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charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <62073d7a-0a4b-4440-90e5-dcce0dec72d7@ideasonboard.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20241028_093251_069990_46E70397 X-CRM114-Status: GOOD ( 66.27 ) 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 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: > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>>> +/* > >>>>>>>>> + * 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