Hi, On 10/30/23 23:11, Jayant Chowdhary wrote: > Hi, > > On 10/28/23 07:09, Jayant Chowdhary wrote: >> 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. > I got caught up with some work which is taking longer than expected. Apologies for the > delay :) I'm testing some things out right now. I hope to be able to post a patch in the > next couple of days. Thanks for your patience. I posted another patch at https://lore.kernel.org/linux-usb/20231102060120.1159112-1-jchowdhary@xxxxxxxxxx/T/#u. I've not split this into 2 patches since here, we have a common function that handles both the bulk and isoc cases and I feel they're logically related. Thank you Jayant
