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

From: Thierry Escande
Date: Tue Oct 18 2016 - 12:09:10 EST

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)
+ if (V4L2_TYPE_IS_OUTPUT(q->type))
+ q->dma_dir = DMA_TO_DEVICE;
+ else
+ q->dma_dir = q->use_dma_bidirectional

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


#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.