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

Tomi Valkeinen
2024-10-28T15:59:21Z

Hi Laurent,

On 28/10/2024 17:17, Laurent Pinchart wrote:
> On Mon, Oct 28, 2024 at 12:30:45PM +0100, Hans Verkuil wrote:
>> On 28/10/2024 12:25, Tomi Valkeinen wrote:
>>> On 28/10/2024 13:13, Hans Verkuil wrote:
>>>> On 28/10/2024 12:05, Tomi Valkeinen wrote:
>>>>> On 28/10/2024 12:11, Hans Verkuil wrote:
>>>>>> On 28/10/2024 10:21, Tomi Valkeinen wrote:
>>>>>>> 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.
>
> That's partly true only. Even if the hardware requires 2 buffers, a
> driver can allocate scratch buffers to lower the requirement for
> userspace. Setting min_queued_buffers to 1 is usually fine, as there are
> few use cases for userspace to start the hardware before a buffer is
> available to capture a frame to. A value of 2 is much more problematic,
> as it prevents operating with a single buffer. I know using a single
> buffer results in frame drops, but there are resource-constrained
> systems where application don't always need all the frames (such as the
> Raspberry Pi Zero for instance). I very strongly encourage drivers to
> never set a min_queued_buffers value higher than 1.
>
>>>>>>
>>>>>> 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.
>
> This is exactly why I dislike min_reqbufs_allocation when set based on
> this logic, it encodes assumption on userspace use cases that a capture
> driver really shouldn't make.
>
>>>>>>
>>>>>> 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,
>
> I agree it's better to handle it in the core than in drivers, even if I
> dislike the feature in the first place.
>
>>>>>> 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.
>
> On a side note, we should transition libcamera to use VIDIOC_CREATE_BUFS
> unconditionally.
>
>>>>>>
>>>>>> 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.
>
> It's not that uncommon, see above.
>
>>>
>>> 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.
>>
>> Exactly. If you really want that, then the driver has to be adapted in the way that Laurent
>> suggested, i.e. with one or more scratch buffers. But that is not always possible, esp. with
>> older hardware without an IOMMU.
>
> Drivers can always allocate a full-frame scratch buffer in the worst
> case. That can waste memory though, which is less than ideal.
>
>>> 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.
>
> Don't add the +1. We should give userspace the information it needs to
> make informed decisions, not make decisions on its behalf.
>
>>>>>
>>>>> 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".
>
> Or just don't set min_reqbufs_allocation ? This is a new driver, and it
> requires a device-specific userspace to operate the ISP. I don't think
> we need to care about applications blindly calling VIDIOC_REQBUFS(1) and
> expecting to get more buffers.

It doesn't require a device-specific userspace for plain CSI-2 capture.

If I understood right, the expected behavior for VIDIOC_REQBUFS is to
return enough buffers for "smooth streaming". So even if device-specific
userspace would be required, doesn't it still make sense to have
min_reqbufs_allocation = 3?

Or is your point that even a device-specific userspace, which knows
exactly what it's doing, would use VIDIOC_REQBUFS, instead of
VIDIOC_CREATE_BUFS?

Also, if I don't set min_reqbufs_allocation, VIDIOC_REQBUFS(1) would
still allocate two buffers, not one.

Tomi