Date:   Fri, 17 Nov 2017 15:49:23 +0900
From:   Alexandre Courbot <acourbot@...omium.org>
To:     Gustavo Padovan <gustavo@...ovan.org>
Cc:     <linux-media@...r.kernel.org>, Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        Pawel Osciak <pawel@...iak.com>,
        Sakari Ailus <sakari.ailus@....fi>,
        Brian Starkey <brian.starkey@....com>,
        Thierry Escande <thierry.escande@...labora.com>,
        <linux-kernel@...r.kernel.org>,
        Gustavo Padovan <gustavo.padovan@...labora.com>
Subject: Re: [RFC v5 07/11] [media] vb2: add in-fence support to QBUF

Hi Gustavo,

I am coming a bit late in this series' review, so apologies if some of my
comments have already have been discussed in an earlier revision.

On Thursday, November 16, 2017 2:10:53 AM JST, Gustavo Padovan wrote:
> From: Gustavo Padovan <gustavo.padovan@...labora.com>
>
> Receive in-fence from userspace and add support for waiting on them
> before queueing the buffer to the driver. Buffers can't be queued to the
> driver before its fences signal. And a buffer can't be queue to the driver
> out of the order they were queued from userspace. That means that even if
> it fence signal it must wait all other buffers, ahead of it in the queue,
> to signal first.
>
> To make that possible 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.
>
> v6:
> 	- With fences always keep the order userspace queues the buffers.
> 	- Protect in_fence manipulation with a lock (Brian Starkey)
> 	- check if fences have the same context before adding a fence array
> 	- Fix last_fence ref unbalance in __set_in_fence() (Brian Starkey)
> 	- Clean up fence if __set_in_fence() fails (Brian Starkey)
> 	- treat -EINVAL from dma_fence_add_callback() (Brian Starkey)
>
> v5:	- use fence_array to keep buffers ordered in vb2 core when
> 	needed (Brian Starkey)
> 	- keep backward compat on the reserved2 field (Brian Starkey)
> 	- protect fence callback removal with lock (Brian Starkey)
>
> 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@...labora.com>
> ---
>  drivers/media/v4l2-core/Kconfig          |   1 +
>  drivers/media/v4l2-core/videobuf2-core.c | 202 
> ++++++++++++++++++++++++++++---
>  drivers/media/v4l2-core/videobuf2-v4l2.c |  29 ++++-
>  include/media/videobuf2-core.h           |  17 ++-
>  4 files changed, 231 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..26de4c80717d 100644
> --- a/drivers/media/v4l2-core/videobuf2-core.c
> +++ b/drivers/media/v4l2-core/videobuf2-core.c
> @@ -23,6 +23,7 @@
>  #include <linux/sched.h>
>  #include <linux/freezer.h>
>  #include <linux/kthread.h>
> +#include <linux/dma-fence-array.h>
>  
>  #include <media/videobuf2-core.h>
>  #include <media/v4l2-mc.h>
> @@ -346,6 +347,7 @@ 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 +1224,9 @@ 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 +1278,23 @@ 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;
> +	unsigned long flags;
> +
> +	/* count num of buffers ready in front of the queued_list */
> +	list_for_each_entry(vb, &q->queued_list, queued_entry) {
> +		spin_lock_irqsave(&vb->fence_cb_lock, flags);
> +		if (!vb->in_fence || dma_fence_is_signaled(vb->in_fence))
> +			ready_count++;
> +		spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> +	}
> +
> +	return ready_count;
> +}
> +
>  int vb2_core_prepare_buf(struct vb2_queue *q, unsigned int index, void *pb)
>  {
>  	struct vb2_buffer *vb;
> @@ -1361,9 +1383,87 @@ 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;
> +	}
> +
> +	/*
> +	 * We always guarantee the ordering of buffers queued from
> +	 * userspace to be the same it is queued to the driver. For that
> +	 * 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 (fence && q->last_fence) {
> +		struct dma_fence **fences;
> +		struct dma_fence_array *arr;
> +
> +		if (fence->context == q->last_fence->context) {
> +			if (fence->seqno - q->last_fence->seqno <= INT_MAX) {
> +				dma_fence_put(q->last_fence);
> +				q->last_fence = dma_fence_get(fence);
> +			} else {
> +				dma_fence_put(fence);
> +				fence = dma_fence_get(q->last_fence);
> +			}
> +			return fence;
> +		}
> +
> +		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);
> +		if (!arr) {
> +			kfree(fences);
> +			return ERR_PTR(-ENOMEM);
> +		}
> +
> +		fence = &arr->base;
> +
> +		q->last_fence = dma_fence_get(fence);
> +	} else if (!fence && q->last_fence) {
> +		fence = dma_fence_get(q->last_fence);
> +	}
> +
> +	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;
> +	}

Is this block necessary? IIUC the callback will never be set on buffers 
without
an input fence, so (!vb->in_fence) should never be satisfied.

> +
> +	dma_fence_put(vb->in_fence);
> +	vb->in_fence = NULL;
> +
> +	if (q->start_streaming_called)
> +		__enqueue_in_driver(vb);
> +	spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> +}
> +
> +int vb2_core_qbuf(struct vb2_queue *q, unsigned int index, void *pb,
> +		  struct dma_fence *fence)
>  {
>  	struct vb2_buffer *vb;
> +	unsigned long flags;
>  	int ret;
>  
>  	vb = q->bufs[index];
> @@ -1372,16 +1472,18 @@ 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 +1500,83 @@ 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)) {
> +		dma_fence_put(fence);
> +		ret = PTR_ERR(vb->in_fence);
> +		goto err;
> +	}
> +	fence = NULL;
> +
> +	/*
> +	 * 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 (vb->in_fence && !q->start_streaming_called &&
> +	    __get_num_ready_buffers(q) == q->min_buffers_needed - 1)
> +		dma_fence_wait(vb->in_fence, true);

Mmm, that's a tough call. Userspace may unexpectingly block due to this 
(which
fences are supposed to prevent), not sure how much of a problem this may 
be.

> +
>  	/*
>  	 * 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.
> +	 */
> +	spin_lock_irqsave(&vb->fence_cb_lock, flags);
> +	if (vb->in_fence) {
> +		ret = dma_fence_add_callback(vb->in_fence, &vb->fence_cb,
> +					     vb2_qbuf_fence_cb);
> +		if (ret == -EINVAL) {
> +			spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> +			goto err;
> +		} else if (!ret) {
> +			goto fill;
> +		}
> +
> +		dma_fence_put(vb->in_fence);
> +		vb->in_fence = NULL;

I suppose the last two lines are supposed to be called when
dma_fence_add_callback() returns -ENOENT, because the fence has already 
been
signaled? In that case I think the code would be more readable if you made 
it
more explicit, something like:

// fence already signaled?
if (ret == -ENOENT) {
	dma_fence_put(vb->in_fence);
	vb->in_fence = NULL;
} else if (ret != 0) {
	spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
	goto err;
}

and that way you can get rid of the short-ranged fill: label.

> +	}
> +
> +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);

Since that code was previously called when vb2_start_streaming() wasn't, we
had a guarantee that the buffer was only queued once. Now I wonder if we 
could
not run into a situation where __enqueue_in_driver is called twice for the 
last
queued buffer, once by vb2_start_streaming() and once here, now that 
vb2_start_streaming() has set q->start_streaming_called to 1?

> +	spin_unlock_irqrestore(&vb->fence_cb_lock, flags);
> +
>  	/* 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 +1787,8 @@ 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 +1816,21 @@ 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 +1892,7 @@ 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 +2412,7 @@ 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 +2591,7 @@ 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 +2694,7 @@ 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) &&

Why do we need to consider both values invalid? Can 0 ever be a valid fence 
fd?

For completeness you may also want to check the case where (b->fence_fd == 
-1 &&
(b->flags & V4L2_BUF_FLAG_IN_FENCE)) and return an error ahead of time, but 
I
suppose the call to sync_file_get_fence() later on will catch this as well.

> +	    !(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);
> +		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 38b9c8dd42c6..5f48c7be7770 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
> @@ -504,6 +514,7 @@ struct vb2_buf_ops {
>   * @last_buffer_dequeued: used in poll() and DQBUF to 
> immediately return if the
>   *		last decoded buffer was already dequeued. Set for capture queues
>   *		when a buffer with the V4L2_BUF_FLAG_LAST is dequeued.
> + * @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
>   */
> @@ -558,6 +569,8 @@ struct vb2_queue {
>  	unsigned int			copy_timestamp:1;
>  	unsigned int			last_buffer_dequeued:1;
>  
> +	struct dma_fence		*last_fence;
> +
>  	struct vb2_fileio_data		*fileio;
>  	struct vb2_threadio_data	*threadio;
>  
> @@ -733,6 +746,7 @@ 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.
> @@ -747,7 +761,8 @@ 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

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