Re: [RFT PATCH v3 6/6] uvcvideo: Move decode processing to process context

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Kieran,

Thank you for the patch.

On Friday, 12 January 2018 11:19:27 EET 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@ideasonboard.com>
> 
> ---
> v2:
>  - Lock full critical section of usb_submit_urb()
> 
> v3:
>  - Fix race on submitting uvc_video_decode_data_work() to work queue.
>  - Rename uvc_decode_op -> uvc_copy_op (Generic to encode/decode)
>  - Rename decodes -> copy_operations
>  - Don't queue work if there is no async task
>  - obtain copy op structure directly in uvc_video_decode_data()
>  - uvc_video_decode_data_work() -> uvc_video_copy_data_work()
> ---
>  drivers/media/usb/uvc/uvc_queue.c |  12 +++-
>  drivers/media/usb/uvc/uvc_video.c | 116 +++++++++++++++++++++++++++----
>  drivers/media/usb/uvc/uvcvideo.h  |  24 ++++++-
>  3 files changed, 138 insertions(+), 14 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);

As discussed today I believe you'll rework this part so I won't comment on it.

>  }
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c
> b/drivers/media/usb/uvc/uvc_video.c index 3878bec3276e..fb6b5af17380 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -1058,21 +1058,74 @@ static int uvc_video_decode_start(struct
> uvc_streaming *stream, return data[0];
>  }
> 
> -static void uvc_video_decode_data(struct uvc_streaming *stream,
> +static void uvc_video_copy_packets(struct uvc_urb *uvc_urb)

Maybe uvc_video_copy_data() as the function doesn't copy the full packets but 
the data only ?

> +{
> +	unsigned int i;
> +
> +	for (i = 0; i < uvc_urb->async_operations; i++) {
> +		struct uvc_copy_op *op = &uvc_urb->copy_operations[i];
> +
> +		memcpy(op->dst, op->src, op->len);
> +
> +		/* Release reference taken on this buffer */
> +		uvc_queue_buffer_release(op->buf);
> +	}
> +}
> +
> +/*
> + * 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

s/safe/safe./

Could you explain what safe work means ?

> + */
> +static void uvc_video_copy_data_work(struct work_struct *work)
> +{
> +	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;
> +	int ret;
> +
> +	uvc_video_copy_packets(uvc_urb);
> +
> +	/*
> +	 * 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_urb *uvc_urb,
>  		struct uvc_buffer *buf, const __u8 *data, int len)
>  {
> -	unsigned int maxlen, nbytes;
> -	void *mem;
> +	unsigned int active_op = uvc_urb->async_operations;
> +	struct uvc_copy_op *decode = &uvc_urb->copy_operations[active_op];
> +	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 +1133,8 @@ static void uvc_video_decode_data(struct uvc_streaming
> *stream, buf->error = 1;
>  		buf->state = UVC_BUF_STATE_READY;
>  	}
> +
> +	uvc_urb->async_operations++;
>  }
> 
>  static void uvc_video_decode_end(struct uvc_streaming *stream,
> @@ -1187,7 +1242,7 @@ 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(uvc_urb, buf, mem + ret,
>  			urb->iso_frame_desc[i].actual_length - ret);
> 
>  		/* Process the header again. */
> @@ -1248,9 +1303,9 @@ static void uvc_video_decode_bulk(struct uvc_urb
> *uvc_urb, * sure buf is never dereferenced if NULL.
>  	 */
> 
> -	/* Process video data. */
> +	/* Prepare video data for processing. */
>  	if (!stream->bulk.skip_payload && buf != NULL)
> -		uvc_video_decode_data(stream, buf, mem, len);
> +		uvc_video_decode_data(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.
> @@ -1322,6 +1377,7 @@ 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;
> +	unsigned long flags;
>  	int ret;
> 
>  	switch (urb->status) {
> @@ -1344,12 +1400,39 @@ static void uvc_video_complete(struct urb *urb)
> 
>  	buf = uvc_queue_get_current_buffer(queue);
> 
> +	/* Re-initialise the URB async work. */
> +	uvc_urb->async_operations = 0;
> +
> +	/*
> +	 * Process the URB headers, and optionally queue expensive memcpy tasks
> +	 * to be 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);
> +	/* Without any queued work, we must submit the URB. */

How about "If no async work is needed, resubmit the URB immediately." ?

> +	if (!uvc_urb->async_operations) {
> +		ret = usb_submit_urb(uvc_urb->urb, GFP_ATOMIC);
> +		if (ret < 0)
> +			uvc_printk(KERN_ERR,
> +				   "Failed to resubmit video URB (%d).\n",
> +				   ret);
> +		return;
> +	}
> +
> +	/*
> +	 * When the stream is stopped, all URBs are freed as part of the call to
> +	 * uvc_stop_streaming() and must not be handled asynchronously. In that
> +	 * event we can safely complete the packet work directly in this
> +	 * context, without resubmitting the URB.
> +	 */
> +	spin_lock_irqsave(&queue->irqlock, flags);
> +	if (!(queue->flags & UVC_QUEUE_STOPPING)) {
> +		INIT_WORK(&uvc_urb->work, uvc_video_copy_data_work);
> +		queue_work(stream->async_wq, &uvc_urb->work);
> +	} else {
> +		uvc_video_copy_packets(uvc_urb);
>  	}
> +	spin_unlock_irqrestore(&queue->irqlock, flags);
>  }
> 
>  /*
> @@ -1620,6 +1703,11 @@ 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;

Shouldn't you call destroy_workqueue() somewhere ?

>  	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..4a814da03913 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_copy_op: Context structure to schedule asynchronous memcpy
> + *
> + * @buf: active buf object for this operation
> + * @dst: copy destination address
> + * @src: copy source address
> + * @len: copy length
> + */
> +struct uvc_copy_op {
> +	struct uvc_buffer *buf;
> +	void *dst;
> +	const __u8 *src;
> +	int len;

Can the length be negative ?

> +};
> +
> +/**
>   * 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
> + * @async_operations: counter to indicate the number of copy operations
> + * @copy_operations: 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 async_operations;
> +	struct uvc_copy_op copy_operations[UVC_MAX_PACKETS];
> +	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. */

-- 
Regards,

Laurent Pinchart