How do you put an cap on the amount of bandwidth a USB device can allocate on Linux?
I have a couple of cheap USB webcams that I'm trying to running simeltaneously. Running
v4l2-ctl --list-formats-ext --device=/dev/videoN shows they both support several uncompressed resolutions at both 30 and 15 FPS.
However, even if I configure one to capture at 15 FPS at 160x120 resolution, it still allocates 480 Mbps of bandwidth, usually preventing me from using any other USB devices, much less the second webcam. Attempting to capture from the second webcam at the same FPS/resolution results in the error:
libv4l2: error turning on stream: No space left on device
VIDIOC_STREAMON: No space left on device
Googling this error usually results in answers like, "you're SOL, maybe buy a camera that supports MJPG?".
But this makes absolutely no sense.
Capturing uncompressed 160x120 RGB equals 160*120*3 = 57600 bytes per frame. At 15 FPS, that requires at least 864000 bytes per second of bandwidth (i.e. a whopping 0.864 Mbps or 6.912 Mbits)! I have USB2 hub, which supports 480 Mbit/s. I should have enough bandwidth to run dozens of these webcams at once, yet running a single one consumes almost all 480 Mbits of my USB hub!
Since the camera doesn't need 480 Mbps to stream 160x120, but it's telling driver to allocate that much, is there any way, at the OS level, to force the driver to allocate a certain amount?