Re: [PATCH v21 2/9] videobuf2: Add min_reqbufs_allocation field to vb2_queue structure

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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@collabora.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