Re: [PATCH 2/2] [media] vb2: Add support for use_dma_bidirectional queue flag

[Thierry Escande]

Hi Sakari,

On 17/10/2016 12:06, Sakari Ailus wrote:
> Hi Thierry,
>
> Thanks for the set. A few comments below.
>
> On Fri, Oct 14, 2016 at 02:08:14PM +0200, Thierry Escande wrote:
> > From: Pawel Osciak <posciak@xxxxxxxxxxxx>
> >
> > When this flag is set for CAPTURE queues by the driver on calling
> > vb2_queue_init(), it forces the buffers on the queue to be
> > allocated/mapped with DMA_BIDIRECTIONAL direction flag, instead of
> > DMA_FROM_DEVICE. This allows the device not only to write to the
> > buffers, but also read out from them. This may be useful e.g. for codec
> > hardware, which may be using CAPTURE buffers as reference to decode
> > other buffers.
>
> Just out of curiosity --- when do you return these buffers back to the user?
> Once they're no longer needed as reference frames?
Tbh, I don't now. This is used by a rockchip vpu driver not yet 
upstreamed in the chromeos v4.4 kernel tree. Pawel might answer this 
question I guess.

> > This flag is ignored for OUTPUT queues, as we don't want to allow HW to
> > be able to write to OUTPUT buffers.
> >
> > Signed-off-by: Pawel Osciak <posciak@xxxxxxxxxxxx>
> > Tested-by: Pawel Osciak <posciak@xxxxxxxxxxxx>
> > Reviewed-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > Signed-off-by: Thierry Escande <thierry.escande@xxxxxxxxxxxxx>
> > ---
> >  drivers/media/v4l2-core/videobuf2-v4l2.c | 8 ++++++--
> >  include/media/videobuf2-core.h           | 4 ++++
> >  2 files changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/media/v4l2-core/videobuf2-v4l2.c b/drivers/media/v4l2-core/videobuf2-v4l2.c
> > index fde1e2d..9255291 100644
> > --- a/drivers/media/v4l2-core/videobuf2-v4l2.c
> > +++ b/drivers/media/v4l2-core/videobuf2-v4l2.c
> > @@ -659,8 +659,12 @@ int vb2_queue_init(struct vb2_queue *q)
> >  	 * queues will always initialize waiting_for_buffers to false.
> >  	 */
> >  	q->quirk_poll_must_check_waiting_for_buffers = true;
> > -	q->dma_dir = V4L2_TYPE_IS_OUTPUT(q->type)
> > -		   ? DMA_TO_DEVICE : DMA_FROM_DEVICE;
> > +
> > +	if (V4L2_TYPE_IS_OUTPUT(q->type))
> > +		q->dma_dir = DMA_TO_DEVICE;
> > +	else
> > +		q->dma_dir = q->use_dma_bidirectional
> > +			   ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;
> >
> >  	return vb2_core_queue_init(q);
> >  }
> > diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> > index 38410dd..e613c74 100644
> > --- a/include/media/videobuf2-core.h
> > +++ b/include/media/videobuf2-core.h
> > @@ -433,6 +433,9 @@ struct vb2_buf_ops {
> >   * @quirk_poll_must_check_waiting_for_buffers: Return POLLERR at poll when QBUF
> >   *              has not been called. This is a vb1 idiom that has been adopted
> >   *              also by vb2.
> > + * @use_dma_bidirectional:	use DMA_BIDIRECTIONAL for CAPTURE buffers; this
> > + *				allows HW to read from the CAPTURE buffers in
> > + *				addition to writing; ignored for OUTPUT queues
> >   * @lock:	pointer to a mutex that protects the vb2_queue struct. The
> >   *		driver can set this to a mutex to let the v4l2 core serialize
> >   *		the queuing ioctls. If the driver wants to handle locking
> > @@ -500,6 +503,7 @@ struct vb2_queue {
> >  	unsigned			fileio_write_immediately:1;
> >  	unsigned			allow_zero_bytesused:1;
> >  	unsigned		   quirk_poll_must_check_waiting_for_buffers:1;
> > +	unsigned			use_dma_bidirectional:1;
>
> This field is in the same struct as dma_dir which it directly affects.
>
> How about adding a macro instead to give you the queue DMA direction
> instead?
>
> E.g.
>
> #define vb2_dma_dir(q) \
> 	(V4L2_TYPE_IS_OUTPUT((q)->type) ? DMA_TO_DEVICE : \
> 	 (q)->use_dma_bidirectional ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE)
>
> I would call this capture_dma_bidirectional as it only affects capture. Or
> simply choose DMA_BIDIRECTIONAL whenever the flag is set.
Sure. Will do.

Regards,
 Thierry