How to force the device assignment with V4l V2.0?

How to force the device assignment with V4l V2.0?

[Ian Pickworth]

I'm having a play with the latest code from hg on the 2.6.27 kernel,
Gentoo installation. This is triggered by the gspca driver being
absorbed into the main tree (which is a great move by the way).

I have two devices - a CX88 based Hauppauge TV PCI card, and a USB
webcam. In the "old" style drivers, I could force the loading of the two
modules (cx8800 and gspca) in a set sequence, using blacklist and
modules.autoload. This is enough to ensure that cx88 gets /dev/video0,
and the usb webcam gets /dev/video1.

However, when trying a recent hg snapshot, the sequence of loading the
modules does not change what the V4l driver is doing when loaded.
Looking at dmesg, I see that the new drivers are doing quite a lot of
detecting work themselves - it looks like they pick up the USB device
first regardless of the module blacklist/load sequence I have specified.

So, question is: Is there a preferred way of forcing the sequence of
device assignment in V4L these days? I need the cx88 to be /dev/video0
and the USB webcam to be /dev/video1 - otherwise all sorts of programs
get confused.

Many thanks
Regards
Ian

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Re: How to force the device assignment with V4l V2.0?

[Pádraig Brady]

Ian Pickworth wrote:
> I have two devices - a CX88 based Hauppauge TV PCI card, and a USB
> webcam. In the "old" style drivers, I could force the loading of the two
> modules (cx8800 and gspca) in a set sequence, using blacklist and
> modules.autoload. This is enough to ensure that cx88 gets /dev/video0,
> and the usb webcam gets /dev/video1.

I use udev rules to give persistent names.

Here is my /etc/udev/rules.d/video.rules file,
which creates /dev/webcam and /dev/tvtuner as appropriate.

KERNEL=="video*" SYSFS{name}=="USB2.0 Camera", NAME="video%n", SYMLINK+="webcam"
KERNEL=="video*" SYSFS{name}=="em28xx*", NAME="video%n", SYMLINK+="tvtuner"

To find distinguishing attributes to match on use:

echo /sys/class/video4linux/video* | xargs -n1 udevinfo -a -p

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Re: How to force the device assignment with V4l V2.0?

[Ian Pickworth]

Pádraig Brady wrote:
> Ian Pickworth wrote:
>> I have two devices - a CX88 based Hauppauge TV PCI card, and a USB
>> webcam. In the "old" style drivers, I could force the loading of the two
>> modules (cx8800 and gspca) in a set sequence, using blacklist and
>> modules.autoload. This is enough to ensure that cx88 gets /dev/video0,
>> and the usb webcam gets /dev/video1.
> 
> I use udev rules to give persistent names.
> 
> Here is my /etc/udev/rules.d/video.rules file,
> which creates /dev/webcam and /dev/tvtuner as appropriate.
> 
> KERNEL=="video*" SYSFS{name}=="USB2.0 Camera", NAME="video%n", SYMLINK+="webcam"
> KERNEL=="video*" SYSFS{name}=="em28xx*", NAME="video%n", SYMLINK+="tvtuner"
> 
> To find distinguishing attributes to match on use:
> 
> echo /sys/class/video4linux/video* | xargs -n1 udevinfo -a -p

I did think about that, but the problem is that applications want to see
/dev/video(n) style names. Especially TV Time that (I think) uses
/dev/video - set to link to /dev/video0 by the standard udev rules.

I could be wrong - how does one get these programs to recognize the
custom links created by udev rules? Or, is it possible to get udev to
change /dev/video(n) assignment based on udev rules? I'm thinking not,
since one of the things you can match on is KERNEL.

Regards
Ian

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Re: How to force the device assignment with V4l V2.0?

[Ian Pickworth]

Ian Pickworth wrote:
> Pádraig Brady wrote:
>   
>> Ian Pickworth wrote:
>>     
>>> I have two devices - a CX88 based Hauppauge TV PCI card, and a USB
>>> webcam. In the "old" style drivers, I could force the loading of the two
>>> modules (cx8800 and gspca) in a set sequence, using blacklist and
>>> modules.autoload. This is enough to ensure that cx88 gets /dev/video0,
>>> and the usb webcam gets /dev/video1.
>>>       
>> I use udev rules to give persistent names.
>>
>> Here is my /etc/udev/rules.d/video.rules file,
>> which creates /dev/webcam and /dev/tvtuner as appropriate.
>>
>> KERNEL=="video*" SYSFS{name}=="USB2.0 Camera", NAME="video%n", SYMLINK+="webcam"
>> KERNEL=="video*" SYSFS{name}=="em28xx*", NAME="video%n", SYMLINK+="tvtuner"
>>
>> To find distinguishing attributes to match on use:
>>
>> echo /sys/class/video4linux/video* | xargs -n1 udevinfo -a -p
>>     
>
> I did think about that, but the problem is that applications want to see
> /dev/video(n) style names. Especially TV Time that (I think) uses
> /dev/video - set to link to /dev/video0 by the standard udev rules.
>   
A bit more research and I can see how to set TVTIME to use a different

device:
    In ~/.tvtime/tvtime.xml set this line:
    <option name="V4LDevice" value="/dev/v4l/video0"/>
    or whatever the device is you want.

So I can have a go at the udev solution - I'll try it a bit later.

Thanks
Regards
Ian


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Re: How to force the device assignment with V4l V2.0?

[Brandon Philips]

On Fri, Oct 17, 2008 at 6:41 AM, Ian Pickworth <ian@pickworth.me.uk> wrote:
> I'm having a play with the latest code from hg on the 2.6.27 kernel,
> Gentoo installation. This is triggered by the gspca driver being
> absorbed into the main tree (which is a great move by the way).
>
> I have two devices - a CX88 based Hauppauge TV PCI card, and a USB
> webcam. In the "old" style drivers, I could force the loading of the two
> modules (cx8800 and gspca) in a set sequence, using blacklist and
> modules.autoload. This is enough to ensure that cx88 gets /dev/video0,
> and the usb webcam gets /dev/video1.
>
> However, when trying a recent hg snapshot, the sequence of loading the
> modules does not change what the V4l driver is doing when loaded.
> Looking at dmesg, I see that the new drivers are doing quite a lot of
> detecting work themselves - it looks like they pick up the USB device
> first regardless of the module blacklist/load sequence I have specified.
>
> So, question is: Is there a preferred way of forcing the sequence of
> device assignment in V4L these days? I need the cx88 to be /dev/video0
> and the USB webcam to be /dev/video1 - otherwise all sorts of programs
> get confused.

If you have an up to date udev you should have rules that creates
persistent symlinks in /dev/v4l/by-path/

See the patches and rules here:
http://marc.info/?l=linux-video&m=120936768608839&w=2

Cheers,

Brandon

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