One misconception of queueing request to the usb gadget controller is, that the amount of data that one usb_request is representing is the same as the hardware is able to transfer in one interval. This exact idea applies to the uvc_video gadget that assumes that the request needs to have the size as the gadgets bandwidth parameters are configured with. (maxpacket, multiplier and burst) Although it is actually no problem for the hardware to queue a big request, in the case of isochronous transfers it is impractical to queue big amount of data per request to the hardware. Especially if the necessary bandwidth was increased on purpose to maintain high amounts of data. E.g. in the dwc3-controller it has the negative impact that the endpoint FIFO will not be filled fast enough by the internal hardware engine. Since each transfer buffer (TRB) represents one big request, the hardware will need a long time to prefill the hardware FIFO. This can be avoided by queueing more smaller requests which will then lead to smaller TRBs which the hardware engine can keep up with. This patch is simply dropping the maxburst as an multiplier for the size calculation and therefor decreases the overall maximum request size. Signed-off-by: Michael Grzeschik <m.grzeschik@xxxxxxxxxxxxxx> --- This patch is created as an result from the discussion in the following thread: https://lore.kernel.org/linux-usb/20231031231841.dbhtrgqounu2rvgn@xxxxxxxxxxxx/ drivers/usb/gadget/function/uvc_queue.c | 1 - drivers/usb/gadget/function/uvc_video.c | 1 - 2 files changed, 2 deletions(-) diff --git a/drivers/usb/gadget/function/uvc_queue.c b/drivers/usb/gadget/function/uvc_queue.c index 0aa3d7e1f3cc32..1d3c3c09ff97cb 100644 --- a/drivers/usb/gadget/function/uvc_queue.c +++ b/drivers/usb/gadget/function/uvc_queue.c @@ -55,7 +55,6 @@ static int uvc_queue_setup(struct vb2_queue *vq, sizes[0] = video->imagesize; req_size = video->ep->maxpacket - * max_t(unsigned int, video->ep->maxburst, 1) * (video->ep->mult); /* We divide by two, to increase the chance to run diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c index 91af3b1ef0d412..c6b61843bad3d7 100644 --- a/drivers/usb/gadget/function/uvc_video.c +++ b/drivers/usb/gadget/function/uvc_video.c @@ -329,7 +329,6 @@ uvc_video_alloc_requests(struct uvc_video *video) BUG_ON(video->req_size); req_size = video->ep->maxpacket - * max_t(unsigned int, video->ep->maxburst, 1) * (video->ep->mult); video->ureq = kcalloc(video->uvc_num_requests, sizeof(struct uvc_request), GFP_KERNEL); -- 2.39.2
