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Groups > comp.os.linux.misc > #7709

Re: assigning specific /dev/videoN to specific USB webcams

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On Sun, 24 Mar 2013 22:54:57 +0000, J G Miller wrote:

> On Sunday, March 24th, 2013, at 21:51:32h +0000, Kevin Snodgrass
> explained:
> 
>> I have 2 DVRs, 1 has 2 identical Hauppauge PVR-250 cards, but they are
>> just straight NTSC cable
> 
> Not changed to digital (as in digital cable) yet?

Yes, but most of those channels are scrambled and I only have 1 cable 
box.  I've had this setup since 2003/2004 except for the pcHDTV card 
which I added a few years ago.

Giving serious consideration to purchasing an HDHomeRun Prime...

>> So far, the webcam always seems to be /dev/video0, but the PVR-250 and
>> the pcHDTV seem to be randomly assigned /dev/video1 and /dev/video2
> 
> Well probably because the kernel is probing the USB ports before it gets
> on to the PCI slots where the cards are located.

That makes sense, and it's also not a problem.  The problem is the semi-
randomness of /dev/video1 and /dev/video2.

> Your problem is rather more difficult to resolve because both of your
> cards are the same model.  Theoretically it should be possible to create
> a udev rule based on the absolute pci_path_to_device string because they
> are in different slots.

Actually they are different cards.  The PC with 2 PVR-250 cards also has 
identical inputs, so which card is which does not matter.

I mucked around a bit and found out that the /dev/videoX device nodes are 
symlinks from /dev/v4l/by-path/.  Since PCI slot is part of the device 
node name in /dev/v4l/by-path/ it is possible to use those instead of the 
simpler /dev/videoX device node names.  Example, on my system the PVR-250 
is on PCI bus 1, slot 7, so I can access it with /dev/v4l/by-path/pci-0000
\:01\:07.0-video-index0.  I.e., this works:

$ mplayer /dev/v4l/by-path/pci-0000\:01\:07.0-video-index0 

Since the bus/slot will not change unless I move the card I will just 
change my script(s) to use that instead.

>> BTW, does anyone know why the pcHDTV 3000 does NOT use /dev/videoX ? 
>> It uses /dev/dvb/adapterX/dvrX.  Is there a techinical reason why the
>> device driver authors chose that instead of /dev/videoX ?
> 
[snippage]
> 
> So oversimplifying perhaps -- analog video devices use /dev/videoN
> files, whereas digital video devices use /dev/dvb/adapterN directories.

ok, but WHY does V4L assign analog devices /dev/videoX and digital /dev/
dvb/adapterX ?  I guess I'm just less than impressed with the lack of 
consistancy.  Especially since there is a /dev/videoX node associated 
with the pcHDTV card, but it is completely useless as far as I can tell.  
Therefore, what's the point?

> But back on the topic of udev rules, it seems that this is not the
> newgroup that has any udev gurus.
> 
> Can anybody suggest where is the best place to get some help?

And back to the semi-randomness of hardware devices and their device node 
names... It's irritating to me at least, that Linux is NOT consistent 
with device names across reboots.  This causes me major issues 
elsewhere.  My opinion, the user/sysadmin shouldn't have to write scripts 
to work around things like semi-random device node naming from kernel 
modules to achieve simple things like recording from tuner card X vs card 
Y or automated checking with smartctl of hard drive health.  (1 machine 
has 4 drives, if drive 3 has some health issues, but I reboot that 
machine before checking the logs I create nightly, it gets assigned a 
different /dev/sdX name, now i don't know which drive it was that had the 
problem pre-reboot.)

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Thread

assigning specific /dev/videoN to specific USB webcams J G Miller <miller@yoyo.ORG> - 2013-03-17 19:20 +0000
  Re: assigning specific /dev/videoN to specific USB webcams Robert Heller <heller@deepsoft.com> - 2013-03-17 14:46 -0500
    Re: assigning specific /dev/videoN to specific USB webcams J G Miller <miller@yoyo.ORG> - 2013-03-17 20:04 +0000
  Re: assigning specific /dev/videoN to specific USB webcams Kevin Snodgrass <kdsnodgrass@yahoo.com> - 2013-03-24 21:51 +0000
    Re: assigning specific /dev/videoN to specific USB webcams J G Miller <miller@yoyo.ORG> - 2013-03-24 22:54 +0000
      Re: assigning specific /dev/videoN to specific USB webcams Robert Heller <heller@deepsoft.com> - 2013-03-24 18:27 -0500
        Re: assigning specific /dev/videoN to specific USB webcams J G Miller <miller@yoyo.ORG> - 2013-03-25 02:34 +0000
      Re: assigning specific /dev/videoN to specific USB webcams Kevin Snodgrass <kdsnodgrass@yahoo.com> - 2013-03-31 18:16 +0000
        Re: assigning specific /dev/videoN to specific USB webcams J G Miller <miller@yoyo.ORG> - 2013-03-31 19:33 +0000
          Re: assigning specific /dev/videoN to specific USB webcams Kevin Snodgrass <kdsnodgrass@yahoo.com> - 2013-04-11 14:19 +0000
            Re: assigning specific /dev/videoN to specific USB webcams J G Miller <miller@yoyo.ORG> - 2013-04-11 15:21 +0000
  Re: assigning specific /dev/videoN to specific USB webcams Eric Pozharski <whynot@pozharski.name> - 2013-04-01 13:52 +0300
    Re: assigning specific /dev/videoN to specific USB webcams J G Miller <miller@yoyo.ORG> - 2013-04-01 18:24 +0000

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