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.
>
>> 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.
Regards,
Hans
>
> Tomi
>
