Hi, On Wed, 2018-08-15 at 09:57 -0400, Nicolas Dufresne wrote: > Le lundi 06 août 2018 à 10:16 +0200, Paul Kocialkowski a écrit : > > Hi Hans and all, > > > > On Sat, 2018-08-04 at 15:50 +0200, Hans Verkuil wrote: > > > Hi all, > > > > > > While the Request API patch series addresses all the core API issues, there > > > are some high-level considerations as well: > > > > > > 1) How can the application tell that the Request API is supported and for > > > which buffer types (capture/output) and pixel formats? > > > > > > 2) How can the application tell if the Request API is required as opposed to being > > > optional? > > > > > > 3) Some controls may be required in each request, how to let userspace know this? > > > Is it even necessary to inform userspace? > > > > > > 4) (For bonus points): How to let the application know which streaming I/O modes > > > are available? That's never been possible before, but it would be very nice > > > indeed if that's made explicit. > > > > Thanks for bringing up these considerations and questions, which perhaps > > cover the last missing bits for streamlined use of the request API by > > userspace. I would suggest another item, related to 3): > > > > 5) How can applications tell whether the driver supports a specific > > codec profile/level, not only for encoding but also for decoding? It's > > common for low-end embedded hardware to not support the most advanced > > profiles (e.g. H264 high profile). > > Hi Paul, after some discussion with Philip, he sent a proposal patch > that enables profile/level extended CID support to decoders too. The > control is made read-only, the point is not really the CID get/set but > that the controls allow enumerating the supported values. This seems > quite straightforward and easy to use. > > This enumeration is already provided this way some of the existing > sate-full encoders. Sounds great, thanks for looking into it! I looked for the patch in the list, but couldn't find it off-hand. Do you have a link to it? I would like to bind it to the Cedrus VPU driver eventually. Cheers, Paul > > > Since the Request API associates data with frame buffers it makes sense to expose > > > this as a new capability field in struct v4l2_requestbuffers and struct v4l2_create_buffers. > > > > > > The first struct has 2 reserved fields, the second has 8, so it's not a problem to > > > take one for a capability field. Both structs also have a buffer type, so we know > > > if this is requested for a capture or output buffer type. The pixel format is known > > > in the driver, so HAS/REQUIRES_REQUESTS can be set based on that. I doubt we'll have > > > drivers where the request caps would actually depend on the pixel format, but it > > > theoretically possible. For both ioctls you can call them with count=0 at the start > > > of the application. REQBUFS has of course the side-effect of deleting all buffers, > > > but at the start of your application you don't have any yet. CREATE_BUFS has no > > > side-effects. > > > > My initial thoughts on this point were to have flags exposed in > > v4l2_capability, but now that you're saying it, it does make sense for > > the flag to be associated with a buffer rather than the global device. > > > > In addition, I've heard of cases (IIRC it was some Rockchip platforms) > > where the platform has both stateless and stateful VPUs (I think it was > > stateless up to H264 and stateful for H265). This would allow supporting > > these two hardware blocks under the same video device (if that makes > > sense anyway). And even if there's no immediate need, it's always good > > to have this level of granularity (with little drawbacks). > > > > > I propose adding these capabilities: > > > > > > #define V4L2_BUF_CAP_HAS_REQUESTS 0x00000001 > > > #define V4L2_BUF_CAP_REQUIRES_REQUESTS 0x00000002 > > > #define V4L2_BUF_CAP_HAS_MMAP 0x00000100 > > > #define V4L2_BUF_CAP_HAS_USERPTR 0x00000200 > > > #define V4L2_BUF_CAP_HAS_DMABUF 0x00000400 > > > > > > If REQUIRES_REQUESTS is set, then HAS_REQUESTS is also set. > > > > > > At this time I think that REQUIRES_REQUESTS would only need to be set for the > > > output queue of stateless codecs. > > > > > > If capabilities is 0, then it's from an old kernel and all you know is that > > > requests are certainly not supported, and that MMAP is supported. Whether USERPTR > > > or DMABUF are supported isn't known in that case (just try it :-) ). > > > > Sounds good to me! > > > > > Strictly speaking we do not need these HAS_MMAP/USERPTR/DMABUF caps, but it is very > > > easy to add if we create a new capability field anyway, and it has always annoyed > > > the hell out of me that we didn't have a good way to let userspace know what > > > streaming I/O modes we support. And with vb2 it's easy to implement. > > > > I totally agree here, it would be very nice to take the occasion to > > expose to userspace what I/O modes are available. The current try-and- > > see approach works, but this feels much better indeed. > > > > > Regarding point 3: I think this should be documented next to the pixel format. I.e. > > > the MPEG-2 Slice format used by the stateless cedrus codec requires the request API > > > and that two MPEG-2 controls (slice params and quantization matrices) must be present > > > in each request. > > > > > > I am not sure a control flag (e.g. V4L2_CTRL_FLAG_REQUIRED_IN_REQ) is needed here. > > > It's really implied by the fact that you use a stateless codec. It doesn't help > > > generic applications like v4l2-ctl or qv4l2 either since in order to support > > > stateless codecs they will have to know about the details of these controls anyway. > > > > > > So I am inclined to say that it is not necessary to expose this information in > > > the API, but it has to be documented together with the pixel format documentation. > > > > I think this is affected by considerations about codec profile/level > > support. More specifically, some controls will only be required for > > supporting advanced codec profiles/levels, so they can only be > > explicitly marked with appropriate flags by the driver when the target > > profile/level is known. And I don't think it would be sane for userspace > > to explicitly set what profile/level it's aiming at. As a result, I > > don't think we can explicitly mark controls as required or optional. > > > > I also like the idea that it should instead be implicit and that the > > documentation should detail which specific stateless metadata controls > > are required for a given profile/level. > > > > As for controls validation, the approach followed in the Cedrus driver > > is to check that the most basic controls are filled and allow having > > missing controls for those that match advanced profiles. > > > > Since this approach feels somewhat generic enough to be applied to all > > stateless VPU drivers, maybe this should be made a helper in the > > framework? > > > > In addition, I see a need for exposing the maximum profile/level that > > the driver supports for decoding. I would suggest reusing the already- > > existing dedicated controls used for encoding for this purpose. For > > decoders, they would be used to expose the (read-only) maximum > > profile/level that is supported by the hardware and keep using them as a > > settable value in a range (matching the level of support) for encoders. > > > > This is necessary for userspace to determine whether a given video can > > be decoded in hardware or not. Instead of half-way decoding the video > > (ending up in funky results), this would easily allow skipping hardware > > decoding and e.g. falling back on software decoding. > > > > What do you think? > > > > Cheers, > > > > Paul > > -- Paul Kocialkowski, Bootlin (formerly Free Electrons) Embedded Linux and kernel engineering https://bootlin.com
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