Message-ID: <20241028163243.GB26852@pendragon.ideasonboard.com>
Date: Mon, 28 Oct 2024 18:32:43 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Hans Verkuil <hverkuil@...all.nl>, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org,
	Naushir Patuck <naush@...pberrypi.com>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Jacopo Mondi <jacopo.mondi@...asonboard.com>,
	Kieran Bingham <kieran.bingham@...asonboard.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Raspberry Pi Kernel Maintenance <kernel-list@...pberrypi.com>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>
Subject: Re: [PATCH v6 3/4] media: raspberrypi: Add support for RP1-CFE

On Mon, Oct 28, 2024 at 05:32:27PM +0200, Tomi Valkeinen wrote:
> 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?

"Smooth streaming" is use case-dependent, you will need different number
of buffers for different use cases. That's why I don't like hardcoding
this in a video capture driver. I'd rather expose information about the
driver behaviour (in particular, how many buffers it will hold on
without returning anything to userspace until a new buffer gets queued)
and let applications make a decision. I don't expect applications
relying on VIDIOC_REQBUFS(1) to work out-of-the-box on Pi 5 anyway, as
the media graph needs to be configured.

> 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?

I expect a device-specific userspace not to require drivers to make
policy decisions on its behalf.

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

-- 
Regards,

Laurent Pinchart

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