Re: [PATCH] usb: gadget: uvc: reserve frame interval headroom for the zero length backlog

[Daehwan Jung]

On Tue, Oct 13, 2026 at 02:15:36PM -0700, Avichal Rakesh wrote:
> 
> On 10/11/26 8:40 PM, Daehwan Jung wrote:
> > uvc_video_prep_requests() spreads a frame over every service interval of
> > the negotiated frame interval:
> > 
> > 	nreq = DIV_ROUND_UP(video->interval, interval_duration);
> > 
> > and uvc_buffer_prepare() then splits every buffer into exactly
> > reqs_per_frame requests - only the request size varies, never the count.
> > So transmitting one frame always occupies nreq service intervals, 100% of
> > the frame interval, with nothing left over.
> > 
> > That budget cannot be met. While no video buffer is available,
> > uvcg_video_hw_submit() deliberately keeps a backlog of zero length
> > requests queued to the endpoint:
> > 
> > 	if (list_empty(&video->req_free) ||
> > 	    (atomic_read(&video->queued) > UVCG_REQ_MAX_ZERO_COUNT)) {
> > 
> > Up to UVCG_REQ_MAX_ZERO_COUNT + 1 of them are in flight whenever the
> > encoder is late, each occupying a service interval that carries no
> > payload, and the ring is FIFO so they all drain before the next frame's
> > payload moves. The frame period is therefore reqs_per_frame plus that
> > backlog, always longer than the frame interval the host asked for. The
> > UVCG_REQ_MAX_ZERO_COUNT compensation already in this function only sizes
> > the request pool; it buys the backlog no time.
> > 
> > At 60fps over SuperSpeed with bInterval 1 the backlog is 33 of the 133
> > intervals a frame interval has - 4.125ms, a quarter of the budget. The
> > waste is a fixed count, so it hurts in inverse proportion to the frame
> > interval: 24.8% at 60fps down to 6.2% at 15fps.
> > 
> > Reserve it. Measured on a SuperSpeed gadget (isoc IN, wMaxPacketSize
> > 1024, bMaxBurst 2, bInterval 1) running the Android DeviceAsWebcam
> > service, as reported by the CTS Verifier "Camera - Webcam" test, next to
> > the rate the microframe accounting predicts:
> > 
> >    target   predicted  measured  |  predicted  measured
> >              (before)  (before)  |   (after)    (after)
> >      60        47.9       46     |    59.7        56
> >      30        26.7       26     |    30.0        28
> >      24        21.8       22     |    24.5        22
> >      15        14.1       13     |    16.0        13
> 
> We recently fixed this exact issue in Android's DeviceAsWebcam client. In
> our case, DeviceAsWebcam was only queuing a frame when the previous one was
> returned.
> 
> With the changes to uvc_video_prep_requests(), a frame stays queued for one
> frame interval, and subsequent ioctl/scheduling delays meant that the gadget
> driver had to insert empty requests between two frames bringing down the
> effective FPS.
> 
> Our fix was to maintain two queued buffers to UVC Gadget Driver ensuring
> that the UVC gadget driver has a frame ready to send immediately after one
> is complete. This prevents insertion of zero length requests between two
> consecutive frames (fixing the FPS degradation you see) and gives
> DeviceAsWebcam a little bit of breathing room for ioctl and scheduling
> delays.
> 
> It is not clear if you are working with Android or not. If not, it is well
> worth checking if you have the same issue as us instead of adding an
> arbitrary headroom here. The numbers you report are extremely similar to
> what we were seeing.
> 
> If this was motivated from Android's DeviceAsWebcam, take a look at
> https://android-review.googlesource.com/c/platform/packages/services/DeviceAsWebcam/+/4353888
> and the comments in that change.
> 
> The change itself has been abandoned but it has context on the underlying
> issue, fix, and where to find the fix until it makes its way to AOSP.
> 
> Best Regards,
> Avi
> 

Thanks for sharing it. I'm working with Android.
Let me check AOSP.

Best Regards,
Daehwan Jung