Re: [PATCH v6 3/4] media: raspberrypi: Add support for RP1-CFE

Hans Verkuil
2024-10-28T10:13:16Z

On 28/10/2024 10:21, Tomi Valkeinen wrote:
> Hi,
>
> On 24/10/2024 11:20, Hans Verkuil wrote:
>> Hi Tomi,
>>
>> I know this driver is already merged, but while checking for drivers that use
>> q->max_num_buffers I stumbled on this cfe code:
>>
>> <snip>
>>
>>> +/*
>>> + * vb2 ops
>>> + */
>>> +
>>> +static int cfe_queue_setup(struct vb2_queue *vq, unsigned int *nbuffers,
>>> +               unsigned int *nplanes, unsigned int sizes[],
>>> +               struct device *alloc_devs[])
>>> +{
>>> +    struct cfe_node *node = vb2_get_drv_priv(vq);
>>> +    struct cfe_device *cfe = node->cfe;
>>> +    unsigned int size = is_image_node(node) ?
>>> +                    node->vid_fmt.fmt.pix.sizeimage :
>>> +                    node->meta_fmt.fmt.meta.buffersize;
>>> +
>>> +    cfe_dbg(cfe, "%s: [%s] type:%u\n", __func__, node_desc[node->id].name,
>>> +        node->buffer_queue.type);
>>> +
>>> +    if (vq->max_num_buffers + *nbuffers < 3)
>>> +        *nbuffers = 3 - vq->max_num_buffers;
>>
>> This makes no sense: max_num_buffers is 32, unless explicitly set when vb2_queue_init
>> is called. So 32 + *nbuffers is never < 3.
>>
>> If the idea is that at least 3 buffers should be allocated by REQBUFS, then set
>> q->min_reqbufs_allocation = 3; before calling vb2_queue_init and vb2 will handle this
>> for you.
>>
>> Drivers shouldn't modify *nbuffers, except in very rare circumstances, especially
>> since the code is almost always wrong.
>
> Looking at this, the original code in the old BSP tree was, which somehow, along the long way, got turned into the above:
>
> if (vq->num_buffers + *nbuffers < 3)
>         *nbuffers = 3 - vq->num_buffers;
>
> So... I think that is the same as "q->min_reqbufs_allocation = 3"?
>
> The distinction between min_queued_buffers and min_reqbufs_allocation, or rather the need for the latter, still escapes me. If the HW/SW requires N buffers to be queued, why would we require
> allocating more than N buffers?

min_queued_buffers is easiest to explain: that represents the requirements of the DMA
engine, i.e. how many buffers much be queued before the DMA engine can be started.
Typically it is 0, 1 or 2.

min_reqbufs_allocation is the minimum number of buffers that will be allocated when
calling VIDIOC_REQBUFS in order for userspace to be able to stream without blocking
or dropping frames.

Typically this is 3 for video capture: one buffer is being DMAed, another is queued up
and the third is being processed by userspace. But sometimes drivers have other
requirements.

The reason is that some applications will just call VIDIOC_REQBUFS with count=1 and
expect it to be rounded up to whatever makes sense. See the VIDIOC_REQBUFS doc in
https://hverkuil.home.xs4all.nl/spec/userspace-api/v4l/vidioc-reqbufs.html

"It can be smaller than the number requested, even zero, when the driver runs out of
free memory. A larger number is also possible when the driver requires more buffers
to function correctly."

How drivers implement this is a mess, and usually the code in the driver is wrong as
well. In particular they often did not take VIDIOC_CREATE_BUFS into account, i.e.
instead of 'if (vq->num_buffers + *nbuffers < 3)' they would do 'if (*nbuffers < 3)'.

When we worked on the support for more than 32 buffers we added min_reqbufs_allocation
to let the core take care of this. In addition, this only applies to VIDIOC_REQBUFS,
if you want full control over the number of allocated buffers, then use VIDIOC_CREATE_BUFS,
with this ioctl the number of buffers will never be more than requested, although it
may be less if you run out of memory.

I really should go through all existing drivers and fix them up if they try to
handle this in the queue_setup function, I suspect a lot of them are quite messy.

One thing that is missing in the V4L2 uAPI is a way to report the minimum number of
buffers that need to be allocated, i.e. min_queued_buffers + 1. Since if you want
to use CREATE_BUFS you need that information so you know that you have to create
at least that number of buffers. We have the V4L2_CID_MIN_BUFFERS_FOR_CAPTURE control,
but it is effectively codec specific. This probably should be clarified.

I wonder if it wouldn't be better to add a min_num_buffers field to
struct v4l2_create_buffers and set it to min_queued_buffers + 1.

Regards,

Hans

>
>  Tomi
>