Hi, On 10/28/23 04:10, Michael Grzeschik wrote: > On Fri, Oct 27, 2023 at 10:58:11AM -0400, Alan Stern wrote: >> On Fri, Oct 27, 2023 at 03:39:44PM +0200, Michael Grzeschik wrote: >>> On Fri, Oct 27, 2023 at 02:47:52PM +0300, Laurent Pinchart wrote: >>> > On Fri, Oct 27, 2023 at 01:10:21PM +0200, Michael Grzeschik wrote: >>> > > On Fri, Oct 27, 2023 at 10:51:17AM +0300, Laurent Pinchart wrote: >>> > > > On Thu, Oct 26, 2023 at 09:56:35PM +0000, Jayant Chowdhary wrote: >>> > > >> This patch is based on top of >>> > > >> https://lore.kernel.org/linux-usb/20230930184821.310143-1-arakesh@xxxxxxxxxx/T/#t: >>> > > >> >>> > > >> When we use an async work queue to perform the function of pumping >>> > > >> usb requests to the usb controller, it is possible that thread scheduling >>> > > >> affects at what cadence we're able to pump requests. This could mean usb >>> > > >> requests miss their uframes - resulting in video stream flickers on the host >>> > > >> device. >>> > > >> >>> > > >> In this patch, we move the pumping of usb requests to >>> > > >> 1) uvcg_video_complete() complete handler for both isoc + bulk >>> > > >> endpoints. We still send 0 length requests when there is no uvc buffer >>> > > >> available to encode. >>> > > > >>> > > > This means you will end up copying large amounts of data in interrupt >>> > > > context. The work queue was there to avoid exactly that, as it will >>> > > > introduce delays that can affect other parts of the system. I think this >>> > > > is a problem. >>> > > >>> > > Regarding Thin's argument about possible scheduling latency that is already >>> > > introducing real errors, this seemed like a good solution. >>> > > >>> > > But sure, this potential latency introduced in the interrupt context can >>> > > trigger other side effects. >>> > > >>> > > However I think we need some compromise since both arguments are very valid. >>> > >>> > Agreed. >>> > >>> > > Any ideas, how to solve this? >>> > >>> > I'm afraid not. >>> >>> We discussed this and came to the conclusion that we could make use of >>> kthread_create and sched_setattr with an attr->sched_policy = SCHED_DEADLINE >>> here instead of the workqueue. This way we would ensure that the worker >>> would be triggered with hard definitions. >>> >>> Since the SG case is not that heavy on the completion handler, we could >>> also make this kthread conditionaly to the memcpy case. >> >> If you don't mind a naive suggestion from someone who knows nothing >> about the driver... >> >> An attractive possibility is to have the work queue (or kthread) do the >> time-consuming copying, but leave the submission up to the completion >> handler. If the data isn't ready (or there's no data to send) when the >> handler runs, then queue a 0-length request. >> >> That will give you the best of both worlds: low latency while in >> interrupt context and a steady, constant flow of USB transfers at all >> times. The question of how to schedule the work queue or kthread is a >> separate matter, not directly relevant to this design decision. > > That's it. This is probably the best way to tackle the overall problem. > > So we leave the call of the encode callback to the worker, that will > probably still can be a workqueue. The complete callback is calling > the explicit uvcg_video_ep_queue when prepared requests are available > and if there is nothing pending it will just enqueue zero requests. > > Thank you Alan, this makes so much sense! > > Jayant, Laurent: Do you agree? > If yes, Jayant will you change the patch accordingly? > > Thanks for all the discussion Greg, Michael, Laurent and Alan. Apologies for not responding earlier since I am OOO. While I haven't tried this out this does seem like a very good idea. Thank you Alan! I will aim to make changes and post a patch on Monday night PST. Jayant
