On 28/10/2024 16:52, Laurent Pinchart wrote: > Hi Hans, > > On Mon, Oct 28, 2024 at 12:10:22PM +0100, Hans Verkuil wrote: >> Hi all, >> >> This mail thread uncovered some corner cases around how many buffers should be allocated >> if VIDIOC_REQBUFS with count = 1 is called: >> >> https://lore.kernel.org/linux-media/20241003-rp1-cfe-v6-0-d6762edd98a8@xxxxxxxxxxxxxxxx/T/#mc2210597d92b5a0f09fabdac2f7307128aaa9bd8 > > I'll repeat below some comments I've made in that thread, as they're > better discussed in the context of this RFC. > >> When it comes to the minimum number of buffers there are a number of limitations: >> >> 1) The DMA engine needs at least N buffers to be queued before it can start. Typically >> this is 0, 1 or 2, and a driver sets this via the vb2_queue min_queued_buffers field. >> So if min_queued_buffers = 1, then the DMA engine needs one buffer at all times to >> DMA to. Allocating just one buffer would mean the DMA engine can never return that >> buffer to userspace (it would just keep recycling the same buffer over and over), so >> the minimum must be min_queued_buffers + 1. > > I think you're mixing hardware and driver constraints here. Drivers can > use scratch buffers to relax the hardware requirements, and allow > userspace operation with less buffers than strictly required by the > hardware. > > The cost of allocating such scratch buffers vary depending on the > device. When an IOMMU is available, or when the device has a line stride > that can be set to 0 and supports race-free programming of the stride > and buffer addresses, the scratch buffer can be as small as a single > page or a single line. In other cases, a full-frame scratch buffer is > required, which is costly, and the decision on whether or not to > allocate such a scratch buffer should probably be taken with userspace > being involved. I honestly don't see why you would want to spend a lot of time on adding scratch buffer support just to save a bit of memory. Is the use-case of capturing just a single buffer so common? To me it seems that it only makes sense to spend effort on this if you only need to capture a single buffer and never need to stream more buffers. Can you describe the use-case of capturing just a single buffer? Is that just for testing libcamera? Or is it something that happens all the time during normal libcamera operation? Supporting scratch buffers is a lot of effort for something that is not needed for normal streaming. > > min_queued_buffers describes how the device operates from a userspace > point of view, so I don't think it should be considered or documented as > being a hardware requirement, but a driver requirement. It's a hardware and/or driver requirement. It is absolutely not a userspace requirement. Normal userspace applications that use VIDIOC_REQBUFS and just stream video will never notice this. > >> 2) Historically VIDIOC_REQBUFS is expected to increase the count value to a number that >> ensures the application can smoothly process the video stream. Typically this will >> be 3 or 4 (if min_queued_buffers == 2): min_queued_buffers are used by the DMA engine, >> one buffer is queued up in vb2, ready to be used by the DMA engine as soon as it >> returns a filled buffer to userspace, and one buffer is processed by userspace. >> >> This is to support applications that call VIDIOC_REQBUFS with count = 1 and leave it >> to the driver to increment it to a workable value. > > Do we know what those applications are ? I'm not disputing the fact that > this may need to be supported to avoid breaking old userspace, but I > also think this feature should be phased out for new drivers, especially > drivers that require a device-specific userspace and therefore won't > work out of the box with old applications. xawtv is one: it will call REQBUFS with count = 2 (so this would fail for any driver that sets min_queued_buffers to 2), and with count = 1 if it wants to capture just a single frame. 'git grep min_queued_buffers|grep -v videobuf|wc' gives me 83 places where it is set. Some of those are likely wrong (min_queued_buffers has been abused as a replacement for min_reqbufs_allocation), but still that's quite a lot. Mostly these are older drivers for hardware without an IOMMU and typically for SDTV capture. So memory is not a consideration for those drivers since a SDTV buffer is quite small. > >> 3) Stateful codecs in particular have additional requirements beyond the DMA engine >> limits due to the fact that they have to keep track of reference buffers and other >> codec limitations. As such more buffers are needed, and that number might also vary >> based on the specific codec used. The V4L2_CID_MIN_BUFFERS_FOR_CAPTURE/OUTPUT >> controls are used to report that. Support for this is required by the stateful codec >> API. >> >> The documentation of these controls suggest that these are generic controls, but >> as of today they are only used by stateful codec drivers. >> >> 4) Some corner cases (mainly/only SDR, I think) where you need more than the usual >> 3 or 4 buffers since the buffers arrive at a high frequency. > > High frame rates is an important feature, but it's also a can of worms. > V4L2 is lacking the ability to batch multiple frames, we will have to > address that. Hopefully it could be decoupled from this RFC. It's a separate issue indeed. I just mentioned it because I know SDR drivers use this. They are rarely used, though. > >> Rather than have drivers try to correct the count value (typically incorrectly), the >> vb2_queue min_reqbufs_allocation field was added to set the minimum number of >> buffers that VIDIOC_REQBUFS should allocate if count is less than that. > > Even if I dislike this feature, I agree it's better implemented through > min_reqbufs_allocation than by manual calculations in drivers. > >> VIDIOC_CREATE_BUFS is not affected by that: if you use CREATE_BUFS you take full control >> of how many buffers you want to create. It might create fewer buffers if you run out of >> memory, but never more than requested. >> >> But what is missing is that if you use CREATE_BUFS you need to know the value of >> min_queued_buffers + 1, and that is not exposed. >> >> I would propose to add a min_num_buffers field to struct v4l2_create_buffers >> and add a V4L2_BUF_CAP_SUPPORTS_MIN_NUM_BUFFERS flag to signal the presence of >> that field. And vb2 can set it to min_queued_buffers + 1. > > This would require allocating a buffer first to get the value. Wouldn't > a read-only control be better ? No. You can call CREATE_BUFS with count = 0: in that case it does nothing, except filling in all those capabilities. It was designed with that in mind so you have an ioctl that can return all that information. > > Furthermore, I would rather provide the min_queued_buffers value instead > of min_queued_buffers + 1. The V4L2 API should provide userspace with > information it needs to make informed decisions, but not make those > decisions in behalf of userspace. It's up to applications to add 1 or > more buffers depending on their use case. I would definitely want more opinions on this. What's the point of returning min_queued_buffers and then creating that many buffers and still not be able to stream? Can you think of a scenario (e.g. in libcamera or elsewhere) where that makes sense? Also, will the average V4L2 user have the knowledge to understand that? You have that knowledge, but I think for anyone else it would be really confusing. > > I think we also need to discuss policies regarding scratch buffer > allocation in the context of this RFC. When the hardware supports small > scratch buffers, I would like to make it mandatory for drivers to do so > and support min_queued_buffers = 0. I would first like to know the use-case (as I mentioned above). For the type of drivers I mostly work with (video receivers), it would just be a lot of work for no gain. But perhaps for camera pipelines it does make sense? > When scratch buffers are expensive, do we want to still support them in > the kernel, perhaps in a way controlled by userspace ? A userspace that > can guarantee it will always provide min_queued_buffers + 1 buffers > could indicate so and avoid scratch buffer allocation, while a userspace > that can't provide that guarantee would get scratch buffers from the > kernel. That is really the difference between using VIDIOC_REQBUFS and VIDIOC_CREATE_BUFS. I.e., userspace can already choose this. Just to clarify the reason for this RFC: the current situation is messy. There is a lot of history and a lot of older drivers do not always do the right thing. With this RFC I would like to get a consensus of how it should work. After that I want to implement any missing bits and improve the documentation, and finally go through the drivers and at least try to make them behave consistently. Also I want to improve v4l2-compliance to test more corner cases, especially if you use CREATE_BUFS instead of REQBUFS (I already have a patch for that ready). The work Benjamin did on increasing the max number of supported buffers and the REMOVE_BUFS ioctl uncovered a lot of that messy history, and it is clear we need to try and clarify how it should work. >> The second proposal is to explicitly document that the V4L2_CID_MIN_BUFFERS_FOR_CAPTURE/OUTPUT >> are for stateful codec support only, at least for now. I just discovered that v4l2-compliance and v4l2-ctl do not honor these controls for stateful codecs. That's something that needs to be fixed. There is also one other item that I would like to discuss: the vb2 queue_setup callback is currently used for both REQBUFS and CREATE_BUFS, and it remains confusing for drivers how to use it exactly. I am inclined to redesign that part, most likely splitting it in two: either one callback for REQBUFS and one for CREATE_BUFS, or alternatively, one callback when allocating buffers for the first time (so REQBUFS and when CREATE_BUFS is called for the first time, i.e. when no buffers are allocated yet), and one callback when adding additional buffers. I would have to think about this, and probably experiment a bit. Regards, Hans >> >> If this is in place, then min_reqbufs_allocation should be set to a sane number of >> buffers (i.e. typically 3 or 4), and if you want precise control, use VIDIOC_CREATE_BUFS. >
