linux-kernel - Re: [PATCH 2/2] [media] vb2: Add support for use_dma

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Message-ID: <664b9541-e7ab-f1b3-b12c-677db0e63fc0@collabora.com>
Date:   Tue, 18 Oct 2016 18:08:53 +0200
From:   Thierry Escande <thierry.escande@...labora.com>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>
Subject: Re: [PATCH 2/2] [media] vb2: Add support for use_dma_bidirectional
 queue flag

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@...omium.org>
>>
>> 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@...omium.org>
>> Tested-by: Pawel Osciak <posciak@...omium.org>
>> Reviewed-by: Tomasz Figa <tfiga@...omium.org>
>> Signed-off-by: Thierry Escande <thierry.escande@...labora.com>
>> ---
>>  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