Messages in this thread Patch in this message | | | Date | Thu, 2 Nov 2023 16:07:28 +0000 | Subject | Re: [PATCH v3] usb:gadget:uvc Do not use worker thread to queue isoc usb requests | From | Dan Scally <> |
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Hi Jayant - thanks for the patch
On 02/11/2023 06:01, Jayant Chowdhary wrote: > When we use an async work queue to perform the function of pumping > usb requests to the usb controller, it is possible that amongst other > factors, thread scheduling affects at what cadence we're able to pump > requests. This could mean isoc usb requests miss their uframes - resulting > in video stream flickers on the host device. > > To avoid this, we make the async_wq thread only produce isoc usb_requests > with uvc buffers encoded into them. The process of queueing to the > endpoint is done by the uvc_video_complete() handler. In case no > usb_requests are ready with encoded information, we just queue a zero > length request to the endpoint from the complete handler. > > For bulk endpoints the async_wq thread still queues usb requests to the > endpoint. > > Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de> > Signed-off-by: Jayant Chowdhary <jchowdhary@google.com> > Suggested-by: Avichal Rakesh <arakesh@google.com> > Suggested-by: Alan Stern <stern@rowland.harvard.edu> > --- > Based on top of > https://lore.kernel.org/linux-usb/20230930184821.310143-1-arakesh@google.com/T/#t: > v1->v2: Added self Signed-Off-by and addressed review comments > v2->v3: Encode to usb requests in async_wq; queue to ep in complete handler > for isoc transfers. > > drivers/usb/gadget/function/uvc.h | 8 + > drivers/usb/gadget/function/uvc_video.c | 187 +++++++++++++++++++----- > 2 files changed, 156 insertions(+), 39 deletions(-) > > diff --git a/drivers/usb/gadget/function/uvc.h b/drivers/usb/gadget/function/uvc.h > index e8d4c87f1e09..82c783410554 100644 > --- a/drivers/usb/gadget/function/uvc.h > +++ b/drivers/usb/gadget/function/uvc.h > @@ -105,7 +105,15 @@ struct uvc_video { > bool is_enabled; /* tracks whether video stream is enabled */ > unsigned int req_size; > struct list_head ureqs; /* all uvc_requests allocated by uvc_video */ > + > + /* USB requests video pump thread can encode into*/
"USB requests that the video pump thread can encode into", and a space before the closing */ please (and the same a few more times below).
> struct list_head req_free; > + > + /* > + * USB requests video pump thread has already encoded into. These are > + * ready to be queued to the endpoint. > + */ > + struct list_head req_ready; > spinlock_t req_lock; > > unsigned int req_int_count; > diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c > index 53feb790a4c3..c84183e9afcc 100644 > --- a/drivers/usb/gadget/function/uvc_video.c > +++ b/drivers/usb/gadget/function/uvc_video.c > @@ -268,6 +268,98 @@ static int uvcg_video_ep_queue(struct uvc_video *video, struct usb_request *req) > return ret; > } > > +/* This function must be called with video->req_lock held*/ > +static int uvcg_video_usb_req_queue(struct uvc_video *video, > + struct usb_request *req, bool queue_to_ep) { Brace on a new line please - same a few more times below > + bool is_bulk = video->max_payload_size; empty line here > + if (!video->is_enabled) { > + uvc_video_free_request(req->context, video->ep); > + return -ENODEV; > + } > + if (queue_to_ep) { > + struct uvc_request *ureq = req->context; > + /* > + * With USB3 handling more requests at a higher speed, we can't > + * afford to generate an interrupt for every request. Decide to > + * interrupt: > + * > + * - When no more requests are available in the free queue, as > + * this may be our last chance to refill the endpoint's > + * request queue. > + * > + * - When this is request is the last request for the video > + * buffer, as we want to start sending the next video buffer > + * ASAP in case it doesn't get started already in the next > + * iteration of this loop. > + * > + * - Four times over the length of the requests queue (as > + * indicated by video->uvc_num_requests), as a trade-off > + * between latency and interrupt load. > + */ > + if (list_empty(&video->req_free) || ureq->last_buf || > + !(video->req_int_count % > + DIV_ROUND_UP(video->uvc_num_requests, 4))) { > + video->req_int_count = 0; > + req->no_interrupt = 0; > + } else { > + req->no_interrupt = 1; > + } > + video->req_int_count++; > + return uvcg_video_ep_queue(video, req); > + } else { > + /* > + * If we're not queing to the ep, for isoc we're queing > + * to the req_ready list, otherwise req_free. > + */ > + struct list_head *list = > + is_bulk ? &video->req_free : &video->req_ready; > + list_add_tail(&req->list, list); > + } > + return 0; > +} > + > +static int uvcg_video_ep_queue_zero_length(struct usb_request *req, > + struct uvc_video *video) { > + req->length = 0; > + return uvcg_video_ep_queue(video, req); > +} Not sure this is worth its own function > + > +/* Must only be called from uvcg_video_enable - since after that we only want to > + * queue requests to the endpoint from the uvc_video_complete complete handler. > + * This function is needed in order to 'kick start' the flow of requests from > + * gadget driver to the usb controller. > + */ > +static void uvc_video_ep_queue_initial_requests(struct uvc_video *video) { > + struct usb_request *req = NULL; > + unsigned long flags = 0; > + unsigned int count = 0; > + int ret = 0; > + /* We only queue half of the free list since we still want to have > + * some free usb_requests in the free list for the video_pump async_wq > + * thread to encode uvc buffers into. Otherwise we could get into a > + * situation where the free list does not have any usb requests to > + * encode into - we always end up queueing 0 length requests to the > + * end point. > + */ > + unsigned half_list_size = video->uvc_num_requests / 2; > + spin_lock_irqsave(&video->req_lock, flags); > + /* Take these requests off the free list and queue them all to the > + * endpoint. Since we queue the requests with the req_lock held, > + */
This comment seems to be incomplete? You also want an opening /* on its own line:
/* * Multi line comments * look like this */
> + while (count < half_list_size) { > + req = list_first_entry(&video->req_free, struct usb_request, > + list); > + list_del(&req->list); > + ret = uvcg_video_ep_queue_zero_length(req, video); > + if (ret < 0) { > + uvcg_queue_cancel(&video->queue, /*disconnect*/0); > + break; > + } > + count++; > + } > + spin_unlock_irqrestore(&video->req_lock, flags); > +} > +
So if I'm understanding the new starting sequence right for an isoc endpoint there's an initial flight of half the requests (between 2 and 32) that are queued as zero length - the very first one to .complete() being re-queued as a zero length request before the workqueue is started and encodes data into the _other_ half of the requests which were left in req_free and putting them into req_ready. At that point the .complete()s being run start to pick requests off req_ready instead and they get sent out with data...does that sound right?
What are the implications of those initial 3-33 zero length requests? What kind of latency can that introduce to the start of the video stream?
> static void > uvc_video_complete(struct usb_ep *ep, struct usb_request *req) > { > @@ -276,6 +368,8 @@ uvc_video_complete(struct usb_ep *ep, struct usb_request *req) > struct uvc_video_queue *queue = &video->queue; > struct uvc_buffer *last_buf = NULL; > unsigned long flags; > + bool is_bulk = video->max_payload_size; > + int ret = 0; > > spin_lock_irqsave(&video->req_lock, flags); > if (!video->is_enabled) { > @@ -329,7 +423,38 @@ uvc_video_complete(struct usb_ep *ep, struct usb_request *req) > * back to req_free > */ > if (video->is_enabled) { > - list_add_tail(&req->list, &video->req_free); > + /* > + * Here we check whether any request is available in the ready > + * list. If it is, queue it to the ep and add the current > + * usb_request to the req_free list - for video_pump to fill in. > + * Otherwise, just use the current usb_request to queue a 0 > + * length request to the ep. Since we always add to the req_free > + * list if we dequeue from the ready list, there will never > + * be a situation where the req_free list is completely out of > + * requests and cannot recover. > + */ > + struct usb_request *to_queue = req; > + to_queue->length = 0; > + if (!list_empty(&video->req_ready)) { > + to_queue = list_first_entry(&video->req_ready, > + struct usb_request, list); > + list_del(&to_queue->list); > + /* Add it to the free list. */ > + list_add_tail(&req->list, &video->req_free); > + } > + /* > + * Queue to the endpoint. The actual queueing to ep will > + * only happen on one thread - the async_wq for bulk endpoints > + * and this thread for isoc endpoints. > + */ > + ret = uvcg_video_usb_req_queue(video, to_queue, > + /*queue_to_ep*/!is_bulk);
In principle in-line comments are fine, but I don't think the parameter name is worth a comment
> + if(ret < 0) { > + uvcg_queue_cancel(queue, 0); > + } > + /* Queue work to the wq as well since its possible that a buffer > + * may not have been completed. > + */
The phrasing of this implies this is a bit of defensive programming, but if we don't queue to the wq here then doesn't that mean it'll never run?
> queue_work(video->async_wq, &video->pump); > } else { > uvc_video_free_request(ureq, ep); > @@ -347,6 +472,7 @@ uvc_video_free_requests(struct uvc_video *video) > > INIT_LIST_HEAD(&video->ureqs); > INIT_LIST_HEAD(&video->req_free); > + INIT_LIST_HEAD(&video->req_ready); > video->req_size = 0; > return 0; > } > @@ -424,8 +550,7 @@ static void uvcg_video_pump(struct work_struct *work) > struct usb_request *req = NULL; > struct uvc_buffer *buf; > unsigned long flags; > - bool buf_done; > - int ret; > + int ret = 0; > > while (true) { > if (!video->ep->enabled) > @@ -454,7 +579,6 @@ static void uvcg_video_pump(struct work_struct *work) > > if (buf != NULL) { > video->encode(req, video, buf); > - buf_done = buf->state == UVC_BUF_STATE_DONE; > } else if (!(queue->flags & UVC_QUEUE_DISCONNECTED) && !is_bulk) { > /* > * No video buffer available; the queue is still connected and > @@ -462,7 +586,6 @@ static void uvcg_video_pump(struct work_struct *work) > * prevent missed ISOC transfers. > */ > req->length = 0; > - buf_done = false; > } else { > /* > * Either the queue has been disconnected or no video buffer > @@ -473,45 +596,26 @@ static void uvcg_video_pump(struct work_struct *work) > break; > } > > - /* > - * With USB3 handling more requests at a higher speed, we can't > - * afford to generate an interrupt for every request. Decide to > - * interrupt: > - * > - * - When no more requests are available in the free queue, as > - * this may be our last chance to refill the endpoint's > - * request queue. > - * > - * - When this is request is the last request for the video > - * buffer, as we want to start sending the next video buffer > - * ASAP in case it doesn't get started already in the next > - * iteration of this loop. > - * > - * - Four times over the length of the requests queue (as > - * indicated by video->uvc_num_requests), as a trade-off > - * between latency and interrupt load. > - */ > - if (list_empty(&video->req_free) || buf_done || > - !(video->req_int_count % > - DIV_ROUND_UP(video->uvc_num_requests, 4))) { > - video->req_int_count = 0; > - req->no_interrupt = 0; > - } else { > - req->no_interrupt = 1; > - } > - > - /* Queue the USB request */ > - ret = uvcg_video_ep_queue(video, req); > spin_unlock_irqrestore(&queue->irqlock, flags); > > + /* Queue the USB request.*/ I think just drop this - it was always superfluous. > + spin_lock_irqsave(&video->req_lock, flags); > + /* For bulk end points we queue from the worker thread > + * since we would preferably not want to wait on requests > + * to be ready, in the uvcg_video_complete() handler. > + * For isoc endpoints we add the request to the ready list > + * and only queue it to the endpoint from the complete handler. > + */ > + ret = uvcg_video_usb_req_queue(video, req, is_bulk); > + spin_unlock_irqrestore(&video->req_lock, flags); > + > if (ret < 0) { > uvcg_queue_cancel(queue, 0); > break; > } > > - /* Endpoint now owns the request */ > + /* The request is owned by the endpoint / ready list*/ > req = NULL; > - video->req_int_count++; > } > > if (!req) > @@ -567,7 +671,7 @@ uvcg_video_disable(struct uvc_video *video) > > spin_lock_irqsave(&video->req_lock, flags); > /* > - * Remove all uvc_reqeusts from ureqs with list_del_init > + * Remove all uvc_requests from ureqs with list_del_init This should get fixed in the earlier series. > * This lets uvc_video_free_request correctly identify > * if the uvc_request is attached to a list or not when freeing > * memory. > @@ -579,9 +683,13 @@ uvcg_video_disable(struct uvc_video *video) > list_del(&req->list); > uvc_video_free_request(req->context, video->ep); > } > - keep the empty line please > + list_for_each_entry_safe(req, temp, &video->req_ready, list) { > + list_del(&req->list); > + uvc_video_free_request(req->context, video->ep); > + } and one here too. > INIT_LIST_HEAD(&video->ureqs); > INIT_LIST_HEAD(&video->req_free); > + INIT_LIST_HEAD(&video->req_ready); > video->req_size = 0; > spin_unlock_irqrestore(&video->req_lock, flags); > > @@ -635,7 +743,7 @@ int uvcg_video_enable(struct uvc_video *video) > > video->req_int_count = 0; > > - queue_work(video->async_wq, &video->pump); > + uvc_video_ep_queue_initial_requests(video); > > return ret; > } > @@ -648,6 +756,7 @@ int uvcg_video_init(struct uvc_video *video, struct uvc_device *uvc) > video->is_enabled = false; > INIT_LIST_HEAD(&video->ureqs); > INIT_LIST_HEAD(&video->req_free); > + INIT_LIST_HEAD(&video->req_ready); > spin_lock_init(&video->req_lock); > INIT_WORK(&video->pump, uvcg_video_pump); >
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