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

[Hans Verkuil <hverkuil-cisco@xs4all.nl>]

On 22/08/2024 03:29, Tomasz Figa wrote:
> On Thu, Aug 22, 2024 at 8:28 AM Laurent Pinchart
> <laurent.pinchart@ideasonboard.com> wrote:
>>
>> 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 ?
>>
> 
> 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:

> 
> Best,
> Tomasz
> 
>>> -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.

>>> +
>>> +     /*
>>> +      * 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.

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.

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);
>>> @@ -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
>