On Mon, 12 Apr 2010, Sarah Sharp wrote: > I've been trying out the patches to enable isochronous transfers under > xHCI, and they work fine on my USB speaker. However, I've been having > trouble getting my high speed USB webcam to work. The NEC Express Card > I have rejects the first alternate setting that the uvcvideo driver > tries to install (altsetting 11), saying that it takes too much > bandwidth. This happens even when I plug the device directly into the > roothub with no other devices plugged in. > > I would like to know if this is correct behavior for the host, as I > can't believe a device would advertise an alternate setting that took up > too much bandwidth by itself. Device descriptors and a log snippet are > attached. The webcam is a high-speed device, right? The descriptors call for an isochronous-IN endpoint with 3x1020 bytes (period 1 uframe) and an isochronous-IN endpoint with 36 bytes (period 8 uframes). That combination does not exceed the high-speed bandwidth limit, so the controller should accept it. Broadly speaking, at 480 Mb/s you get 60 KB/ms = 7500 B/uframe. The periodic bandwidth limit is 80% of the total, or 6000 B/uframe. Taking bitstuffing into account (6/7) leaves 5142 B/uframe available for data, and overhead eats up some of that. It's still much more than the 3*1020 + 36 = 3096 bytes needed by the webcam. > The other problem is that uvcvideo then gives up on the device when > installing the alt setting fails, rather than trying the next less > resource-intensive alternate setting. The past, submit_urb() might fail > if there wasn't enough bandwidth for the isochronous transfers, but > under an xHCI host controller, it will fail sooner, when > usb_set_interface() is called. That needs to be fixed in all the USB > video drivers. > > I figured out how to patch the gspca driver, but not uvcvideo. The > patch looks a bit hackish; can with experience with that driver look it > over? Can anyone tell me where to look for the usb_set_interface() in > uvcvideo? Not at the moment, sorry. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
