The problem:
>From reading around this list and elsewhere, it appears that you simply
cannot run more than one webcam video stream per USB root hub.
When enabled for video streaming, the webcam reserves the full
isochronous bandwidth needed to stream data at the selected resolution
and framerate, regardless of whether your application acquires the full
framerate or just samples at one frame per second. Hence, trying to set
a low framerate in your user application has no effect to reduce the
bandwidth from the webcam. The webcam is still streaming at the maximum
framerate regardless.
I'm guessing that the video stream is being 'pushed' out by the webcam
firmware and so the UVC driver must take all that it is given...
*A possible solution* ?
Instead, could the UVC driver 'simulate' a reduced framerate by instead
using the "STILL_IMAGE_FRAME" mode of the webcam and grab for itself a
set number of images per second? Or even only when polled by a read from
the user application?
On *nix, "everything is a file"... Could the uvc driver accept reading
of /dev/videoX by the command "cp" so that still image data is copied,
formatted for a jpg or png?
Could the "quirks" setting be abused to set the image grab rate? Or some
other neater method?
Further details:
I'm trying to use two webcams simultaneously:
iManufacturer 1 Sweex
iProduct 2 WC060 Series HD Webcam
On separate hubs, they work fine. On the same hub, the second one to
start shows the error:
"Error starting stream VIDIOC_STREAMON: No space left on device".
I've only got the one root hub on the system I want to use for the two
webcams!
Their descriptors show:
bFrameIntervalType 2
dwFrameInterval( 0) 2000000
dwFrameInterval( 1) 4000000
Can I tweak the UVC driver to preferentially choose the slower frame
rate available?
Aside: The MJPG format doesn't seem to work, nor are any compression
settings visible...
Given a few hints or a patch, I can hack the kernel module to test :-)
This is running on Gentoo, kernel 2.6.34 using the in-kernel uvc module.
Any comment/ideas welcomed.
Regards,
Martin
>From lsusb -v (excerpt):
VideoStreaming Interface Descriptor:
bLength 34
bDescriptorType 36
bDescriptorSubtype 5 (FRAME_UNCOMPRESSED)
bFrameIndex 9
bmCapabilities 0x00
Still image unsupported
wWidth 1600
wHeight 1200
dwMinBitRate 768000
dwMaxBitRate 196608000
dwMaxVideoFrameBufferSize 3840000
dwDefaultFrameInterval 2000000
bFrameIntervalType 2
dwFrameInterval( 0) 2000000
dwFrameInterval( 1) 4000000
VideoStreaming Interface Descriptor:
bLength 42
bDescriptorType 36
bDescriptorSubtype 3 (STILL_IMAGE_FRAME)
bEndpointAddress 0
bNumImageSizePatterns 9
wWidth( 0) 640
wHeight( 0) 480
wWidth( 1) 352
wHeight( 1) 288
wWidth( 2) 320
wHeight( 2) 240
wWidth( 3) 176
wHeight( 3) 144
wWidth( 4) 160
wHeight( 4) 120
wWidth( 5) 800
wHeight( 5) 600
wWidth( 6) 1280
wHeight( 6) 960
wWidth( 7) 1280
wHeight( 7) 1024
wWidth( 8) 1600
wHeight( 8) 1200
bNumCompressionPatterns 9
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