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

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

Hi,

On 28/10/2024 13:13, Hans Verkuil wrote:
> On 28/10/2024 12:05, Tomi Valkeinen wrote:
>> 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?
> 
> The DMA engine always uses min_queued_buffers, so if there are only that many buffers,
> then it can never return a buffer to userspace! So you need one more. That's the absolute
> minimum. For smooth capture you need two more to allow time for userspace to process the
> buffer.

Hmm, ok, I see. Well, I guess my "I want to capture just a single frame" 
is not a very common case.

Can I queue one buffer, start streaming, stop streaming, and get the 
filled buffer? But then I guess I don't when the buffer has been filled, 
i.e. when to call stop streaming.

So, never mind, I don't actually have any use case for this, just wondering.

>>
>>> 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.
> 
> I've posted an RFC for this.

Thanks, I'll check it out.

For the original issue in this thread, I think the correct fix is to 
remove the lines from cfe_queue_setup(), and add 
"q->min_reqbufs_allocation = 3".

I'll send a patch for that.

  Tomi