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

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

Hi Hans,

On 28/10/2024 12:11, Hans Verkuil wrote:
> 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)'.

Thanks, this was educational!

So. If I have a driver that has min_queued_buffers = 1, I can use 
VIDIOC_CREATE_BUFS to allocate a single buffer, and then capture just 
one buffer, right? Whereas VIDIOC_REQBUFS would give me (probably) three 
(or two, if the driver does not set min_reqbufs_allocation). Three 
buffers makes sense for full streaming, of course.

> 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

Hmm, so what I wrote above is not correct? One needs min_queued_buffers 
+ 1? Why is that?

> 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.

I think this makes sense (although I still don't get the +1).

However, based on the experiences from adding the streams features to 
various ioctls, let's be very careful =). The new 'min_num_buffers' can 
be filled with garbage by the userspace. If we define the 
'min_num_buffers' field to be always filled by the kernel, and any value 
provided from the userspace to be ignored, I think it should work.

  Tomi