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Message-ID: <f8e34951-5152-4158-8e68-c55aa156999a@xs4all.nl>
Date: Mon, 15 Jan 2024 10:16:43 +0100
From: Hans Verkuil <hverkuil@...all.nl>
To: Benjamin Gaignard <benjamin.gaignard@...labora.com>, mchehab@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
kernel@...labora.com
Subject: Re: [PATCH v16 1/8] videobuf2: Add min_reqbufs_allocation field to
vb2_queue structure
Hi Benjamin,
Some comments below:
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.
You should also mention that it defaults to min_queued_buffers + 1
instead of min_queued_buffers since otherwise you would not have a
buffer available for userspace (or something along those lines).
This is a change that needs to be documented in the commit message.
>
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.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);
> + 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
In this comment it still mentions @min_queued_buffers instead of
@min_queued_buffers + 1.
> * 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.
There is not actually any check for that. But I think such a check should
be added: it makes it much easier to do a 'git grep min_reqbufs_allocation'
and only get those drivers that really set it to a non-standard value.
> + * 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
"workable buffer count": nice phrase, I like it!
> + * 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.
Also a nice final sentence. This is also something that is a nice check to add
to v4l2-compliance! A patch would be welcome...
I think the sentences need to be reordered. How about this (alignment needs to
be fixed):
* @min_reqbufs_allocation: the minimum number of buffers to be allocated when
* calling VIDIOC_REQBUFS. Note that VIDIOC_CREATE_BUFS will *not*
* modify the requested buffer count and does not use this field.
* Drivers can set this if there has to
* be a certain number of buffers available for the hardware to
* work effectively. This allows calling VIDIOC_REQBUFS with a buffer
* count of 1 and it will be automatically adjusted to a workable
* buffer count. If set, then @min_reqbufs_allocation must be
* larger than @min_queued_buffers + 1.
* 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];
>
Regards,
Hans
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