Hi,
in my class I use ten C-TECH CAM-07HD cameras, detected as
ID 0402:8841 ALi Corp. Newmine Camera.
The camera works OK with resolutions <= 320x240@60fps.
Bigger MJPEG resolutions were laggy, with lots of errors in kern.log:
usb 1-2: USB isochronous frame lost (-75)
xhci_hcd 0000:15:00.0: WARN: buffer overrun event for slot 9 ep 4 on endpoint
Bigger YUY2 resolutions did not get any data. Quirks doesn't help.
Behavior is the same on (Ubuntu) kernels 6.5.0, 6.2.0, 5.4.0,
both on 2.0 and 3.0 USB ports.
So I start USB scanning and finally find the reason:
- camera sends four isochronous interface descriptors for wMaxPacketSize 1024 bytes
and linux uvcvideo module always chooses bAlternateSetting 4.
- Windows 10 always chooses bAlternateSetting 1 for the camera
and bigger resolutions are working OK there.
(I attached listing from lspci -v -d 0402:8841 )
This original line of code from drivers/media/usb/uvc/uvc_video.c
if (psize >= bandwidth && psize <= best_psize) {
chooses last alt setting in case the same wMaxPacketSize values.
If I force using settings 1, 2 or 3 instead in uvc_video.c,
my REALLY BAD camera starts working OK like a charm, without log errors.
So I offer you to change the line above in some future uvc_video.c to:
if (psize >= bandwidth && psize < best_psize) { //first preferency
which I probe completely solved this problem
and as Windows does the same will not harm other types of cameras.
sincerely
Karel Janda
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