Re: [PATCHv4 0/2] Document memory-to-memory video codec interfaces
From: Hans Verkuil
Date: Mon Jun 03 2019 - 10:06:32 EST
These changes + the stateless decoder v4 patch are now available here:
Easier to read than a patch :-)
On 6/3/19 1:28 PM, Hans Verkuil wrote:
> Since Thomasz was very busy with other things, I've taken over this
> patch series. This v4 includes his draft changes and additional changes
> from me.
> This series attempts to add the documentation of what was discussed
> during Media Workshops at LinuxCon Europe 2012 in Barcelona and then
> later Embedded Linux Conference Europe 2014 in DÃsseldorf and then
> eventually written down by Pawel Osciak and tweaked a bit by Chrome OS
> video team (but mostly in a cosmetic way or making the document more
> precise), during the several years of Chrome OS using the APIs in
> Note that most, if not all, of the API is already implemented in
> existing mainline drivers, such as s5p-mfc or mtk-vcodec. Intention of
> this series is just to formalize what we already have.
> Thanks everyone for the huge amount of useful comments to previous
> versions of this series. Much of the credits should go to Pawel Osciak
> too, for writing most of the original text of the initial RFC.
> This v4 incorporates all known comments (let me know if I missed
> something!) and should be complete for the decoder.
> For the encoder there are two remaining TODOs for the API:
> 1) Setting the frame rate so bitrate control can make sense, since
> they need to know this information.
> Suggested solution: require support for ENUM_FRAMEINTERVALS for the
> coded pixelformats and S_PARM(OUTPUT). Open question: some drivers
> (mediatek, hva, coda) require S_PARM(OUTPUT), some (venus) allow both
> S_PARM(CAPTURE) and S_PARM(OUTPUT). I am inclined to allow both since
> this is not a CAPTURE vs OUTPUT thing, it is global to both queues.
> 2) Interactions between OUTPUT and CAPTURE formats.
> The main problem is what to do if the capture sizeimage is too small
> for the OUTPUT resolution when streaming starts.
> Proposal: width and height of S_FMT(OUTPUT) are used to
> calculate a minimum sizeimage (app may request more). This is
> V4L2_FMT_FLAG_FIXED_RESOLUTION is always set for codec formats
> for the encoder (i.e. we don't support mid-stream resolution
> changes for now) and V4L2_EVENT_SOURCE_CHANGE is not
> supported. See https://patchwork.linuxtv.org/patch/56478/ for
> the patch adding this flag.
> Of course, if we start to support mid-stream resolution
> changes (or other changes that require a source change event),
> then this flag should be dropped by the encoder driver and
> documentation on how to handle the source change event should
> be documented in the encoder spec. I prefer to postpone this
> until we have an encoder than can actually do mid-stream
> resolution changes.
> If sizeimage of the OUTPUT is too small for the CAPTURE
> resolution and V4L2_EVENT_SOURCE_CHANGE is not supported,
> then the second STREAMON (either CAPTURE or OUTPUT) will
> return -ENOMEM since there is not enough memory to do the
> If V4L2_FMT_FLAG_FIXED_RESOLUTION is set (i.e. that should
> be the case for all current encoders), then any bitrate controls
> will be limited in range to what the current state (CAPTURE and
> OUTPUT formats and frame rate) supports.
> Comments regarding these two encoder proposals are welcome!
> Changes since v3:
> - Lots of stylistic fixes and fixing typos/grammar/etc.
> - width/height for S_FMT(OUTPUT):
> Expects that the output width and height is always a valid
> resolution (i.e. never 0x0), and G/S/TRY_FMT and REQBUFS will use that
> instead of returning an error. Note that this resolution is a placeholder
> until the actual resolution is parsed from the stream.
> - Dropped step 3 (Query the minimum number of buffers required for the CAPTURE
> queue via VIDIOC_G_CTRL().) in the Capture Setup section. It seems to be
> a left-over from earlier versions. The same information is also in Step 10,
> so no need to have this in two places.
> - Added step 5 in the Capture Setup section: set COMPOSE rectangle if needed.
> - VIDIO_DECODER_CMD: document EBUSY return while draining the queue.
> - width/height for S_FMT(CAPTURE): The width/height for the CAPTURE format
> are marked as read-only and are based on the encoders current state such
> as the OUTPUT format.
> - Drop TGT_COMPOSE support in the encoder: there are currently
> no encoders that can do composing/scaling.
> - Document corner cases in the Drain sequence
> - Document error handling.
> - VIDIO_ENCODER_CMD: document EBUSY return while draining the queue.
> Changes since v2:
> - Specified that the initial source change event is signaled
> regardless of whether the client-set format matches the
> stream format.
> - Dropped V4L2_CID_MIN_BUFFERS_FOR_OUTPUT since it's meaningless
> for the bitstream input buffers of decoders.
> - Explicitly stated that VIDIOC_REQBUFS is not allowed on CAPTURE
> if the stream information is not available.
> - Described decode error handling.
> - Mentioned that timestamps can be observed after a seek to
> determine whether the CAPTURE buffers originated from before
> or after the seek.
> - Explicitly stated that after a pair of V4L2_DEC_CMD_STOP and
> V4L2_DEC_CMD_START, the decoder is not reset and preserves
> all the state.
> - Specified that width and height of CAPTURE format are ignored
> and always zero.
> - Explicitly noted the common use case for the CROP target with
> macroblock-unaligned video resolutions.
> - Added a reference to Request API.
> - Dropped V4L2_CID_MIN_BUFFERS_FOR_CAPTURE since it's meaningless
> for the bitstream output buffers of encoders.
> - Explicitly stated that after a pair of V4L2_ENC_CMD_STOP and
> V4L2_ENC_CMD_START, the encoder is not reset and preserves
> all the state.
> - Dropped format enumeration from "Initialization", since it's already
> a part of "Querying capabilities".
> - Many spelling, grammar, stylistic, etc. changes.
> - Changed the style of note blocks.
> - Rebased onto Hans' documentation cleanup series.
> - Moved the interfaces under the "Video Memory-To-Memory Interface"
> For changes since v1 see the v2:
> For changes since RFC see the v1:
> Tomasz Figa (2):
> media: docs-rst: Document memory-to-memory video decoder interface
> media: docs-rst: Document memory-to-memory video encoder interface
> Documentation/media/uapi/v4l/dev-decoder.rst | 1084 +++++++++++++++++
> Documentation/media/uapi/v4l/dev-encoder.rst | 608 +++++++++
> Documentation/media/uapi/v4l/dev-mem2mem.rst | 9 +-
> Documentation/media/uapi/v4l/pixfmt-v4l2.rst | 10 +
> Documentation/media/uapi/v4l/v4l2.rst | 12 +-
> .../media/uapi/v4l/vidioc-decoder-cmd.rst | 41 +-
> .../media/uapi/v4l/vidioc-encoder-cmd.rst | 51 +-
> 7 files changed, 1779 insertions(+), 36 deletions(-)
> create mode 100644 Documentation/media/uapi/v4l/dev-decoder.rst
> create mode 100644 Documentation/media/uapi/v4l/dev-encoder.rst