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Date: Mon, 15 Jan 2024 11:27:05 +0100
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Subject: Re: [PATCH v16 1/8] videobuf2: Add min_reqbufs_allocation field to
 vb2_queue structure
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To: Benjamin Gaignard <benjamin.gaignard@collabora.com>, mchehab@kernel.org
Cc: linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
 kernel@collabora.com
References: <20231215090813.15610-1-benjamin.gaignard@collabora.com>
 <20231215090813.15610-2-benjamin.gaignard@collabora.com>
From: Hans Verkuil <hverkuil@xs4all.nl>
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In-Reply-To: <20231215090813.15610-2-benjamin.gaignard@collabora.com>
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On 15/12/2023 10:08, Benjamin Gaignard wrote:
> Add 'min_reqbufs_allocation' field in vb2_queue structure so drivers
> can specificy the minimum number of buffers to allocate when calling
> VIDIOC_REQBUFS.
> If used this minimum should be higher than the minimum number of
> queued buffers needed to start streaming.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
> ---
>  drivers/media/common/videobuf2/videobuf2-core.c | 10 ++++++++--
>  include/media/videobuf2-core.h                  | 13 +++++++++++++
>  2 files changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 41a832dd1426..a183edf11586 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -865,7 +865,7 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
>  	/*
>  	 * Make sure the requested values and current defaults are sane.
>  	 */
> -	num_buffers = max_t(unsigned int, *count, q->min_queued_buffers);
> +	num_buffers = max_t(unsigned int, *count, q->min_reqbufs_allocation);
>  	num_buffers = min_t(unsigned int, num_buffers, q->max_num_buffers);
>  	memset(q->alloc_devs, 0, sizeof(q->alloc_devs));
>  	/*
> @@ -917,7 +917,7 @@ int vb2_core_reqbufs(struct vb2_queue *q, enum vb2_memory memory,
>  	 * There is no point in continuing if we can't allocate the minimum
>  	 * number of buffers needed by this vb2_queue.
>  	 */
> -	if (allocated_buffers < q->min_queued_buffers)
> +	if (allocated_buffers < q->min_reqbufs_allocation)
>  		ret = -ENOMEM;
>  
>  	/*
> @@ -2521,6 +2521,12 @@ int vb2_core_queue_init(struct vb2_queue *q)
>  	if (WARN_ON(q->supports_requests && q->min_queued_buffers))
>  		return -EINVAL;
>  
> +	q->min_reqbufs_allocation = max_t(unsigned int,
> +					  q->min_reqbufs_allocation,
> +					  q->min_queued_buffers + 1);

I have been thinking about this a bit more. The problem is that if
min_queued_buffers == 0, then min_reqbufs_allocation becomes 1.

But the minimum requirement should really be 2: one buffer is used to
capture a new frame, the other is processed by userspace.

So this should be modified as well. I would write this as 'if' statements
as well:

	if (q->min_reqbufs_allocation < q->min_queued_buffers + 1)
		q->min_reqbufs_allocation = q->min_queued_buffers + 1;
	if (q->min_reqbufs_allocation < 2)
		q->min_reqbufs_allocation = 2;

You can add comments before each 'if' to clarify why.

I'm also wondering what this change to calculating q->min_reqbufs_allocation
when the driver leaves it to 0 shouldn't be done in a separate patch, either
before or after this one. We're changing the default behavior of REQBUFS
slightly, and mixing that in with this patch feels wrong.

Note that __vb2_init_fileio() also needs to be updated: it currently uses
the same calculation of the initial 'count' value for reqbufs, but that
can now just use q->min_reqbufs_allocation directly.

Regards,

	Hans

> +	if (WARN_ON(q->min_reqbufs_allocation > q->max_num_buffers))
> +		return -EINVAL;
> +
>  	INIT_LIST_HEAD(&q->queued_list);
>  	INIT_LIST_HEAD(&q->done_list);
>  	spin_lock_init(&q->done_lock);
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index 56719a26a46c..7b84b4e2e273 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -553,6 +553,18 @@ struct vb2_buf_ops {
>   *		VIDIOC_REQBUFS will ensure at least @min_queued_buffers
>   *		buffers will be allocated. Note that VIDIOC_CREATE_BUFS will not
>   *		modify the requested buffer count.
> + * @min_reqbufs_allocation: the minimum number of buffers to be allocated when
> + *		calling VIDIOC_REQBUFS. Drivers can set this if there has to
> + *		be a certain number of buffers available for the hardware to
> + *		work effectively. If set, then @min_reqbufs_allocation must be
> + *		larger than @min_queued_buffers + 1.
> + *		This field is only used by VIDIOC_REQBUFS. This allows calling
> + *		that ioctl with a buffer count of 1 and it will be automatically
> + *		adjusted to a workable buffer count. VIDIOC_CREATE_BUFS will not
> + *		modify the requested buffer count.
> + *		If this field is > 3, then it is highly recommended that the
> + *		driver implements the V4L2_CID_MIN_BUFFERS_FOR_CAPTURE/OUTPUT
> + *		control.
>   */
>  /*
>   * Private elements (won't appear at the uAPI book):
> @@ -618,6 +630,7 @@ struct vb2_queue {
>  	u32				timestamp_flags;
>  	gfp_t				gfp_flags;
>  	u32				min_queued_buffers;
> +	u32				min_reqbufs_allocation;
>  
>  	struct device			*alloc_devs[VB2_MAX_PLANES];
>