On 10/04/2018 10:11 AM, Alexandre Courbot wrote: > This patch documents the protocol that user-space should follow when > communicating with stateless video decoders. It is based on the > following references: > > * The current protocol used by Chromium (converted from config store to > request API) > > * The submitted Cedrus VPU driver > > As such, some things may not be entirely consistent with the current > state of drivers, so it would be great if all stakeholders could point > out these inconsistencies. :) > > This patch is supposed to be applied on top of the Request API V18 as > well as the memory-to-memory video decoder interface series by Tomasz > Figa. > > Changes since V1: > > * Applied fixes received as feedback, > * Moved controls descriptions to the extended controls file, > * Document reference frame management and referencing (need Hans' feedback on > that). > > Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxxxx> > --- > .../media/uapi/v4l/dev-stateless-decoder.rst | 348 ++++++++++++++++++ > Documentation/media/uapi/v4l/devices.rst | 1 + > .../media/uapi/v4l/extended-controls.rst | 25 ++ > .../media/uapi/v4l/pixfmt-compressed.rst | 54 ++- > 4 files changed, 424 insertions(+), 4 deletions(-) > create mode 100644 Documentation/media/uapi/v4l/dev-stateless-decoder.rst > > diff --git a/Documentation/media/uapi/v4l/dev-stateless-decoder.rst b/Documentation/media/uapi/v4l/dev-stateless-decoder.rst <snip> > +Buffer management during decoding > +================================= > +Contrary to stateful decoder drivers, a stateless decoder driver does not > +perform any kind of buffer management. In particular, it guarantees that > +``CAPTURE`` buffers will be dequeued in the same order as they are queued. This > +allows user-space to know in advance which ``CAPTURE`` buffer will contain a > +given frame, and thus to use that buffer ID as the key to indicate a reference > +frame. > + > +This also means that user-space is fully responsible for not queuing a given > +``CAPTURE`` buffer for as long as it is used as a reference frame. Failure to do > +so will overwrite the reference frame's data while it is still in use, and > +result in visual corruption of future frames. > + > +Note that this applies to all types of buffers, and not only to > +``V4L2_MEMORY_MMAP`` ones, as drivers supporting ``V4L2_MEMORY_DMABUF`` will > +typically maintain a map of buffer IDs to DMABUF handles for reference frame > +management. Queueing a buffer will result in the map entry to be overwritten > +with the new DMABUF handle submitted in the :c:func:`VIDIOC_QBUF` ioctl. The more I think about this, the more I believe that relying on capture buffer indices is wrong. It's easy enough if there is a straightforward 1-1 relationship, but what if you have H264 slices as Nicolas mentioned and it becomes a N-1 relationship? Yes, you can still do this in userspace, but it becomes a lot more complicated. And what if in the future instead of having one capture buffer per decoded frame there will be multiple capture buffers per decoded frame, each with a single slice (for example)? I would feel much happier if we used a 'cookie' to refer to buffers. The next problem would be where to put it. I dislike abusing the timestamp field for this. Part of the reason is that there will be changes there to fix the year 2038 issue, and I am not entirely sure what that will do to how this field is handled since there may be conversions from a pre-2038 timeval to a 2038-ready timeval. So a union with the timestamp field and a cookie field (+ BUF_FLAG_COOKIE) would work best IMHO. Regards, Hans
