Re: [RFT PATCH v2 6/6] uvcvideo: Move decode processing to process context

From: Kieran Bingham
Date: Thu Jan 11 2018 - 12:54:56 EST

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Hi Kieran !

Thanking you for the patch might be rather self serving here :D

On 09/01/18 13:09, Kieran Bingham wrote:
> Newer high definition cameras, and cameras with multiple lenses such as
> the range of stereo-vision cameras now available have ever increasing
> data rates.
>
> The inclusion of a variable length packet header in URB packets mean
> that we must memcpy the frame data out to our destination 'manually'.
> This can result in data rates of up to 2 gigabits per second being
> processed.
>
> To improve efficiency, and maximise throughput, handle the URB decode
> processing through a work queue to move it from interrupt context, and
> allow multiple processors to work on URBs in parallel.
>
> Signed-off-by: Kieran Bingham <kieran.bingham@xxxxxxxxxxxxxxxx>
>
> ---
> v2:
> - Lock full critical section of usb_submit_urb()
>
> drivers/media/usb/uvc/uvc_queue.c | 12 +++-
> drivers/media/usb/uvc/uvc_video.c | 111 +++++++++++++++++++++++++------
> drivers/media/usb/uvc/uvcvideo.h | 24 +++++++-
> 3 files changed, 129 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/media/usb/uvc/uvc_queue.c b/drivers/media/usb/uvc/uvc_queue.c
> index 5a9987e547d3..598087eeb5c2 100644
> --- a/drivers/media/usb/uvc/uvc_queue.c
> +++ b/drivers/media/usb/uvc/uvc_queue.c
> @@ -179,10 +179,22 @@ static void uvc_stop_streaming(struct vb2_queue *vq)
> struct uvc_video_queue *queue = vb2_get_drv_priv(vq);
> struct uvc_streaming *stream = uvc_queue_to_stream(queue);
>
> + /* Prevent new buffers coming in. */
> + spin_lock_irq(&queue->irqlock);
> + queue->flags |= UVC_QUEUE_STOPPING;
> + spin_unlock_irq(&queue->irqlock);
> +
> + /*
> + * All pending work should be completed before disabling the stream, as
> + * all URBs will be free'd during uvc_video_enable(s, 0).
> + */
> + flush_workqueue(stream->async_wq);
> +
> uvc_video_enable(stream, 0);
>
> spin_lock_irq(&queue->irqlock);
> uvc_queue_return_buffers(queue, UVC_BUF_STATE_ERROR);
> + queue->flags &= ~UVC_QUEUE_STOPPING;
> spin_unlock_irq(&queue->irqlock);
> }
>
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index 3878bec3276e..a9ddc4f27012 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -1058,21 +1058,67 @@ static int uvc_video_decode_start(struct uvc_streaming *stream,
> return data[0];
> }
>
> -static void uvc_video_decode_data(struct uvc_streaming *stream,
> - struct uvc_buffer *buf, const __u8 *data, int len)
> +/*
> + * uvc_video_decode_data_work: Asynchronous memcpy processing
> + *
> + * Perform memcpy tasks in process context, with completion handlers
> + * to return the URB, and buffer handles.
> + *
> + * The work submitter must pre-determine that the work is safe
> + */
> +static void uvc_video_decode_data_work(struct work_struct *work)

This handles asynchronous memory copy work - which could equally be encode work
- so _decode_ might not be an appropriate name here.

> {
> - unsigned int maxlen, nbytes;
> - void *mem;
> + struct uvc_urb *uvc_urb = container_of(work, struct uvc_urb, work);
> + struct uvc_streaming *stream = uvc_urb->stream;
> + struct uvc_video_queue *queue = &stream->queue;
> + unsigned int i;
> + int ret;
> +
> + for (i = 0; i < uvc_urb->packets; i++) {
> + struct uvc_decode_op *op = &uvc_urb->decodes[i];
> +
> + memcpy(op->dst, op->src, op->len);
> +
> + /* Release reference taken on this buffer */
> + uvc_queue_buffer_release(op->buf);
> + }
> +
> + /*
> + * Prevent resubmitting URBs when shutting down to ensure that no new
> + * work item will be scheduled after uvc_stop_streaming() flushes the
> + * work queue.
> + */
> + spin_lock_irq(&queue->irqlock);
> + if (!(queue->flags & UVC_QUEUE_STOPPING)) {
> + ret = usb_submit_urb(uvc_urb->urb, GFP_ATOMIC);
> + if (ret < 0)
> + uvc_printk(KERN_ERR,
> + "Failed to resubmit video URB (%d).\n",
> + ret);
> + }
> + spin_unlock_irq(&queue->irqlock);
> +}
> +
> +static void uvc_video_decode_data(struct uvc_decode_op *decode,
> + struct uvc_urb *uvc_urb, struct uvc_buffer *buf,
> + const __u8 *data, int len)
> +{
> + unsigned int maxlen;
>
> if (len <= 0)
> return;
>
> - /* Copy the video data to the buffer. */
> maxlen = buf->length - buf->bytesused;
> - mem = buf->mem + buf->bytesused;
> - nbytes = min((unsigned int)len, maxlen);
> - memcpy(mem, data, nbytes);
> - buf->bytesused += nbytes;
> +
> + /* Take a buffer reference for async work */
> + kref_get(&buf->ref);
> +
> + decode->buf = buf;
> + decode->src = data;
> + decode->dst = buf->mem + buf->bytesused;
> + decode->len = min_t(unsigned int, len, maxlen);
> +
> + buf->bytesused += decode->len;
>
> /* Complete the current frame if the buffer size was exceeded. */
> if (len > maxlen) {
> @@ -1080,6 +1126,8 @@ static void uvc_video_decode_data(struct uvc_streaming *stream,
> buf->error = 1;
> buf->state = UVC_BUF_STATE_READY;
> }
> +
> + uvc_urb->packets++;
> }
>
> static void uvc_video_decode_end(struct uvc_streaming *stream,
> @@ -1162,6 +1210,8 @@ static void uvc_video_decode_isoc(struct uvc_urb *uvc_urb,
> int ret, i;
>
> for (i = 0; i < urb->number_of_packets; ++i) {
> + struct uvc_decode_op *op = &uvc_urb->decodes[uvc_urb->packets];
> +

This structure is only ever used by the uvc_video_decode_data() function, and it
could be obtained in there directly.

> if (urb->iso_frame_desc[i].status < 0) {
> uvc_trace(UVC_TRACE_FRAME, "USB isochronous frame "
> "lost (%d).\n", urb->iso_frame_desc[i].status);
> @@ -1187,7 +1237,7 @@ static void uvc_video_decode_isoc(struct uvc_urb *uvc_urb,
> continue;
>
> /* Decode the payload data. */
> - uvc_video_decode_data(stream, buf, mem + ret,
> + uvc_video_decode_data(op, uvc_urb, buf, mem + ret,
> urb->iso_frame_desc[i].actual_length - ret);
>
> /* Process the header again. */
> @@ -1248,9 +1298,12 @@ static void uvc_video_decode_bulk(struct uvc_urb *uvc_urb,
> * sure buf is never dereferenced if NULL.
> */
>
> - /* Process video data. */
> - if (!stream->bulk.skip_payload && buf != NULL)
> - uvc_video_decode_data(stream, buf, mem, len);
> + /* Prepare video data for processing. */
> + if (!stream->bulk.skip_payload && buf != NULL) {
> + struct uvc_decode_op *op = &uvc_urb->decodes[0];

Again - no reason to obtain this op here.

> +
> + uvc_video_decode_data(op, uvc_urb, buf, mem, len);
> + }
>
> /* Detect the payload end by a URB smaller than the maximum size (or
> * a payload size equal to the maximum) and process the header again.
> @@ -1322,7 +1375,8 @@ static void uvc_video_complete(struct urb *urb)
> struct uvc_streaming *stream = uvc_urb->stream;
> struct uvc_video_queue *queue = &stream->queue;
> struct uvc_buffer *buf = NULL;
> - int ret;
> + unsigned long flags;
> + bool stopping;
>
> switch (urb->status) {
> case 0:
> @@ -1342,14 +1396,30 @@ static void uvc_video_complete(struct urb *urb)
> return;
> }
>
> + /*
> + * Simply accept and discard completed URBs without processing when the
> + * stream is being shutdown. URBs will be freed as part of the
> + * uvc_video_enable(s, 0) action, so we must not queue asynchronous
> + * work based upon them.
> + */
> + spin_lock_irqsave(&queue->irqlock, flags);
> + stopping = queue->flags & UVC_QUEUE_STOPPING;
> + spin_unlock_irqrestore(&queue->irqlock, flags);
> +
> + if (stopping)
> + return;
> +

We can race here if the stream gets stopped at this point...! (Thanks Laurent)

> buf = uvc_queue_get_current_buffer(queue);
>
> + /* Re-initialise the URB packet work */
> + uvc_urb->packets = 0;
> +
> + /* Process the URB headers, but work is deferred to a work queue */
> stream->decode(uvc_urb, buf);
>
> - if ((ret = usb_submit_urb(urb, GFP_ATOMIC)) < 0) {
> - uvc_printk(KERN_ERR, "Failed to resubmit video URB (%d).\n",
> - ret);
> - }
> + /* Handle any heavy lifting required */
> + INIT_WORK(&uvc_urb->work, uvc_video_decode_data_work);> + queue_work(stream->async_wq, &uvc_urb->work);

The encode operations don't currently schedule any work to be done - but the URB
isn't submitted until the work queue runs, which adds unnecessary latency to
non-async functionality.!

> }
>
> /*
> @@ -1620,6 +1690,11 @@ static int uvc_init_video(struct uvc_streaming *stream, gfp_t gfp_flags)
>
> uvc_video_stats_start(stream);
>
> + stream->async_wq = alloc_workqueue("uvcvideo", WQ_UNBOUND | WQ_HIGHPRI,
> + 0);
> + if (!stream->async_wq)
> + return -ENOMEM;
> +
> if (intf->num_altsetting > 1) {
> struct usb_host_endpoint *best_ep = NULL;
> unsigned int best_psize = UINT_MAX;
> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> index 6a18dbfc3e5b..13dc0b2bb8b9 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -411,6 +411,7 @@ struct uvc_buffer {
>
> #define UVC_QUEUE_DISCONNECTED (1 << 0)
> #define UVC_QUEUE_DROP_CORRUPTED (1 << 1)
> +#define UVC_QUEUE_STOPPING (1 << 2)
>
> struct uvc_video_queue {
> struct vb2_queue queue;
> @@ -483,12 +484,30 @@ struct uvc_stats_stream {
> };
>
> /**
> + * struct uvc_decode_op: Context structure to schedule asynchronous memcpy
> + *
> + * @buf: active buf object for this decode
> + * @dst: copy destination address
> + * @src: copy source address
> + * @len: copy length
> + */
> +struct uvc_decode_op {
> + struct uvc_buffer *buf;
> + void *dst;
> + const __u8 *src;
> + int len;
> +};
> +
> +/**
> * struct uvc_urb - URB context management structure
> *
> * @urb: the URB described by this context structure
> * @stream: UVC streaming context
> * @buffer: memory storage for the URB
> * @dma: DMA coherent addressing for the urb_buffer
> + * @packets: counter to indicate the number of copy operations
> + * @decodes: work descriptors for asynchronous copy operations
> + * @work: work queue entry for asynchronous decode
> */
> struct uvc_urb {
> struct urb *urb;
> @@ -496,6 +515,10 @@ struct uvc_urb {
>
> char *buffer;
> dma_addr_t dma;
> +
> + unsigned int packets;
> + struct uvc_decode_op decodes[UVC_MAX_PACKETS];

Whilst not currently, the encode functions could also use this system - so
perhaps decodes isn't the right name here.

struct uvc_copy_op copy_operations[UVC_MAX_PACKETS];

might be a better description

> + struct work_struct work;
> };
>
> struct uvc_streaming {
> @@ -528,6 +551,7 @@ struct uvc_streaming {
> /* Buffers queue. */
> unsigned int frozen : 1;
> struct uvc_video_queue queue;
> + struct workqueue_struct *async_wq;
> void (*decode)(struct uvc_urb *uvc_urb, struct uvc_buffer *buf);
>
> /* Context data used by the bulk completion handler. */
>

Next message: Willy Tarreau: "Re: Backport of KPTI to 2.6.32 available"
Previous message: Andy Lutomirski: "Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti_disable is set"
In reply to: Kieran Bingham: "[RFT PATCH v2 6/6] uvcvideo: Move decode processing to process context"
Next in thread: Kieran Bingham: "[RFT PATCH v2 1/6] uvcvideo: Refactor URB descriptors"
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