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Message-ID: <20240821232819.GA18600@pendragon.ideasonboard.com>
Date: Thu, 22 Aug 2024 02:28:19 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Benjamin Gaignard <benjamin.gaignard@...labora.com>
Cc: mchehab@...nel.org, tfiga@...omium.org, m.szyprowski@...sung.com,
ezequiel@...guardiasur.com.ar, p.zabel@...gutronix.de,
hverkuil-cisco@...all.nl, nicolas@...fresne.ca,
linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-rockchip@...ts.infradead.org, kernel@...labora.com
Subject: Re: [PATCH v21 2/9] videobuf2: Add min_reqbufs_allocation field to
vb2_queue structure
Hi Benjamin,
On Thu, Mar 14, 2024 at 04:32:19PM +0100, Benjamin Gaignard wrote:
> Add 'min_reqbufs_allocation' field in the vb2_queue structure so drivers
> can specify the minimum number of buffers to allocate when calling
> VIDIOC_REQBUFS.
> When initializing the queue, v4l2 core makes sure that the following
> constraints are respected:
> - the minimum number of buffers to allocate must be at least 2 because
> one buffer is used by the hardware while the other is being processed
> by userspace.
This breaks userspace for the Renesas vsp1 driver :-( Unit tests fail,
as some of them rely on operation with a single buffer.
The vsp1 is a memory-to-memory processing engine, so operating with a
single buffer is fine in some use cases. I would argue that for live
capture devices there are valid use cases to operate with a single
buffer too. Changing this by default will break use cases.
How can we relax this check ? Should I simply submit a patch that lowers
the minimum to one buffer ?
> -if the driver needs 'min_queued_buffers' in the queue before calling
> start_streaming(), then the minimum requirement is 'min_queued_buffers + 1'
> to keep at least one buffer available for userspace.
>
> Simplify __vb2_init_fileio() by using 'min_reqbufs_allocation' directly
> to avoid duplicating the minimum number of buffers to allocate computation.
>
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
> ---
> .../media/common/videobuf2/videobuf2-core.c | 38 +++++++++++--------
> include/media/videobuf2-core.h | 15 +++++++-
> 2 files changed, 37 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index d8b3c04cb3b5..58c495b253ce 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -866,7 +866,7 @@ 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));
> /*
> @@ -918,7 +918,7 @@ 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;
>
> /*
> @@ -2524,6 +2524,25 @@ int vb2_core_queue_init(struct vb2_queue *q)
> if (WARN_ON(q->supports_requests && q->min_queued_buffers))
> return -EINVAL;
>
> + /*
> + * The minimum requirement is 2: one buffer is used
> + * by the hardware while the other is being processed by userspace.
> + */
> + if (q->min_reqbufs_allocation < 2)
> + q->min_reqbufs_allocation = 2;
> +
> + /*
> + * If the driver needs 'min_queued_buffers' in the queue before
> + * calling start_streaming() then the minimum requirement is
> + * 'min_queued_buffers + 1' to keep at least one buffer available
> + * for userspace.
> + */
> + if (q->min_reqbufs_allocation < q->min_queued_buffers + 1)
> + 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);
> @@ -2717,7 +2736,6 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
> struct vb2_fileio_data *fileio;
> struct vb2_buffer *vb;
> int i, ret;
> - unsigned int count = 0;
>
> /*
> * Sanity check
> @@ -2738,18 +2756,8 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
> if (q->streaming || vb2_get_num_buffers(q) > 0)
> return -EBUSY;
>
> - /*
> - * Start with q->min_queued_buffers + 1, driver can increase it in
> - * queue_setup()
> - *
> - * 'min_queued_buffers' buffers need to be queued up before you
> - * can start streaming, plus 1 for userspace (or in this case,
> - * kernelspace) processing.
> - */
> - count = max(2, q->min_queued_buffers + 1);
> -
> dprintk(q, 3, "setting up file io: mode %s, count %d, read_once %d, write_immediately %d\n",
> - (read) ? "read" : "write", count, q->fileio_read_once,
> + (read) ? "read" : "write", q->min_reqbufs_allocation, q->fileio_read_once,
> q->fileio_write_immediately);
>
> fileio = kzalloc(sizeof(*fileio), GFP_KERNEL);
> @@ -2763,7 +2771,7 @@ static int __vb2_init_fileio(struct vb2_queue *q, int read)
> * Request buffers and use MMAP type to force driver
> * to allocate buffers by itself.
> */
> - fileio->count = count;
> + fileio->count = q->min_reqbufs_allocation;
> fileio->memory = VB2_MEMORY_MMAP;
> fileio->type = q->type;
> q->fileio = fileio;
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index 667bf9ee1101..4a8b9135cec8 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -549,9 +549,21 @@ struct vb2_buf_ops {
> * @start_streaming can be called. Used when a DMA engine
> * cannot be started unless at least this number of buffers
> * have been queued into the driver.
> - * VIDIOC_REQBUFS will ensure at least @min_queued_buffers
> + * VIDIOC_REQBUFS will ensure at least @min_queued_buffers + 1
> * 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. 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.
> * @alloc_devs: &struct device memory type/allocator-specific per-plane device
> */
> /*
> @@ -622,6 +634,7 @@ 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,
Laurent Pinchart
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