Message-ID: <20241028151713.GI24052@pendragon.ideasonboard.com>
Date: Mon, 28 Oct 2024 17:17:13 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Hans Verkuil <hverkuil@...all.nl>
Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
	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 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.

> > 
> > I'll send a patch for that.

-- 
Regards,

Laurent Pinchart

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