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Message-ID: <20240822122033.GA11152@pendragon.ideasonboard.com>
Date: Thu, 22 Aug 2024 15:20:33 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Hans Verkuil <hverkuil-cisco@...all.nl>
Cc: Tomasz Figa <tfiga@...omium.org>,
Benjamin Gaignard <benjamin.gaignard@...labora.com>,
mchehab@...nel.org, m.szyprowski@...sung.com,
ezequiel@...guardiasur.com.ar, p.zabel@...gutronix.de,
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
Hello,
On Thu, Aug 22, 2024 at 08:11:17AM +0200, Hans Verkuil wrote:
> On 22/08/2024 03:29, Tomasz Figa wrote:
> > On Thu, Aug 22, 2024 at 8:28 AM Laurent Pinchart wrote:
> >> 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 ?
> >>
> >
> > Uhm, I didn't notice this when reading this series. (Actually if I
> > recall correctly, originally it didn't have this restriction.)
> >
> > Indeed, I don't see what's wrong with just having 1 buffer. If
> > performance is not a concern, it's okay to just serialize the
> > operation on 1 buffer.
> >
> > If you would be so nice to send it, I think a patch that changes
> > min_reqbufs_allocation to max(max(1, min_reqbufs_allocation),
> > min_queued_buffers)) would be good.
> >
> > That said, Benjamin, Hans, are we missing something by any chance? :)
>
> See my proposal below:
>
> >>> -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)
>
> This should be:
>
> if (!q->min_reqbufs_allocation)
>
> >>> + q->min_reqbufs_allocation = 2;
>
> and vsp1 should set q->min_reqbufs_allocation to 1.
That would work to fix the issue with vsp1, but I don't think it's
enough. See below.
> >>> +
> >>> + /*
> >>> + * 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;
>
> The reasoning behind all this is that VIDIOC_REBUFS is expected to allocate
> a sane (i.e. workable and efficient) number of buffers.
>
> So if the DMA engine requires at least X buffers queued, then the minimum
> reqbufs allocation is at least q->min_queued_buffers + 1, otherwise you
> would never be able to get a buffer back.
>
> That's also why q->min_reqbufs_allocation was set to a minimum of 2: one
> buffer in flight, one buffer processed by userspace. That made the queue_setup
> call simpler for quite a few drivers that manually set the buffer count to 2.
> For most drivers, allocating 2 buffers makes perfect sense.
For devices that can absolutely not work with less than two buffers,
setting the minimum to 2 is fine. That is however not the majority use
case, and that's why setting the default in the vb2 core, overriding all
the drivers that haven't been patched, doesn't sound like a good idea to
me.
Generally speaking, 2 is in many cases too low. In the common use case
of capture and display, you will need a minimum of 3 buffers. Use cases
are the responsibility of userspace, we shouldn't try to be too smart
here.
I actually want to lower the number of buffers and require camera
drivers to support operating with a single buffer as a general rule.
Drivers that hold on the last buffer until a new one is provided are
very painful to use, they're causing issues with libcamera.
> But if a driver sets q->min_reqbufs_allocation explicitly to 1, then that
> should be honored, and my proposed change above will do that.
>
> Laurent, if you agree with this, just post patches for this.
>
> >>> +
> >>> + 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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