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charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <563347aa-4155-47e1-b71a-0107aed83eb6@xs4all.nl> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20241028_081731_186866_B5B99E69 X-CRM114-Status: GOOD ( 59.43 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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: > >>>>>> > >>>>>> > >>>>>> > >>>>>>> +/* > >>>>>>> + * 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