When selecting an alternate setting, the driver loops over all available alternate settings to find the one with the lowest bandwidth high enough for the selected format and resolution. While all alternate settings should have different packet sizes, some buggy devices report multiple alternate settings with the same size. The driver happens to pick the last one in this case. In theory this should work fine, but in real life we have device bugs. The Ali Corp. Newmine Camera (0402:8841) exposes four alternate settings with the same packet size. The first three seem to work fine, while selecting the last one results in lots of transmission errors. Switch to using the first best alternate setting when multiple are present. This should be safe (last famous words), as sniffing USB traffic with the faulty device shows that Windows 10 picks the first alternate setting, and devices are typically tested on Windows. Reported-by: Karel Janda <karel1@xxxxxxxxxxxx> Closes: https://lore.kernel.org/linux-media/Nh6D0WI--3-9@xxxxxxxxxxxx/ Suggested-by: Karel Janda <karel1@xxxxxxxxxxxx> Signed-off-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> --- Karel, could you please test this patch ? Please also let me know if you are fine having your name recorded in the kernel git history with the Reported-by and Suggested-by tags above. If not, I will drop them. --- drivers/media/usb/uvc/uvc_video.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c index 28dde08ec6c5..7cbf4692bd87 100644 --- a/drivers/media/usb/uvc/uvc_video.c +++ b/drivers/media/usb/uvc/uvc_video.c @@ -1954,7 +1954,7 @@ static int uvc_video_start_transfer(struct uvc_streaming *stream, /* Check if the bandwidth is high enough. */ psize = uvc_endpoint_max_bpi(stream->dev->udev, ep); - if (psize >= bandwidth && psize <= best_psize) { + if (psize >= bandwidth && psize < best_psize) { altsetting = alts->desc.bAlternateSetting; best_psize = psize; best_ep = ep; -- Regards, Laurent Pinchart
