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

From: Hans Verkuil
Date: Tue Jun 11 2019 - 04:40:35 EST

On 6/10/19 5:57 PM, Nicolas Dufresne wrote:
> Le lundi 03 juin 2019 Ã 13:28 +0200, Hans Verkuil a Ãcrit :
>> 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
>> 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.
> Is ENUM_FRAMEINTERVALS really required ? This will be a hint to the
> encoder, so that the encoder round to it's internal precision does not
> seem very important.

I don't like this proposal. Especially the use of S_PARM (I *hate* that ioctl).
I think the frame period should be a control with a min/max range, like
any other control.

FRAMEINTERVALS really refers to the rate at which frames arrive when
capturing, and that's not quite what is happening in an encoder.

For now I want to concentrate on the decoder spec, and I'll come back to
this later.

>> 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 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.
> For H264, mid-stream changes would make sense for the case we'd like
> the statefull encoder to emit multiple PPS at start, so then the switch
> would simply change the PPS index. The problem seems to be how do we
> expose "multiple" resolution in our interface ? As there is currently
> no solution to this, I would not see much use for having this supported
> at the moment.

I agree. That's why I want to postpone that part.

>> 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.
>> 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.

Note that this flag will be inverted: V4L2_FMT_FLAG_DYN_RESOLUTION.
So this text is out of date in that regard.



>> 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:
>> (
>> 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.
>> (
>> - Moved the interfaces under the "Video Memory-To-Memory Interface"
>> section.
>> 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