Hi Tomasz, On Friday, 26 October 2018 14:41:26 EEST Tomasz Figa wrote: > On Thu, Sep 20, 2018 at 11:42 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote: > > Some parts of the V4L2 API are awkward to use and I think it would be > > a good idea to look at possible candidates for that. > > > > Examples are the ioctls that use struct v4l2_buffer: the multiplanar > > support is really horrible, and writing code to support both single and > > multiplanar is hard. We are also running out of fields and the timeval > > isn't y2038 compliant. > > > > A proof-of-concept is here: > > > > https://git.linuxtv.org/hverkuil/media_tree.git/commit/?h=v4l2-buffer&id=a > > 95549df06d9900f3559afdbb9da06bd4b22d1f3 > > > > It's a bit old, but it gives a good impression of what I have in mind. > > On a related, but slightly different note, I'm wondering how we should > handle a case where we have an M format (e.g. NV12M with 2 memory > planes), but only 1 DMA-buf with all planes to import. That generally > means that we have to use the same DMA-buf FD with an offset for each > plane. In theory, v4l2_plane::data_offset could be used for this, but > the documentation says that it should be set by the application only > for OUTPUT planes. Moreover, existing drivers tend to just ignore > it... The following patches may be of interest. https://patchwork.linuxtv.org/patch/29177/ https://patchwork.linuxtv.org/patch/29178/ > There is also the opposite problem. Sometimes the application is given > 3 different FDs but pointing to the same buffer. If it has to work > with a video device that only supports non-M formats, it can either > fail, making it unusable, or blindly assume that they all point to the > same buffer and just give the first FD to the video device (we do it > in Chromium, since our allocator is guaranteed to keep all planes of > planar formats in one buffer, if to be used with V4L2). > > Something that we could do is allowing the QBUF semantics of M formats > for non-M formats, where the application would fill the planes[] array > for all planes with all the FDs it has and the kernel could then > figure out if they point to the same buffer (i.e. resolve to the same > dma_buf struct) or fail if not. > > [...] > > > Do we have more ioctls that could use a refresh? S/G/TRY_FMT perhaps, > > again in order to improve single vs multiplanar handling. > > I'd definitely be more than happy to see the plane handling unified > between non-M and M formats, in general. The list of problems with > current interface: > > 1) The userspace has to hardcode the computations of bytesperline for > chroma planes of non-M formats (while they are reported for M > formats). > > 2) Similarly, offsets of the planes in the buffer for non-M formats > must be explicitly calculated in the application, > > 3) Drivers have to explicitly handle both non-M and M formats or > otherwise they would suffer from issues with application compatibility > or sharing buffers with other devices (one supporting only M and the > other only non-M), > > 4) Inconsistency in the meaning of planes[0].sizeimage for non-M > formats and M formats, making it impossible to use planes[0].sizeimage > to set the luma plane size in the hardware for non-M formats (since > it's the total size of all planes). > > I might have probably forgotten about something, but generally fixing > the 4 above, would be a really big step forward. -- Regards, Laurent Pinchart
