On Fri, Oct 20, 2023 at 11:30:52PM +0000, Thinh Nguyen wrote: > Sorry for the delay response. > > On Sun, Oct 15, 2023, Jayant Chowdhary wrote: > > On 10/12/23 11:50, Thinh Nguyen wrote: > > > The frequency of the request submission should not depend on the > > > video_pump() work thread since it can vary. The frequency of request > > > submission should match with the request completion. We know that > > > request completion rate should be fixed (1 uframe/request + when you > > > don't set no_interrupt). Base on this you can do your calculation on how > > > often you should set no_interrupt and how many requests you must submit. > > > You don't have to wait for the video_pump() to submit 0-length requests. > > > > > > The only variable here is the completion handler delay or system > > > latency, which should not be much and should be within your calculation. > > > > > > Thanks for the suggestion. It indeed makes sense that we do not completely depend on > > video_pump() for sending 0 length requests. I was concerned about > > synchronization needed when we send requests to the dwc3 controller from > > different threads. I see that the dwc3 controller code does internally serialize > > queueing requests, can we expect this from other controllers as well ? > > While it's not explicitly documented, when the gadget driver uses > usb_ep_queue(), the order in which the gadget recieves the request > should be maintained and serialized. Because the order the transfer go > out for the same endpoint can be critical, breaking this will cause > issue. That's right, but if usp_ep_queue() is called from multiple contexts, there's no guarantee it can provide when it comes to the ordering. The caller has to handle it. > > This brings me to another question for Michael - I see > > that we introduced a worker thread for pumping usb requests to the usb endpoint > > in https://urldefense.com/v3/__https://lore.kernel.org/all/20200427151614.10868-1-m.grzeschik@xxxxxxxxxxxxxx/__;!!A4F2R9G_pg!aAnzCopbTbZtUxBK6a6r6_QzV-b2Z2J5o5esPaartZ2jogKijmhqj6OyiKDg-BPhxq8pJHR3HS1Hf8z6YnqfWTon$ > > (I see multiple email addresses, so apologies if I used the incorrect one). > > > > Did we introduce the worker thread to solve some specific deadlock scenarios ? > > Or was it a general mitigation against racy usb request submission from v4l2 buffer > > queuing, stream enable and the video complete handler firing ? > > > > I was chatting with Avi about this, what if we submit requests to the endpoint > > only at two points in the streaming lifecycle - > > 1) The whole 64 (or however many usb requests are allocated) when > > uvcg_video_enable() is called - with 0 length usb_requests. > > 2) In the video complete handler - if a video buffer is available, we encode it > > and submit it to the endpoint. If not, we send a 0 length request. > > > > This way we're really maintaining back pressure and sending requests as soon > > as we can to the dwc3 controller. Encoding is mostly memcpys from what I see > > so hopefully not too heavy on the interrupt handler. I will work on prototyping > > this meanwhile. > > That sounds good to me. I believe Michael already provided some test > patches and you've already done some preliminary tests for that right? -- Regards, Laurent Pinchart
