Re: [PATCHv4 0/2] Document memory-to-memory video codec interfaces

[Nicolas Dufresne <nicolas@ndufresne.ca>]

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Le lundi 03 juin 2019 à 13:28 +0200, Hans Verkuil a écrit :
> 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 Düsseldorf 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
> production.
> 
> 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.

I agree, as long as it's documented. I can imagine how this could be
confusing for new users.

> 
> 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
>    driver-specific.
> 
>    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
>    encode.

You seem confident that we will know immediately if it's too small. But
what I remember is that HW has an interrupt for this, allowing
userspace to allocate a larger buffer and resume.

Should we make the capture queue independent of the streaming state, so
that we can streamoff/reqbufs/.../streamon to resume from an ENOMEM
error ? And shouldn't ENOMEM be returned by the following capture DQBUF
when such an interrupt is raised ?

> 
>    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!
> 
> Regards,
> 
> 	Hans
> 
> Changes since v3:
> 
> - Lots of stylistic fixes and fixing typos/grammar/etc.
> 
> Decoder:
> 
> - 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.
> 
> Encoder:
> 
> - 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:
> (https://lore.kernel.org/patchwork/cover/1002474/)
> Decoder:
>  - 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.
> 
> Encoder:
>  - 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.
> 
> General:
>  - 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.
>    (https://patchwork.kernel.org/cover/10775407/
>     https://patchwork.kernel.org/patch/10776737/)
>  - Moved the interfaces under the "Video Memory-To-Memory Interface"
>    section.
> 
> For changes since v1 see the v2:
> https://lore.kernel.org/patchwork/cover/1002474/
> 
> For changes since RFC see the v1:
> https://patchwork.kernel.org/cover/10542207/
> 
> 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
> 

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