  The PVR-350 menu in
> mythtv-setup never sees the PVR-350.
> 
> Once I removed the webcam and undid my changes to the udev rules,
> everything works fine again.
> 
> Is there some sort of trickery going on under the covers of
> mythtv-setup with regards to the /dev/videoN naming conventions and
> attaching to devices ?

I am guessing you probably have to modify your /etc/modprobe.d/aliases
to tell your webcam to use a different minor number. For example:

	alias char-major-81 videodev
	options ivtv ivtv_first_minor=2

You will see that "ls -l /dev/video*" have a major number of "81", and
various minor numbers (starting at 0 for /dev/video0).

The module driving your webcam ought to have some kind of option to
tell it to "skip" the first few minor numbers. The ivtv line above is
an example; it tells my PVR-500 to start with /dev/video2 (by default
it will want to start at /dev/video0 and conflict with my HD-3000
card, same as you). Each module will have its own configuration
parameter names, so you will have to examine your driver
documentation.

If not, I believe you can force it with something like:

	alias char-major-81-2 webcam

to force your webcam to use /dev/video2 . But I'm not 100% sure about
that.

--Rob
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