Re: [PATCH v2 3/3] usb: gadget: uvc: set req_size and n_requests based on the frame interval

[Avichal Rakesh <arakesh@google.com>]

On 6/22/24 4:48 PM, Michael Grzeschik wrote:
> With the information of the interval frame length it is now possible to
> calculate the number of usb requests by the frame duration. Based on the
> request size and the imagesize we calculate the actual size per request.
> This has calculation has the benefit that the frame data is equally
> distributed over all allocated requests.
> 
> We keep the current req_size calculation as a fallback, if the interval
> callbacks did not set the interval property.
> 
> Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
> 
> ---
> v1 -> v2: - add headersize per request into calculation
> ---
>  drivers/usb/gadget/function/uvc_queue.c | 30 +++++++++++++++++++++++-------
>  drivers/usb/gadget/function/uvc_video.c |  2 +-
>  2 files changed, 24 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/uvc_queue.c b/drivers/usb/gadget/function/uvc_queue.c
> index ce51643fc4639..141e52e34c610 100644
> --- a/drivers/usb/gadget/function/uvc_queue.c
> +++ b/drivers/usb/gadget/function/uvc_queue.c
> @@ -44,7 +44,7 @@ static int uvc_queue_setup(struct vb2_queue *vq,
>  {
>  	struct uvc_video_queue *queue = vb2_get_drv_priv(vq);
>  	struct uvc_video *video = container_of(queue, struct uvc_video, queue);
> -	unsigned int req_size;
> +	unsigned int req_size, max_req_size, header_size;
>  	unsigned int nreq;
>  
>  	if (*nbuffers > UVC_MAX_VIDEO_BUFFERS)
> @@ -54,15 +54,31 @@ static int uvc_queue_setup(struct vb2_queue *vq,
>  
>  	sizes[0] = video->imagesize;
>  
> -	req_size = video->ep->maxpacket
> +	nreq = DIV_ROUND_UP(video->interval, video->ep->desc->bInterval * 1250);

This seems problematic? I am not very well versed in the different USB speeds,
but IIRC fullspeed and highspeed enpoints have different bus intervals, and 
treat bInterval in different units (in frames for fs and in microframes for hs).

We likely need some speed specific logic when calculating nreq.

Assuming this logic is for >= hs, this allocates the exact number of 
usb_requests needed to stream a frame over to the host in one video 
frame interval. With the zero length backpressure still in place, this 
would mean that the actual video frame is sent over a period longer than
on video frame interval. I will try these patches locally, but if you
haven't already, please do check if you run into the problem you
brought up in https://lore.kernel.org/all/ZiWga5Kqno1ICv97@pengutronix.de/.
My guess is that the problem will show up here as well.

> +
> +	header_size = nreq * UVCG_REQUEST_HEADER_LEN;
> +
> +	req_size = DIV_ROUND_UP(video->imagesize + header_size, nreq);
> +
> +	max_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
> -	 * into fewer requests for smaller framesizes.
> -	 */
> -	nreq = DIV_ROUND_UP(DIV_ROUND_UP(sizes[0], 2), req_size);
> -	nreq = clamp(nreq, 4U, 64U);
> +	if (!req_size) {
> +		req_size = max_req_size;
> +
> +		/* We divide by two, to increase the chance to run
> +		 * into fewer requests for smaller framesizes.
> +		 */
> +		nreq = DIV_ROUND_UP(DIV_ROUND_UP(sizes[0], 2), req_size);
> +		nreq = clamp(nreq, 4U, 64U);
> +	} else if (req_size > max_req_size) {
> +		/* The prepared interval length and expected buffer size
> +		 * is not possible to stream with the currently configured
> +		 * isoc bandwidth
> +		 */
> +		return -EINVAL;
> +	}
>  
>  	video->req_size = req_size;
>  	video->uvc_num_requests = nreq;
> diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c
> index 95bb64e16f3da..d197c46e93fb4 100644
> --- a/drivers/usb/gadget/function/uvc_video.c
> +++ b/drivers/usb/gadget/function/uvc_video.c
> @@ -304,7 +304,7 @@ static int uvcg_video_usb_req_queue(struct uvc_video *video,
>  		 */
>  		if (list_empty(&video->req_free) || ureq->last_buf ||
>  			!(video->req_int_count %
> -			DIV_ROUND_UP(video->uvc_num_requests, 4))) {
> +			clamp(DIV_ROUND_UP(video->uvc_num_requests, 4), 4U, 16U))) {
>  			video->req_int_count = 0;
>  			req->no_interrupt = 0;
>  		} else {
>