Re: cx231xx with 3 grabbers

[Mauro Carvalho Chehab <mchehab+samsung@kernel.org>]

Em Tue, 14 May 2019 00:18:31 +0200
Tomasz Borowczyk <tomboro88@gmail.com> escreveu:

> Hello linux-media.
> 
> I use cx231xx driver with my August Vgb100 USB video grabber. It works
> great. It automatically detects my device and I can watch the video
> from analog camera on my Raspberry Pi, and I am very happy about how
> it works.
> 
> But there is a catch. I want to connect 3 such cameras to my Raspberry
> Pi. When there are connected 2 of them, I can watch video from both of
> them without problems. When there are connected 3 adapters, I can
> watch the video from all three cameras, but the picture from one of
> them is flickering and blurring between blue screen and the correct
> data. And no matter in what order I connect them to the Raspberry, the
> problem exists always on the same adapter.
> The same problem exists when I connect the cameras to my laptop
> running Raspbian desktop.
> This could suggest that the one adapter is a little defective, but it
> works okay when there are connected only two adapters, or when it is
> the only connected. Also when I disconnect it and connect again to USB
> port, I can see correct image. But when I start to play picture from
> another camera, and then go back to the "defective" one, it flickers
> again. How can I find the reason of the problem?

The problem is related to the maximum bandwidth that an USB 2.0
provides. Most audio and video devices like cameras use an type of
USB package, called ISOC, with allows reserving bandwidth for them.
It actually reserves a number of USB slots. The maximum is 980 slots
per second, if I'm not mistaken.

The actual number of allocated slots depends on the resolution, 
format, package size, number of frames per second, etc. 

I don't have the numbers for cx231xx, but, with em28xx, a 640x480,
16 bits per pixel, 30 frames per second video uses about 60% of
the available USB 2.0 bandwidth. So, even two cameras at full
res can be too much.

I guess you can consider your self lucky to be able of having
two cameras working :-)

If you need more than that, you'll need to use a machine with
multiple USB buses and connect each camera on a different
bus.

Thanks,
Mauro