Re: [GIT PATCH FOR 2.6.40] uvcvideo patches

[Mauro Carvalho Chehab <mchehab@redhat.com>]

Em 20-05-2011 12:55, Rémi Denis-Courmont escreveu:
> Le vendredi 20 mai 2011 18:32:45 Mauro Carvalho Chehab, vous avez écrit :
>> However, I have serious concerns about media_controller API usage on
>> generic drivers, as it is required that all drivers should be fully
>> configurable via V4L2 API alone, otherwise we'll have regressions, as no
>> generic applications use the media_controller.
> 
> If VLC counts as a generic application, I'd be more than API to use the 
> media_controller (or whatever else) if only to find which ALSA (sub)device, if 
> any, corresponds to the V4L2 node of a given USB webcam or such.
> 
> I don't know any solution at the moment, and this is a major inconvenience on 
> Linux desktop compared to DirectShow.

You don't need the media controller for it.

The proper solution for it is to use the sysfs to identify the alsa sub-device.

For example, I have this on one of my machines:

$ lspci |grep Multimedia
1c:00.0 Multimedia video controller: Conexant Systems, Inc. CX23885 PCI Video and Audio Decoder (rev 04)
37:09.0 Multimedia controller: Philips Semiconductors SAA7131/SAA7133/SAA7135 Video Broadcast Decoder (rev d1)

$ $ lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 008 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 004: ID 0409:005a NEC Corp. HighSpeed Hub
Bus 001 Device 005: ID 0ac8:3330 Z-Star Microelectronics Corp. 
Bus 008 Device 002: ID 2101:020f ActionStar 
Bus 001 Device 003: ID 2040:4200 Hauppauge 
Bus 001 Device 006: ID 0d8c:0126 C-Media Electronics, Inc. 

I wrote an utility in the past that dig into the sysfs stuff to identify it, but
I'm not sure if it is still working fine, as some changes at sysfs might affect it,
as I never intended to maintain such utility.

I'll seek for some time to re-write it with another approach and add it as a 
library, inside v4l2-utils, eventually together with libv4l.

Basically, all V4L devices are under video4linux class:

$ tree /sys/class/video4linux/
/sys/class/video4linux/
├── vbi0 -> ../../devices/pci0000:00/0000:00:1e.0/0000:37:09.0/video4linux/vbi0
├── video0 -> ../../devices/pci0000:00/0000:00:1c.0/0000:1c:00.0/video4linux/video0
├── video1 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-5/1-5:1.0/video4linux/video1
├── video2 -> ../../devices/pci0000:00/0000:00:1e.0/0000:37:09.0/video4linux/video2
└── video3 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-6/1-6.2/1-6.2:1.0/video4linux/video3

And all alsa devices are under sound class:

$ tree /sys/class/sound/
/sys/class/sound/
├── card0 -> ../../devices/pci0000:00/0000:00:1b.0/sound/card0
├── card1 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-5/1-5:1.1/sound/card1
├── card2 -> ../../devices/pci0000:00/0000:00:1e.0/0000:37:09.0/sound/card2
├── card3 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-6/1-6.3/1-6.3:1.0/sound/card3
├── controlC0 -> ../../devices/pci0000:00/0000:00:1b.0/sound/card0/controlC0
├── controlC1 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-5/1-5:1.1/sound/card1/controlC1
├── controlC2 -> ../../devices/pci0000:00/0000:00:1e.0/0000:37:09.0/sound/card2/controlC2
├── controlC3 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-6/1-6.3/1-6.3:1.0/sound/card3/controlC3
├── hwC0D0 -> ../../devices/pci0000:00/0000:00:1b.0/sound/card0/hwC0D0
├── pcmC0D0c -> ../../devices/pci0000:00/0000:00:1b.0/sound/card0/pcmC0D0c
├── pcmC0D0p -> ../../devices/pci0000:00/0000:00:1b.0/sound/card0/pcmC0D0p
├── pcmC1D0c -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-5/1-5:1.1/sound/card1/pcmC1D0c
├── pcmC2D0c -> ../../devices/pci0000:00/0000:00:1e.0/0000:37:09.0/sound/card2/pcmC2D0c
├── pcmC3D0p -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-6/1-6.3/1-6.3:1.0/sound/card3/pcmC3D0p
├── seq -> ../../devices/virtual/sound/seq
└── timer -> ../../devices/virtual/sound/timer

All that such library/utility/function needs do to is to parse the two above sysfs directories
and associate the devices based on the provided aliases.

For example, on the above, we have 4 V4L cards:

PCI card (in this case, a saa7134-based card)
├── vbi0 -> ../../devices/pci0000:00/0000:00:1e.0/0000:37:09.0/video4linux/vbi0
├── video2 -> ../../devices/pci0000:00/0000:00:1e.0/0000:37:09.0/video4linux/video2
└── card2 -> ../../devices/pci0000:00/0000:00:1e.0/0000:37:09.0/sound/card2

All tree are at the PCI device 0000:37:09.0.

USB card (in this example, an em28xx-based card, that uses snd-usb-audio for alsa)
├── video1 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-5/1-5:1.0/video4linux/video1
└── card1 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-5/1-5:1.1/sound/card1

All two are at the USB 1-5 device.

DVB-C/DVB-T PCI card (in this case cx23885, without any alsa-associated device)
└── video0 -> ../../devices/pci0000:00/0000:00:1c.0/0000:1c:00.0/video4linux/video0

This one is at PCI device 0000:1c:00.0.

UVC webcam (with audio provided by snd-usb-audio)
├── video3 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-6/1-6.2/1-6.2:1.0/video4linux/video3
└── card3 -> ../../devices/pci0000:00/0000:00:1a.7/usb1/1-6/1-6.3/1-6.3:1.0/sound/card3

All two are at the USB 1-6 device.

So, it is easy to associate video and audio for each device. You can even associate the volume
controls and the PCM input/outputs using the above info.

Cheers,
Mauro.