On 29/10/2024 09:17, Hans Verkuil wrote: > 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. Actually, this really has to be addressed since this is broken: you can call CREATE_BUFS as a replacement for REQBUFS, but it will act like REQBUFS and the requested sizes are not honored. I added tests for this to v4l2-compliance (locally only), and it fails on everything. It should not be news to anyone that I hate the CREATE_BUFS ioctl API. I posted an RFC for a VIDIOC_ADD_BUFS replacement earlier this year: https://lore.kernel.org/linux-media/243a66ad-6dff-4a43-ab03-e01d1038fe8a@xxxxxxxxx/ I wonder if we should restrict CREATE_BUFS to only be used after calling REQBUFS, and to a proper job for ADD_BUFS. Because given the vb2 design flaw I am not sure if it can be worked around. Or if we even want that. What a mess. Looking at the kernel history, CREATE_BUFS was added back in 2011 and the first very simple v4l2-compliance tests were added in 2012. Moral: whenever a new uAPI is added, make sure it you make really good compliance tests as well. Regards, Hans > > 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. >> > >
