Re: [RFC v4 13/17] [media] vb2: add in-fence support to QBUF

[Brian Starkey]

Hi Gustavo,

On Fri, Oct 20, 2017 at 07:50:08PM -0200, Gustavo Padovan wrote:
> From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
>
> Receive in-fence from userspace and add support for waiting on them
> before queueing the buffer to the driver. Buffers are only queued
> to the driver once they are ready. A buffer is ready when its
> in-fence signals.
>
> For queues that require vb2 to queue buffers to the v4l2 driver in same
> order they are received from userspace we use fence_array to keep that
> ordering. Basically we create a fence_array that contains both the current
> fence and the fence from the previous buffer (which might be a fence array
> as well). The base fence class for the fence_array becomes the new buffer
> fence, waiting on that one guarantees that it won't be queued out of
> order.

The API sounds/looks good to me, makes sense to let driver opt in/out
via the ordered_in_vb2 thing. Thanks for implementing it!

> v5:	- use fence_array to keep buffers ordered in vb2 core when
> 	needed (Brian Stark)
> 	- keep backward compatibility on the reserved2 field (Brian Stark)
> 	- protect fence callback removal with lock (Brian Stark)

Brian Starkey, but close ;-)

> v4:
> 	- Add a comment about dma_fence_add_callback() not returning a
> 	error (Hans)
> 	- Call dma_fence_put(vb->in_fence) if fence signaled (Hans)
> 	- select SYNC_FILE under config VIDEOBUF2_CORE (Hans)
> 	- Move dma_fence_is_signaled() check to __enqueue_in_driver() (Hans)
> 	- Remove list_for_each_entry() in __vb2_core_qbuf() (Hans)
> 	-  Remove if (vb->state != VB2_BUF_STATE_QUEUED) from
> 	vb2_start_streaming() (Hans)
> 	- set IN_FENCE flags on __fill_v4l2_buffer (Hans)
> 	- Queue buffers to the driver as soon as they are ready (Hans)
> 	- call fill_user_buffer() after queuing the buffer (Hans)
> 	- add err: label to clean up fence
> 	- add dma_fence_wait() before calling vb2_start_streaming()
>
> v3:	- document fence parameter
> 	- remove ternary if at vb2_qbuf() return (Mauro)
> 	- do not change if conditions behaviour (Mauro)
>
> v2:
> 	- fix vb2_queue_or_prepare_buf() ret check
> 	- remove check for VB2_MEMORY_DMABUF only (Javier)
> 	- check num of ready buffers to start streaming
> 	- when queueing, start from the first ready buffer
> 	- handle queue cancel
>
> Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
> ---
> drivers/media/v4l2-core/Kconfig          |   1 +
> drivers/media/v4l2-core/videobuf2-core.c | 179 ++++++++++++++++++++++++++++---
> drivers/media/v4l2-core/videobuf2-v4l2.c |  29 ++++-
> include/media/videobuf2-core.h           |  17 ++-
> 4 files changed, 208 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/media/v4l2-core/Kconfig b/drivers/media/v4l2-core/Kconfig
> index a35c33686abf..3f988c407c80 100644
> --- a/drivers/media/v4l2-core/Kconfig
> +++ b/drivers/media/v4l2-core/Kconfig
> @@ -83,6 +83,7 @@ config VIDEOBUF_DVB
> # Used by drivers that need Videobuf2 modules
> config VIDEOBUF2_CORE
> 	select DMA_SHARED_BUFFER
> +	select SYNC_FILE
> 	tristate
>
> config VIDEOBUF2_MEMOPS
> diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
> index 60f8b582396a..78f369dba3e3 100644
> --- a/drivers/media/v4l2-core/videobuf2-core.c
> +++ b/drivers/media/v4l2-core/videobuf2-core.c
> @@ -346,6 +346,7 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
> 		vb->index = q->num_buffers + buffer;
> 		vb->type = q->type;
> 		vb->memory = memory;
> +		spin_lock_init(&vb->fence_cb_lock);
> 		for (plane = 0; plane < num_planes; ++plane) {
> 			vb->planes[plane].length = plane_sizes[plane];
> 			vb->planes[plane].min_length = plane_sizes[plane];
> @@ -1222,6 +1223,9 @@ static void __enqueue_in_driver(struct vb2_buffer *vb)
> {
> 	struct vb2_queue *q = vb->vb2_queue;
>
> +	if (vb->in_fence && !dma_fence_is_signaled(vb->in_fence))
> +		return;
> +
> 	vb->state = VB2_BUF_STATE_ACTIVE;
> 	atomic_inc(&q->owned_by_drv_count);
>
> @@ -1273,6 +1277,20 @@ static int __buf_prepare(struct vb2_buffer *vb, const void *pb)
> 	return 0;
> }
>
> +static int __get_num_ready_buffers(struct vb2_queue *q)
> +{
> +	struct vb2_buffer *vb;
> +	int ready_count = 0;
> +
> +	/* count num of buffers ready in front of the queued_list */
> +	list_for_each_entry(vb, &q->queued_list, queued_entry) {
> +		if (!vb->in_fence || dma_fence_is_signaled(vb->in_fence))
> +			ready_count++;

I think there's still some races on vb->in_fence. Couldn't the
callback on any of the buffers in the queued_list have their callback
run at any moment?

That could make this loop do bad things, and similarly the loop in
vb2_start_streaming() which calls __enqueue_in_driver().

> +	}
> +
> +	return ready_count;
> +}
> +
> int vb2_core_prepare_buf(struct vb2_queue *q, unsigned int index, void *pb)
> {
> 	struct vb2_buffer *vb;
> @@ -1361,7 +1379,74 @@ static int vb2_start_streaming(struct vb2_queue *q)
> 	return ret;
> }
>
> -int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb)
> +static struct dma_fence *__set_in_fence(struct vb2_queue *q,
> +					struct vb2_buffer *vb,
> +					struct dma_fence *fence)
> +{
> +	if (q->last_fence && dma_fence_is_signaled(q->last_fence)) {
> +		dma_fence_put(q->last_fence);
> +		q->last_fence = NULL;
> +	}
> +
> +	/* For drivers that require the ordering of buffers queued from
> +	 * userspace to be the same it is queued to the driver - output
> +	 * some m2m devices - we create a fence array with the fence from
> +	 * the last queued buffer and this one, that way the fence for this
> +	 * buffer can't signal before the last one.
> +	 */
> +	if (q->is_output || q->ordered_in_vb2) {

Just a stylistic thing - you could invert this conditional and return
early, saving a level of indentation below.

> +		if (fence && q->last_fence) {
> +			struct dma_fence **fences;
> +			struct dma_fence_array *arr;
> +
> +			fences = kcalloc(2, sizeof(*fences), GFP_KERNEL);
> +			if (!fences)
> +				return ERR_PTR(-ENOMEM);
> +
> +			fences[0] = fence;
> +			fences[1] = q->last_fence;
> +
> +			arr = dma_fence_array_create(2, fences,
> +						     dma_fence_context_alloc(1),
> +						     1, false);

I'm not sure if it's right to get a new fence_context every time here.
The comment on dma_fence_context_counter sounds like the vb2_queue
should have a single context of its own.

> +			if (!arr) {
> +				kfree(fences);
> +				return ERR_PTR(-ENOMEM);
> +			}
> +
> +			fence = &arr->base;
> +		} else if (!fence && q->last_fence) {
> +			fence = dma_fence_get(q->last_fence);
> +		}
> +
> +		q->last_fence = dma_fence_get(fence);

Seems like the refcounting isn't quite right here.

If you land in "else if (!fence && q->last_fence)", then "fence" gets
a new reference to q->last_fence, and you take another one here -
which looks like one too many.

I _think_ the right thing to do is just give the previous reference on
q->last_fence to "fence", without taking a new one:

	} else if (!fence && q->last_fence) {
		fence = q->last_fence;
	}

That's analogous to the other path where fence is not-NULL, and the
fence-array takes ownership of the existing references on fence and
q->last_fence, without taking new references.

Please double check my logic though!

> +	}
> +
> +	return fence;
> +}
> +
> +static void vb2_qbuf_fence_cb(struct dma_fence *f, struct dma_fence_cb *cb)
> +{
> +	struct vb2_buffer *vb = container_of(cb, struct vb2_buffer, fence_cb);
> +	struct vb2_queue *q = vb->vb2_queue;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&vb->fence_cb_lock, flags);
> +	if (!vb->in_fence) {
> +		spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> +		return;
> +	}
> +
> +	dma_fence_put(vb->in_fence);
> +	vb->in_fence = NULL;
> +	spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> +
> +	if (q->start_streaming_called)
> +		__enqueue_in_driver(vb);
> +}
> +
> +int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb,
> +		  struct dma_fence *fence)
> {
> 	struct vb2_buffer *vb;
> 	int ret;
> @@ -1372,16 +1457,18 @@ int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb)
> 	case VB2_BUF_STATE_DEQUEUED:
> 		ret = __buf_prepare(vb, pb);
> 		if (ret)
> -			return ret;
> +			goto err;
> 		break;
> 	case VB2_BUF_STATE_PREPARED:
> 		break;
> 	case VB2_BUF_STATE_PREPARING:
> 		dprintk(1, "buffer still being prepared\n");
> -		return -EINVAL;
> +		ret = -EINVAL;
> +		goto err;
> 	default:
> 		dprintk(1, "invalid buffer state %d\n", vb->state);
> -		return -EINVAL;
> +		ret = -EINVAL;
> +		goto err;
> 	}
>
> 	/*
> @@ -1398,30 +1485,75 @@ int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb)
>
> 	trace_vb2_qbuf(q, vb);
>
> +	vb->in_fence = __set_in_fence(q, vb, fence);
> +	if (IS_ERR(vb->in_fence)) {
> +		ret = PTR_ERR(vb->in_fence);
> +		goto err;

Between here, __set_in_fence() and vb2_qbuf() there's a reference
leaked on "fence" in the case of error. I'm not sure what semantics
you'd like, but it should be clear whose responsibility it is to clear
up "fence" if __set_in_fence() fails (at the moment, looks like no-one
does).

> +	}

... and if __set_in_fence() succeeds, then vb->in_fence now owns the
reference on "fence" that was passed in, so I guess you should set
fence = NULL here.

> +
> +	/*
> +	 * If it is time to call vb2_start_streaming() wait for the fence
> +	 * to signal first. Of course, this happens only once per streaming.
> +	 * We want to run any step that might fail before we set the callback
> +	 * to queue the fence when it signals.
> +	 */
> +	if (fence && !q->start_streaming_called &&

I think this should be vb->in_fence now, and the same for all further
uses of "fence" in this function.

> +	    __get_num_ready_buffers(q) == q->min_buffers_needed - 1)
> +		dma_fence_wait(fence, true);
> +
> 	/*
> 	 * If streamon has been called, and we haven't yet called
> 	 * start_streaming() since not enough buffers were queued, and
> 	 * we now have reached the minimum number of queued buffers,
> 	 * then we can finally call start_streaming().
> -	 *
> -	 * If already streaming, give the buffer to driver for processing.
> -	 * If not, the buffer will be given to driver on next streamon.
> 	 */
> 	if (q->streaming && !q->start_streaming_called &&
> -	    q->queued_count >= q->min_buffers_needed) {
> +	    __get_num_ready_buffers(q) >= q->min_buffers_needed) {
> 		ret = vb2_start_streaming(q);
> 		if (ret)
> -			return ret;
> -	} else if (q->start_streaming_called) {
> -		__enqueue_in_driver(vb);
> +			goto err;
> +	}
> +
> +	/*
> +	 * For explicit synchronization: If the fence didn't signal
> +	 * yet we setup a callback to queue the buffer once the fence
> +	 * signals, and then, return successfully. But if the fence
> +	 * already signaled we lose the reference we held and queue the
> +	 * buffer to the driver.
> +	 */
> +	if (fence) {
> +		ret = dma_fence_add_callback(fence, &vb->fence_cb,
> +					     vb2_qbuf_fence_cb);
> +		if (!ret)
> +			goto fill;

Just spotted that you should probably handle the -EINVAL case here and
jump to err.

> +
> +		dma_fence_put(fence);
> +		vb->in_fence = NULL;
> 	}
>
> +fill:
> +	/*
> +	 * If already streaming and there is no fence to wait on
> +	 * give the buffer to driver for processing.
> +	 */
> +	if (q->start_streaming_called && !vb->in_fence)
> +		__enqueue_in_driver(vb);

The callback could have run just before this, clearing vb->in_fence
and ending up with __enqueue_in_driver() getting called twice.

> +
> 	/* Fill buffer information for the userspace */
> 	if (pb)
> 		call_void_bufop(q, fill_user_buffer, vb, pb);
>
> 	dprintk(2, "qbuf of buffer %d succeeded\n", vb->index);
> 	return 0;
> +
> +err:
> +	if (vb->in_fence) {
> +		dma_fence_put(vb->in_fence);
> +		vb->in_fence = NULL;
> +	}
> +
> +	return ret;
> +
> }
> EXPORT_SYMBOL_GPL(vb2_core_qbuf);
>
> @@ -1632,6 +1764,8 @@ EXPORT_SYMBOL_GPL(vb2_core_dqbuf);
> static void __vb2_queue_cancel(struct vb2_queue *q)
> {
> 	unsigned int i;
> +	struct vb2_buffer *vb;
> +	unsigned long flags;
>
> 	/*
> 	 * Tell driver to stop all transactions and release all queued
> @@ -1659,6 +1793,21 @@ static void __vb2_queue_cancel(struct vb2_queue *q)
> 	q->queued_count = 0;
> 	q->error = 0;
>
> +	list_for_each_entry(vb, &q->queued_list, queued_entry) {
> +		spin_lock_irqsave(&vb->fence_cb_lock, flags);
> +		if (vb->in_fence) {
> +			dma_fence_remove_callback(vb->in_fence, &vb->fence_cb);
> +			dma_fence_put(vb->in_fence);
> +			vb->in_fence = NULL;
> +		}
> +		spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> +	}
> +
> +	if (q->last_fence) {
> +		dma_fence_put(q->last_fence);
> +		q->last_fence = NULL;
> +	}
> +
> 	/*
> 	 * Remove all buffers from videobuf's list...
> 	 */
> @@ -1720,7 +1869,7 @@ int vb2_core_streamon(struct vb2_queue *q, unsigned int type)
> 	 * Tell driver to start streaming provided sufficient buffers
> 	 * are available.
> 	 */
> -	if (q->queued_count >= q->min_buffers_needed) {
> +	if (__get_num_ready_buffers(q) >= q->min_buffers_needed) {
> 		ret = v4l_vb2q_enable_media_source(q);
> 		if (ret)
> 			return ret;
> @@ -2240,7 +2389,7 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
> 		 * Queue all buffers.
> 		 */
> 		for (i = 0; i < q->num_buffers; i++) {
> -			ret = vb2_core_qbuf(q, i, NULL);
> +			ret = vb2_core_qbuf(q, i, NULL, NULL);
> 			if (ret)
> 				goto err_reqbufs;
> 			fileio->bufs[i].queued = 1;
> @@ -2419,7 +2568,7 @@ static size_t __vb2_perform_fileio(struct vb2_queue *q, char __user *data, size_
>
> 		if (copy_timestamp)
> 			b->timestamp = ktime_get_ns();
> -		ret = vb2_core_qbuf(q, index, NULL);
> +		ret = vb2_core_qbuf(q, index, NULL, NULL);
> 		dprintk(5, "vb2_dbuf result: %d\n", ret);
> 		if (ret)
> 			return ret;
> @@ -2522,7 +2671,7 @@ static int vb2_thread(void *data)
> 		if (copy_timestamp)
> 			vb->timestamp = ktime_get_ns();;
> 		if (!threadio->stop)
> -			ret = vb2_core_qbuf(q, vb->index, NULL);
> +			ret = vb2_core_qbuf(q, vb->index, NULL, NULL);
> 		call_void_qop(q, wait_prepare, q);
> 		if (ret || threadio->stop)
> 			break;
> diff --git a/drivers/media/v4l2-core/videobuf2-v4l2.c b/drivers/media/v4l2-core/videobuf2-v4l2.c
> index 110fb45fef6f..4c09ea007d90 100644
> --- a/drivers/media/v4l2-core/videobuf2-v4l2.c
> +++ b/drivers/media/v4l2-core/videobuf2-v4l2.c
> @@ -23,6 +23,7 @@
> #include <linux/sched.h>
> #include <linux/freezer.h>
> #include <linux/kthread.h>
> +#include <linux/sync_file.h>
>
> #include <media/v4l2-dev.h>
> #include <media/v4l2-fh.h>
> @@ -178,6 +179,12 @@ static int vb2_queue_or_prepare_buf(struct vb2_queue *q, struct v4l2_buffer *b,
> 		return -EINVAL;
> 	}
>
> +	if ((b->fence_fd != 0 && b->fence_fd != -1) &&
> +	    !(b->flags & V4L2_BUF_FLAG_IN_FENCE)) {
> +		dprintk(1, "%s: fence_fd set without IN_FENCE flag\n", opname);
> +		return -EINVAL;
> +	}
> +
> 	return __verify_planes_array(q->bufs[b->index], b);
> }
>
> @@ -203,9 +210,14 @@ static void __fill_v4l2_buffer(struct vb2_buffer *vb, void *pb)
> 	b->timestamp = ns_to_timeval(vb->timestamp);
> 	b->timecode = vbuf->timecode;
> 	b->sequence = vbuf->sequence;
> -	b->fence_fd = -1;
> 	b->reserved = 0;
>
> +	b->fence_fd = -1;
> +	if (vb->in_fence)
> +		b->flags |= V4L2_BUF_FLAG_IN_FENCE;
> +	else
> +		b->flags &= ~V4L2_BUF_FLAG_IN_FENCE;
> +
> 	if (q->is_multiplanar) {
> 		/*
> 		 * Fill in plane-related data if userspace provided an array
> @@ -560,6 +572,7 @@ EXPORT_SYMBOL_GPL(vb2_create_bufs);
>
> int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
> {
> +	struct dma_fence *fence = NULL;
> 	int ret;
>
> 	if (vb2_fileio_is_active(q)) {
> @@ -568,7 +581,19 @@ int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
> 	}
>
> 	ret = vb2_queue_or_prepare_buf(q, b, "qbuf");
> -	return ret ? ret : vb2_core_qbuf(q, b->index, b);
> +	if (ret)
> +		return ret;
> +
> +	if (b->flags & V4L2_BUF_FLAG_IN_FENCE) {
> +		fence = sync_file_get_fence(b->fence_fd);

In DRM a fence_fd of -1 means "already signaled" - should that be
allowed here too?

Cheers,
-Brian

> +		if (!fence) {
> +			dprintk(1, "failed to get in-fence from fd %d\n",
> +				b->fence_fd);
> +			return -EINVAL;
> +		}
> +	}
> +
> +	return vb2_core_qbuf(q, b->index, b, fence);
> }
> EXPORT_SYMBOL_GPL(vb2_qbuf);
>
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index fc333e10e7d8..624ca2dce9ea 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -16,6 +16,7 @@
> #include <linux/mutex.h>
> #include <linux/poll.h>
> #include <linux/dma-buf.h>
> +#include <linux/dma-fence.h>
>
> #define VB2_MAX_FRAME	(32)
> #define VB2_MAX_PLANES	(8)
> @@ -254,11 +255,20 @@ struct vb2_buffer {
> 	 *			all buffers queued from userspace
> 	 * done_entry:		entry on the list that stores all buffers ready
> 	 *			to be dequeued to userspace
> +	 * in_fence:		fence receive from vb2 client to wait on before
> +	 *			using the buffer (queueing to the driver)
> +	 * fence_cb:		fence callback information
> +	 * fence_cb_lock:	protect callback signal/remove
> 	 */
> 	enum vb2_buffer_state	state;
>
> 	struct list_head	queued_entry;
> 	struct list_head	done_entry;
> +
> +	struct dma_fence	*in_fence;
> +	struct dma_fence_cb	fence_cb;
> +	spinlock_t              fence_cb_lock;
> +
> #ifdef CONFIG_VIDEO_ADV_DEBUG
> 	/*
> 	 * Counters for how often these buffer-related ops are
> @@ -508,6 +518,7 @@ struct vb2_buf_ops {
>  * @ordered_in_vb2: set by the driver to tell vb2 te guarantee the order
>  *		of buffer queue from userspace with QBUF() until they are
>  *		queued to the driver.
> + * @last_fence:	last in-fence received. Used to keep ordering.
>  * @fileio:	file io emulator internal data, used only if emulator is active
>  * @threadio:	thread io internal data, used only if thread is active
>  */
> @@ -563,6 +574,8 @@ struct vb2_queue {
> 	unsigned int			ordered_in_driver:1;
> 	unsigned int			ordered_in_vb2:1;
>
> +	struct dma_fence		*last_fence;
> +
> 	struct vb2_fileio_data		*fileio;
> 	struct vb2_threadio_data	*threadio;
>
> @@ -738,6 +751,7 @@ int vb2_core_prepare_buf(struct vb2_queue *q, unsigned int index, void *pb);
>  * @index:	id number of the buffer
>  * @pb:		buffer structure passed from userspace to vidioc_qbuf handler
>  *		in driver
> + * @fence:	in-fence to wait on before queueing the buffer
>  *
>  * Should be called from vidioc_qbuf ioctl handler of a driver.
>  * The passed buffer should have been verified.
> @@ -752,7 +766,8 @@ int vb2_core_prepare_buf(struct vb2_queue *q, unsigned int index, void *pb);
>  * The return values from this function are intended to be directly returned
>  * from vidioc_qbuf handler in driver.
>  */
> -int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb);
> +int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb,
> +		  struct dma_fence *fence);
>
> /**
>  * vb2_core_dqbuf() - Dequeue a buffer to the userspace
> --
> 2.13.6