Re: [PATCH] [media] vb2: move dma-buf unmap from __vb2_dqbuf() to vb2_buffer_done()

[Hans Verkuil]

On 07/20/2016 08:22 PM, Javier Martinez Canillas wrote:
> Currently the dma-buf is unmapped when the buffer is dequeued by userspace
> but it's not used anymore after the driver finished processing the buffer.
> 
> So instead of doing the dma-buf unmapping in __vb2_dqbuf(), it can be made
> in vb2_buffer_done() after the driver notified that buf processing is done.
> 
> Decoupling the buffer dequeue from the dma-buf unmapping has also the side
> effect of making possible to add dma-buf fence support in the future since
> the buffer could be dequeued even before the driver has finished using it.
> 
> Signed-off-by: Javier Martinez Canillas <javier@xxxxxxxxxxxxxxx>
> 
> ---
> Hello,
> 
> I've tested this patch doing DMA buffer sharing between a
> vivid input and output device with both v4l2-ctl and gst:
> 
> $ v4l2-ctl -d0 -e1 --stream-dmabuf --stream-out-mmap
> $ v4l2-ctl -d0 -e1 --stream-mmap --stream-out-dmabuf
> $ gst-launch-1.0 v4l2src device=/dev/video0 io-mode=dmabuf ! v4l2sink device=/dev/video1 io-mode=dmabuf-import
> 
> And I didn't find any issues but more testing will be appreciated.
> 
> Best regards,
> Javier
> 
>  drivers/media/v4l2-core/videobuf2-core.c | 34 +++++++++++++++++++++-----------
>  1 file changed, 22 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
> index 7128b09810be..973331efaf79 100644
> --- a/drivers/media/v4l2-core/videobuf2-core.c
> +++ b/drivers/media/v4l2-core/videobuf2-core.c
> @@ -958,6 +958,22 @@ void *vb2_plane_cookie(struct vb2_buffer *vb, unsigned int plane_no)
>  EXPORT_SYMBOL_GPL(vb2_plane_cookie);
>  
>  /**
> + * __vb2_unmap_dmabuf() - unmap dma-buf attached to buffer planes
> + */
> +static void __vb2_unmap_dmabuf(struct vb2_buffer *vb)
> +{
> +	int i;
> +
> +	for (i = 0; i < vb->num_planes; ++i) {
> +		if (!vb->planes[i].dbuf_mapped)
> +			continue;
> +		call_void_memop(vb, unmap_dmabuf,
> +				vb->planes[i].mem_priv);

Does unmap_dmabuf work in interrupt context? Since vb2_buffer_done can be called from
an irq handler this is a concern.

That said, vb2_buffer_done already calls call_void_memop(vb, finish, vb->planes[plane].mem_priv);
to sync buffers, and that can take a long time as well. So it is not a good idea to
have this in vb2_buffer_done.

What I would like to see is to have vb2 handle this finish() call and the vb2_unmap_dmabuf
in some workthread or equivalent.

It would complicate matters somewhat in vb2, but it would simplify drivers since these
actions would not longer take place in interrupt context.

I think this patch makes sense, but I would prefer that this is moved out of the interrupt
context.

Regards,

	Hans

> +		vb->planes[i].dbuf_mapped = 0;
> +	}
> +}
> +
> +/**
>   * vb2_buffer_done() - inform videobuf that an operation on a buffer is finished
>   * @vb:		vb2_buffer returned from the driver
>   * @state:	either VB2_BUF_STATE_DONE if the operation finished successfully,
> @@ -1028,6 +1044,9 @@ void vb2_buffer_done(struct vb2_buffer *vb, enum vb2_buffer_state state)
>  			__enqueue_in_driver(vb);
>  		return;
>  	default:
> +		if (q->memory == VB2_MEMORY_DMABUF)
> +			__vb2_unmap_dmabuf(vb);
> +
>  		/* Inform any processes that may be waiting for buffers */
>  		wake_up(&q->done_wq);
>  		break;
> @@ -1708,23 +1727,11 @@ EXPORT_SYMBOL_GPL(vb2_wait_for_all_buffers);
>   */
>  static void __vb2_dqbuf(struct vb2_buffer *vb)
>  {
> -	struct vb2_queue *q = vb->vb2_queue;
> -	unsigned int i;
> -
>  	/* nothing to do if the buffer is already dequeued */
>  	if (vb->state == VB2_BUF_STATE_DEQUEUED)
>  		return;
>  
>  	vb->state = VB2_BUF_STATE_DEQUEUED;
> -
> -	/* unmap DMABUF buffer */
> -	if (q->memory == VB2_MEMORY_DMABUF)
> -		for (i = 0; i < vb->num_planes; ++i) {
> -			if (!vb->planes[i].dbuf_mapped)
> -				continue;
> -			call_void_memop(vb, unmap_dmabuf, vb->planes[i].mem_priv);
> -			vb->planes[i].dbuf_mapped = 0;
> -		}
>  }
>  
>  /**
> @@ -1861,6 +1868,9 @@ static void __vb2_queue_cancel(struct vb2_queue *q)
>  			call_void_vb_qop(vb, buf_finish, vb);
>  		}
>  		__vb2_dqbuf(vb);
> +
> +		if (q->memory == VB2_MEMORY_DMABUF)
> +			__vb2_unmap_dmabuf(vb);
>  	}
>  }
>  
>