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@xxxxxxxxxxxxxx>
Signed-off-by: Jayant Chowdhary <jchowdhary@xxxxxxxxxx>
Suggested-by: Avichal Rakesh <arakesh@xxxxxxxxxx>
Suggested-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
---
Based on top of
https://lore.kernel.org/linux-usb/20230930184821.310143-1-arakesh@xxxxxxxxxx/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);
