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SubjectRe: [RFC PATCH 1/2] media: docs-rst: Add decoder UAPI specification to Codec Interfaces
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Thanks Tomasz for this work.

The following is my first read review, please ignore my comments if
they already have been mentioned by others or discussed, I'll catchup
on the appropriate threads later on.

Le mardi 05 juin 2018 à 19:33 +0900, Tomasz Figa a écrit :
> Due to complexity of the video decoding process, the V4L2 drivers of
> stateful decoder hardware require specific sequencies of V4L2 API calls
> to be followed. These include capability enumeration, initialization,
> decoding, seek, pause, dynamic resolution change, flush and end of
> stream.
>
> Specifics of the above have been discussed during Media Workshops at
> LinuxCon Europe 2012 in Barcelona and then later Embedded Linux
> Conference Europe 2014 in Düsseldorf. The de facto Codec API that
> originated at those events was later implemented by the drivers we already
> have merged in mainline, such as s5p-mfc or mtk-vcodec.
>
> The only thing missing was the real specification included as a part of
> Linux Media documentation. Fix it now and document the decoder part of
> the Codec API.
>
> Signed-off-by: Tomasz Figa <tfiga@chromium.org>
> ---
> Documentation/media/uapi/v4l/dev-codec.rst | 771 +++++++++++++++++++++
> Documentation/media/uapi/v4l/v4l2.rst | 14 +-
> 2 files changed, 784 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/media/uapi/v4l/dev-codec.rst b/Documentation/media/uapi/v4l/dev-codec.rst
> index c61e938bd8dc..0483b10c205e 100644
> --- a/Documentation/media/uapi/v4l/dev-codec.rst
> +++ b/Documentation/media/uapi/v4l/dev-codec.rst
> @@ -34,3 +34,774 @@ the codec and reprogram it whenever another file handler gets access.
> This is different from the usual video node behavior where the video
> properties are global to the device (i.e. changing something through one
> file handle is visible through another file handle).
> +
> +This interface is generally appropriate for hardware that does not
> +require additional software involvement to parse/partially decode/manage
> +the stream before/after processing in hardware.
> +
> +Input data to the Stream API are buffers containing unprocessed video
> +stream (Annex-B H264/H265 stream, raw VP8/9 stream) only. The driver is

We should probably use HEVC instead of H265, as this is the name we
have picked for that format.

> +expected not to require any additional information from the client to
> +process these buffers, and to return decoded frames on the CAPTURE queue
> +in display order.

It might confused some users with the fact that first buffer for non-
bytestream formats is special and must contain only the headers (VP8/9
and H264_NO_SC which is also known as H264 AVC, the format used in
ISOMP4). Also, these formats must be framed by userspace, as it's not
possible to divide the frames/nal later on. I would suggest to be a bit
less strict in the introduction here.

> +
> +Performing software parsing, processing etc. of the stream in the driver
> +in order to support stream API is strongly discouraged. In such case use
> +of Stateless Codec Interface (in development) is preferred.
> +
> +Conventions and notation used in this document
> +==============================================
> +
> +1. The general V4L2 API rules apply if not specified in this document
> + otherwise.
> +
> +2. The meaning of words “must”, “may”, “should”, etc. is as per RFC
> + 2119.
> +
> +3. All steps not marked “optional” are required.
> +
> +4. :c:func:`VIDIOC_G_EXT_CTRLS`, :c:func:`VIDIOC_S_EXT_CTRLS` may be used interchangeably with
> + :c:func:`VIDIOC_G_CTRL`, :c:func:`VIDIOC_S_CTRL`, unless specified otherwise.
> +
> +5. Single-plane API (see spec) and applicable structures may be used
> + interchangeably with Multi-plane API, unless specified otherwise.
> +
> +6. i = [a..b]: sequence of integers from a to b, inclusive, i.e. i =
> + [0..2]: i = 0, 1, 2.
> +
> +7. For OUTPUT buffer A, A’ represents a buffer on the CAPTURE queue
> + containing data (decoded or encoded frame/stream) that resulted
> + from processing buffer A.
> +
> +Glossary
> +========
> +
> +CAPTURE
> + the destination buffer queue, decoded frames for
> + decoders, encoded bitstream for encoders;
> + ``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
> + ``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``
> +
> +client
> + application client communicating with the driver
> + implementing this API
> +
> +coded format
> + encoded/compressed video bitstream format (e.g.
> + H.264, VP8, etc.); see raw format; this is not equivalent to fourcc
> + (V4L2 pixelformat), as each coded format may be supported by multiple
> + fourccs (e.g. ``V4L2_PIX_FMT_H264``, ``V4L2_PIX_FMT_H264_SLICE``, etc.)
> +
> +coded height
> + height for given coded resolution
> +
> +coded resolution
> + stream resolution in pixels aligned to codec
> + format and hardware requirements; see also visible resolution
> +
> +coded width
> + width for given coded resolution
> +
> +decode order
> + the order in which frames are decoded; may differ
> + from display (output) order if frame reordering (B frames) is active in
> + the stream; OUTPUT buffers must be queued in decode order; for frame
> + API, CAPTURE buffers must be returned by the driver in decode order;
> +
> +display order
> + the order in which frames must be displayed
> + (outputted); for stream API, CAPTURE buffers must be returned by the
> + driver in display order;
> +
> +EOS
> + end of stream
> +
> +input height
> + height in pixels for given input resolution
> +
> +input resolution
> + resolution in pixels of source frames being input
> + to the encoder and subject to further cropping to the bounds of visible
> + resolution
> +
> +input width
> + width in pixels for given input resolution
> +
> +OUTPUT
> + the source buffer queue, encoded bitstream for
> + decoders, raw frames for encoders; ``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or
> + ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``
> +
> +raw format
> + uncompressed format containing raw pixel data (e.g.
> + YUV, RGB formats)
> +
> +resume point
> + a point in the bitstream from which decoding may
> + start/continue, without any previous state/data present, e.g.: a
> + keyframe (VPX) or SPS/PPS/IDR sequence (H.264); a resume point is
> + required to start decode of a new stream, or to resume decoding after a
> + seek;

I would prefer synchronisation point, but resume point is also good.
The description makes it obvious, thanks.

> +
> +source buffer
> + buffers allocated for source queue
> +
> +source queue
> + queue containing buffers used for source data, i.e.
> +
> +visible height
> + height for given visible resolution

I do believe 'display width/height/resolution' is more common.

> +
> +visible resolution
> + stream resolution of the visible picture, in
> + pixels, to be used for display purposes; must be smaller or equal to
> + coded resolution;
> +
> +visible width
> + width for given visible resolution
> +
> +Decoder
> +=======
> +
> +Querying capabilities
> +---------------------
> +
> +1. To enumerate the set of coded formats supported by the driver, the
> + client uses :c:func:`VIDIOC_ENUM_FMT` for OUTPUT. The driver must always
> + return the full set of supported formats, irrespective of the
> + format set on the CAPTURE queue.
> +
> +2. To enumerate the set of supported raw formats, the client uses
> + :c:func:`VIDIOC_ENUM_FMT` for CAPTURE. The driver must return only the
> + formats supported for the format currently set on the OUTPUT
> + queue.
> + In order to enumerate raw formats supported by a given coded
> + format, the client must first set that coded format on the
> + OUTPUT queue and then enumerate the CAPTURE queue.

As of today, GStreamer expects an initial state, before the first
S_FMT(OUTPUT) that results in all possible formats regardless. Later
on, after S_FMT(OUTPUT) + header buffers has been passed, a new
enumeration is done, and is expected to return a subset (or the same
list). If a better output format then the one chosen by the driver is
found, it will be tried, if not supported, it will simply keep the
driver selected output format. This way, drivers don't need to do extra
work if their output format is completely fixed by the input/headers.
The only upstream driver that have this flexibility is CODA. To be
fair, we don't in GStreamer need to know about the output format, it's
simply exposed to fail earlier if users tries to connect to elements
that are incompatible by nature. We could just remove that initial
probing and it would still work as expected. I think probing all the
output format is not that of a good idea, with the profiles and level
it becomes all very complex.

> +
> +3. The client may use :c:func:`VIDIOC_ENUM_FRAMESIZES` to detect supported
> + resolutions for a given format, passing its fourcc in
> + :c:type:`v4l2_frmivalenum` ``pixel_format``.

Good thing this is a may, since it's all very complex and not that
useful with the levels and profiles. Userspace can figure-out really if
needed.

> +
> + a. Values returned from :c:func:`VIDIOC_ENUM_FRAMESIZES` for coded formats
> + must be maximums for given coded format for all supported raw
> + formats.
> +
> + b. Values returned from :c:func:`VIDIOC_ENUM_FRAMESIZES` for raw formats must
> + be maximums for given raw format for all supported coded
> + formats.
> +
> + c. The client should derive the supported resolution for a
> + combination of coded+raw format by calculating the
> + intersection of resolutions returned from calls to
> + :c:func:`VIDIOC_ENUM_FRAMESIZES` for the given coded and raw formats.
> +
> +4. Supported profiles and levels for given format, if applicable, may be
> + queried using their respective controls via :c:func:`VIDIOC_QUERYCTRL`.
> +
> +5. The client may use :c:func:`VIDIOC_ENUM_FRAMEINTERVALS` to enumerate maximum
> + supported framerates by the driver/hardware for a given
> + format+resolution combination.

I think we'll need to add a section to help with that one. All drivers
supports ranges in fps. Venus have this bug were it sets a range with a
step of 1/1, but because we expose frame intervals instead of
framerate, the result is not as expected. If you want an interval
between 1 and 60 fps, that would be from 1/60s to 1/1s, there is no
valid step that can be used, you are forced to use CONTINUOUS, or
DISCRETE.

> +
> +Initialization sequence
> +-----------------------
> +
> +1. (optional) Enumerate supported OUTPUT formats and resolutions. See
> + capability enumeration.
> +
> +2. Set a coded format on the source queue via :c:func:`VIDIOC_S_FMT`
> +
> + a. Required fields:
> +
> + i. type = OUTPUT

In the introduction, maybe we could say that we use OUTPUT and CAPTURE
to mean both format (with and without MPLANE ?).

> +
> + ii. fmt.pix_mp.pixelformat set to a coded format
> +
> + iii. fmt.pix_mp.width, fmt.pix_mp.height only if cannot be
> + parsed from the stream for the given coded format;
> + ignored otherwise;

GStreamer passes the display size, as the display size found in the
bitstream maybe not match the selected display size by the container
(e.g. ISOMP4/Matroska). I'm not sure what drivers endup doing, it was
not really thought through, we later query the selection to know the
display size. We could follow this new rule by not passing anything and
then simply picking the smallest from bitstream display size and
container display size. I'm just giving a reference of what existing
userspace may be doing at the moment, as we'll have to care about
breaking existing software when implementing this.

> +
> + b. Return values:
> +
> + i. EINVAL: unsupported format.
> +
> + ii. Others: per spec
> +
> + .. note::
> +
> + The driver must not adjust pixelformat, so if
> + ``V4L2_PIX_FMT_H264`` is passed but only
> + ``V4L2_PIX_FMT_H264_SLICE`` is supported, S_FMT will return
> + -EINVAL. If both are acceptable by client, calling S_FMT for
> + the other after one gets rejected may be required (or use
> + :c:func:`VIDIOC_ENUM_FMT` to discover beforehand, see Capability
> + enumeration).

Ok, that's new, in GStreamer we validate that the format haven't been
changed. Should be backward compatible though. What we don't do though
is check back the OUTPUT format after setting the CAPTURE format, that
would seem totally invalid. You mention that this isn't allowed later
on, so that's great.

> +
> +3. (optional) Get minimum number of buffers required for OUTPUT queue
> + via :c:func:`VIDIOC_G_CTRL`. This is useful if client intends to use
> + more buffers than minimum required by hardware/format (see
> + allocation).

I have never seen such restriction on a decoder, though it's optional
here, so probably fine.

> +
> + a. Required fields:
> +
> + i. id = ``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT``
> +
> + b. Return values: per spec.
> +
> + c. Return fields:
> +
> + i. value: required number of OUTPUT buffers for the currently set
> + format;
> +
> +4. Allocate source (bitstream) buffers via :c:func:`VIDIOC_REQBUFS` on OUTPUT
> + queue.
> +
> + a. Required fields:
> +
> + i. count = n, where n > 0.
> +
> + ii. type = OUTPUT
> +
> + iii. memory = as per spec
> +
> + b. Return values: Per spec.
> +
> + c. Return fields:
> +
> + i. count: adjusted to allocated number of buffers
> +
> + d. The driver must adjust count to minimum of required number of
> + source buffers for given format and count passed. The client
> + must check this value after the ioctl returns to get the
> + number of buffers allocated.
> +
> + .. note::
> +
> + Passing count = 1 is useful for letting the driver choose
> + the minimum according to the selected format/hardware
> + requirements.

This raises a question, should V4L2_CID_MIN_BUFFERS_FOR_OUTPUT really
be the minimum, or min+1. Since REQBUFS is likely to allocate min+1 to
be efficient ? Allocating just the minimum, means that the decoder will
always be idle while the userspace is handling an output.

> +
> + .. note::
> +
> + To allocate more than minimum number of buffers (for pipeline
> + depth), use G_CTRL(``V4L2_CID_MIN_BUFFERS_FOR_OUTPUT)`` to
> + get minimum number of buffers required by the driver/format,
> + and pass the obtained value plus the number of additional
> + buffers needed in count to :c:func:`VIDIOC_REQBUFS`.
> +
> +5. Begin parsing the stream for stream metadata via :c:func:`VIDIOC_STREAMON` on
> + OUTPUT queue. This step allows the driver to parse/decode
> + initial stream metadata until enough information to allocate
> + CAPTURE buffers is found. This is indicated by the driver by
> + sending a ``V4L2_EVENT_SOURCE_CHANGE`` event, which the client
> + must handle.

GStreamer still uses legacy path, expecting G_FMT to block if there is
headers in the queue. Do we want to document this legacy method or not
?

> +
> + a. Required fields: as per spec.
> +
> + b. Return values: as per spec.
> +
> + .. note::
> +
> + Calling :c:func:`VIDIOC_REQBUFS`, :c:func:`VIDIOC_STREAMON`
> + or :c:func:`VIDIOC_G_FMT` on the CAPTURE queue at this time is not
> + allowed and must return EINVAL.
> +
> +6. This step only applies for coded formats that contain resolution
> + information in the stream.
> + Continue queuing/dequeuing bitstream buffers to/from the
> + OUTPUT queue via :c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`. The driver
> + must keep processing and returning each buffer to the client
> + until required metadata to send a ``V4L2_EVENT_SOURCE_CHANGE``
> + for source change type ``V4L2_EVENT_SRC_CH_RESOLUTION`` is
> + found. There is no requirement to pass enough data for this to
> + occur in the first buffer and the driver must be able to
> + process any number
> +
> + a. Required fields: as per spec.
> +
> + b. Return values: as per spec.
> +
> + c. If data in a buffer that triggers the event is required to decode
> + the first frame, the driver must not return it to the client,
> + but must retain it for further decoding.
> +
> + d. Until the resolution source event is sent to the client, calling
> + :c:func:`VIDIOC_G_FMT` on the CAPTURE queue must return -EINVAL.
> +
> + .. note::
> +
> + No decoded frames are produced during this phase.
> +
> +7. This step only applies for coded formats that contain resolution
> + information in the stream.
> + Receive and handle ``V4L2_EVENT_SOURCE_CHANGE`` from the driver
> + via :c:func:`VIDIOC_DQEVENT`. The driver must send this event once
> + enough data is obtained from the stream to allocate CAPTURE
> + buffers and to begin producing decoded frames.
> +
> + a. Required fields:
> +
> + i. type = ``V4L2_EVENT_SOURCE_CHANGE``
> +
> + b. Return values: as per spec.
> +
> + c. The driver must return u.src_change.changes =
> + ``V4L2_EVENT_SRC_CH_RESOLUTION``.
> +
> +8. This step only applies for coded formats that contain resolution
> + information in the stream.
> + Call :c:func:`VIDIOC_G_FMT` for CAPTURE queue to get format for the
> + destination buffers parsed/decoded from the bitstream.
> +
> + a. Required fields:
> +
> + i. type = CAPTURE
> +
> + b. Return values: as per spec.
> +
> + c. Return fields:
> +
> + i. fmt.pix_mp.width, fmt.pix_mp.height: coded resolution
> + for the decoded frames
> +
> + ii. fmt.pix_mp.pixelformat: default/required/preferred by
> + driver pixelformat for decoded frames.
> +
> + iii. num_planes: set to number of planes for pixelformat.
> +
> + iv. For each plane p = [0, num_planes-1]:
> + plane_fmt[p].sizeimage, plane_fmt[p].bytesperline as
> + per spec for coded resolution.
> +
> + .. note::
> +
> + Te value of pixelformat may be any pixel format supported,
> + and must
> + be supported for current stream, based on the information
> + parsed from the stream and hardware capabilities. It is
> + suggested that driver chooses the preferred/optimal format
> + for given configuration. For example, a YUV format may be
> + preferred over an RGB format, if additional conversion step
> + would be required.
> +
> +9. (optional) Enumerate CAPTURE formats via :c:func:`VIDIOC_ENUM_FMT` on
> + CAPTURE queue.
> + Once the stream information is parsed and known, the client
> + may use this ioctl to discover which raw formats are supported
> + for given stream and select on of them via :c:func:`VIDIOC_S_FMT`.
> +
> + a. Fields/return values as per spec.
> +
> + .. note::
> +
> + The driver must return only formats supported for the
> + current stream parsed in this initialization sequence, even
> + if more formats may be supported by the driver in general.
> + For example, a driver/hardware may support YUV and RGB
> + formats for resolutions 1920x1088 and lower, but only YUV for
> + higher resolutions (e.g. due to memory bandwidth
> + limitations). After parsing a resolution of 1920x1088 or
> + lower, :c:func:`VIDIOC_ENUM_FMT` may return a set of YUV and RGB
> + pixelformats, but after parsing resolution higher than
> + 1920x1088, the driver must not return (unsupported for this
> + resolution) RGB.
> +
> + However, subsequent resolution change event
> + triggered after discovering a resolution change within the
> + same stream may switch the stream into a lower resolution;
> + :c:func:`VIDIOC_ENUM_FMT` must return RGB formats again in that case.
> +
> +10. (optional) Choose a different CAPTURE format than suggested via
> + :c:func:`VIDIOC_S_FMT` on CAPTURE queue. It is possible for the client
> + to choose a different format than selected/suggested by the
> + driver in :c:func:`VIDIOC_G_FMT`.
> +
> + a. Required fields:
> +
> + i. type = CAPTURE
> +
> + ii. fmt.pix_mp.pixelformat set to a coded format
> +
> + b. Return values:
> +
> + i. EINVAL: unsupported format.
> +
> + c. Calling :c:func:`VIDIOC_ENUM_FMT` to discover currently available formats
> + after receiving ``V4L2_EVENT_SOURCE_CHANGE`` is useful to find
> + out a set of allowed pixelformats for given configuration,
> + but not required.
> +
> +11. (optional) Acquire visible resolution via :c:func:`VIDIOC_G_SELECTION`.
> +
> + a. Required fields:
> +
> + i. type = CAPTURE
> +
> + ii. target = ``V4L2_SEL_TGT_CROP``
> +
> + b. Return values: per spec.
> +
> + c. Return fields
> +
> + i. r.left, r.top, r.width, r.height: visible rectangle; this must
> + fit within coded resolution returned from :c:func:`VIDIOC_G_FMT`.
> +
> +12. (optional) Get minimum number of buffers required for CAPTURE queue
> + via :c:func:`VIDIOC_G_CTRL`. This is useful if client intends to use
> + more buffers than minimum required by hardware/format (see
> + allocation).

Should not be optional if the driver have this restriction.

> +
> + a. Required fields:
> +
> + i. id = ``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE``
> +
> + b. Return values: per spec.
> +
> + c. Return fields:
> +
> + i. value: minimum number of buffers required to decode the stream
> + parsed in this initialization sequence.
> +
> + .. note::
> +
> + Note that the minimum number of buffers must be at least the
> + number required to successfully decode the current stream.
> + This may for example be the required DPB size for an H.264
> + stream given the parsed stream configuration (resolution,
> + level).
> +
> +13. Allocate destination (raw format) buffers via :c:func:`VIDIOC_REQBUFS` on the
> + CAPTURE queue.
> +
> + a. Required fields:
> +
> + i. count = n, where n > 0.
> +
> + ii. type = CAPTURE
> +
> + iii. memory = as per spec
> +
> + b. Return values: Per spec.
> +
> + c. Return fields:
> +
> + i. count: adjusted to allocated number of buffers.
> +
> + d. The driver must adjust count to minimum of required number of
> + destination buffers for given format and stream configuration
> + and the count passed. The client must check this value after
> + the ioctl returns to get the number of buffers allocated.
> +
> + .. note::
> +
> + Passing count = 1 is useful for letting the driver choose
> + the minimum.
> +
> + .. note::
> +
> + To allocate more than minimum number of buffers (for pipeline
> + depth), use G_CTRL(``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE)`` to
> + get minimum number of buffers required, and pass the obtained
> + value plus the number of additional buffers needed in count
> + to :c:func:`VIDIOC_REQBUFS`.
> +
> +14. Call :c:func:`VIDIOC_STREAMON` to initiate decoding frames.
> +
> + a. Required fields: as per spec.
> +
> + b. Return values: as per spec.
> +
> +Decoding
> +--------
> +
> +This state is reached after a successful initialization sequence. In
> +this state, client queues and dequeues buffers to both queues via
> +:c:func:`VIDIOC_QBUF` and :c:func:`VIDIOC_DQBUF`, as per spec.
> +
> +Both queues operate independently. The client may queue and dequeue
> +buffers to queues in any order and at any rate, also at a rate different
> +for each queue. The client may queue buffers within the same queue in
> +any order (V4L2 index-wise). It is recommended for the client to operate
> +the queues independently for best performance.
> +
> +Source OUTPUT buffers must contain:
> +
> +- H.264/AVC: one or more complete NALUs of an Annex B elementary
> + stream; one buffer does not have to contain enough data to decode
> + a frame;
> +
> +- VP8/VP9: one or more complete frames.
> +
> +No direct relationship between source and destination buffers and the
> +timing of buffers becoming available to dequeue should be assumed in the
> +Stream API. Specifically:
> +
> +- a buffer queued to OUTPUT queue may result in no buffers being
> + produced on the CAPTURE queue (e.g. if it does not contain
> + encoded data, or if only metadata syntax structures are present
> + in it), or one or more buffers produced on the CAPTURE queue (if
> + the encoded data contained more than one frame, or if returning a
> + decoded frame allowed the driver to return a frame that preceded
> + it in decode, but succeeded it in display order)
> +
> +- a buffer queued to OUTPUT may result in a buffer being produced on
> + the CAPTURE queue later into decode process, and/or after
> + processing further OUTPUT buffers, or be returned out of order,
> + e.g. if display reordering is used
> +
> +- buffers may become available on the CAPTURE queue without additional
> + buffers queued to OUTPUT (e.g. during flush or EOS)

There is no mention of timestamp passing and
V4L2_BUF_FLAG_TIMESTAMP_COPY. These though are rather important
respectively to match decoded frames with appropriate metadata and to
discard stored metadata from the userspace queue.

Unlike the suggestion here, most decoder are frame base, it would be
nice to check if this is an actual firmware limitation in certain
cases.

> +
> +Seek
> +----
> +
> +Seek is controlled by the OUTPUT queue, as it is the source of bitstream
> +data. CAPTURE queue remains unchanged/unaffected.
> +
> +1. Stop the OUTPUT queue to begin the seek sequence via
> + :c:func:`VIDIOC_STREAMOFF`.
> +
> + a. Required fields:
> +
> + i. type = OUTPUT
> +
> + b. The driver must drop all the pending OUTPUT buffers and they are
> + treated as returned to the client (as per spec).
> +
> +2. Restart the OUTPUT queue via :c:func:`VIDIOC_STREAMON`
> +
> + a. Required fields:
> +
> + i. type = OUTPUT
> +
> + b. The driver must be put in a state after seek and be ready to
> + accept new source bitstream buffers.
> +
> +3. Start queuing buffers to OUTPUT queue containing stream data after
> + the seek until a suitable resume point is found.
> +
> + .. note::
> +
> + There is no requirement to begin queuing stream
> + starting exactly from a resume point (e.g. SPS or a keyframe).
> + The driver must handle any data queued and must keep processing
> + the queued buffers until it finds a suitable resume point.
> + While looking for a resume point, the driver processes OUTPUT
> + buffers and returns them to the client without producing any
> + decoded frames.

I have some doubts that this actually works. What you describe here is
a flush/reset seqeuence. The drivers I have worked with totally forgets
about their state after STREAMOFF on any queues. The initialization
process need to happen again. Though, adding this support with just
resetting the OUTPUT queue should be backward compatible. GStreamer
always STREAMOFF on both sides.

> +
> +4. After a resume point is found, the driver will start returning
> + CAPTURE buffers with decoded frames.
> +
> + .. note::
> +
> + There is no precise specification for CAPTURE queue of when it
> + will start producing buffers containing decoded data from
> + buffers queued after the seek, as it operates independently
> + from OUTPUT queue.

Also, in practice it is totally un-reliable to start from random point.
Some decoder will produce corrupted frame, some will wait, you never
known. Seek code in ffmpeg, gstreamer, vlc, etc. always pick a good
sync point. Then marks the extra as "decode only", hence the need for
matching input/output for metadata, and drops the extra.
> +
> + - The driver is allowed to and may return a number of remaining CAPTURE
> + buffers containing decoded frames from before the seek after the
> + seek sequence (STREAMOFF-STREAMON) is performed.

This is not a proper seek. That's probably why we also streamoff the
capture queue to get rid of these ancient buffers. This seems only
useful if you are trying to do seamless seeking (aka non flushing
seek), which is a very niche use case.

> +
> + - The driver is also allowed to and may not return all decoded frames
> + queued but not decode before the seek sequence was initiated.
> + E.g. for an OUTPUT queue sequence: QBUF(A), QBUF(B),
> + STREAMOFF(OUT), STREAMON(OUT), QBUF(G), QBUF(H), any of the
> + following results on the CAPTURE queue is allowed: {A’, B’, G’,
> + H’}, {A’, G’, H’}, {G’, H’}.
> +
> +Pause
> +-----
> +
> +In order to pause, the client should just cease queuing buffers onto the
> +OUTPUT queue. This is different from the general V4L2 API definition of
> +pause, which involves calling :c:func:`VIDIOC_STREAMOFF` on the queue. Without
> +source bitstream data, there is not data to process and the hardware
> +remains idle. Conversely, using :c:func:`VIDIOC_STREAMOFF` on OUTPUT queue
> +indicates a seek, which 1) drops all buffers in flight and 2) after a
> +subsequent :c:func:`VIDIOC_STREAMON` will look for and only continue from a
> +resume point. This is usually undesirable for pause. The
> +STREAMOFF-STREAMON sequence is intended for seeking.
> +
> +Similarly, CAPTURE queue should remain streaming as well, as the
> +STREAMOFF-STREAMON sequence on it is intended solely for changing buffer
> +sets
> +
> +Dynamic resolution change
> +-------------------------
> +
> +When driver encounters a resolution change in the stream, the dynamic
> +resolution change sequence is started.
> +
> +1. On encountering a resolution change in the stream. The driver must
> + first process and decode all remaining buffers from before the
> + resolution change point.
> +
> +2. After all buffers containing decoded frames from before the
> + resolution change point are ready to be dequeued on the
> + CAPTURE queue, the driver sends a ``V4L2_EVENT_SOURCE_CHANGE``
> + event for source change type ``V4L2_EVENT_SRC_CH_RESOLUTION``.
> + The last buffer from before the change must be marked with
> + :c:type:`v4l2_buffer` ``flags`` flag ``V4L2_BUF_FLAG_LAST`` as in the flush
> + sequence.
> +
> + .. note::
> +
> + Any attempts to dequeue more buffers beyond the buffer marked
> + with ``V4L2_BUF_FLAG_LAST`` will result in a -EPIPE error from
> + :c:func:`VIDIOC_DQBUF`.
> +
> +3. After dequeuing all remaining buffers from the CAPTURE queue, the
> + client must call :c:func:`VIDIOC_STREAMOFF` on the CAPTURE queue. The
> + OUTPUT queue remains streaming (calling STREAMOFF on it would
> + trigger a seek).
> + Until STREAMOFF is called on the CAPTURE queue (acknowledging
> + the event), the driver operates as if the resolution hasn’t
> + changed yet, i.e. :c:func:`VIDIOC_G_FMT`, etc. return previous
> + resolution.

It's a bit more complicated, if the resolution goes bigger, the encoded
buffer size needed to fit a full frame may be bidger. Reallocation of
the output queue may be needed. In some memory constraint device, we'll
also want to reallocate if it's going smaller. FFMPEG implement
something clever for selected the size, CODA driver does the same but
in the driver. It's a bit of a mess.

In the long term, for gapless change (specially with CMA) we might want
to support using larger buffer in the CAPTURE queue to avoid
reallocation. OMX supports this.

> +
> +4. The client frees the buffers on the CAPTURE queue using
> + :c:func:`VIDIOC_REQBUFS`.
> +
> + a. Required fields:
> +
> + i. count = 0
> +
> + ii. type = CAPTURE
> +
> + iii. memory = as per spec
> +
> +5. The client calls :c:func:`VIDIOC_G_FMT` for CAPTURE to get the new format
> + information.
> + This is identical to calling :c:func:`VIDIOC_G_FMT` after
> + ``V4L2_EVENT_SRC_CH_RESOLUTION`` in the initialization
> + sequence and should be handled similarly.
> +
> + .. note::
> +
> + It is allowed for the driver not to support the same
> + pixelformat as previously used (before the resolution change)
> + for the new resolution. The driver must select a default
> + supported pixelformat and return it from :c:func:`VIDIOC_G_FMT`, and
> + client must take note of it.
> +
> +6. (optional) The client is allowed to enumerate available formats and
> + select a different one than currently chosen (returned via
> + :c:func:`VIDIOC_G_FMT)`. This is identical to a corresponding step in
> + the initialization sequence.
> +
> +7. (optional) The client acquires visible resolution as in
> + initialization sequence.
> +
> +8. (optional) The client acquires minimum number of buffers as in
> + initialization sequence.
> +
> +9. The client allocates a new set of buffers for the CAPTURE queue via
> + :c:func:`VIDIOC_REQBUFS`. This is identical to a corresponding step in
> + the initialization sequence.
> +
> +10. The client resumes decoding by issuing :c:func:`VIDIOC_STREAMON` on the
> + CAPTURE queue.
> +
> +During the resolution change sequence, the OUTPUT queue must remain
> +streaming. Calling :c:func:`VIDIOC_STREAMOFF` on OUTPUT queue will initiate seek.
> +
> +The OUTPUT queue operates separately from the CAPTURE queue for the
> +duration of the entire resolution change sequence. It is allowed (and
> +recommended for best performance and simplcity) for the client to keep
> +queuing/dequeuing buffers from/to OUTPUT queue even while processing
> +this sequence.
> +
> +.. note::
> +
> + It is also possible for this sequence to be triggered without
> + change in resolution if a different number of CAPTURE buffers is
> + required in order to continue decoding the stream.

Perhaps the driver should be queried for the new display resolution
through G_SELECTION ?

> +
> +Flush
> +-----
> +
> +Flush is the process of draining the CAPTURE queue of any remaining

Ok, call this Drain if it's the process of draining, it's really
confusing as GStreamer makes a distinction between flush (getting rid
of, like a reset) and draining (which involved displaying the
remaining, but stop producing new data).

> +buffers. After the flush sequence is complete, the client has received
> +all decoded frames for all OUTPUT buffers queued before the sequence was
> +started.
> +
> +1. Begin flush by issuing :c:func:`VIDIOC_DECODER_CMD`.
> +
> + a. Required fields:
> +
> + i. cmd = ``V4L2_DEC_CMD_STOP``
> +
> +2. The driver must process and decode as normal all OUTPUT buffers
> + queued by the client before the :c:func:`VIDIOC_DECODER_CMD` was
> + issued.
> + Any operations triggered as a result of processing these
> + buffers (including the initialization and resolution change
> + sequences) must be processed as normal by both the driver and
> + the client before proceeding with the flush sequence.
> +
> +3. Once all OUTPUT buffers queued before ``V4L2_DEC_CMD_STOP`` are
> + processed:
> +
> + a. If the CAPTURE queue is streaming, once all decoded frames (if
> + any) are ready to be dequeued on the CAPTURE queue, the
> + driver must send a ``V4L2_EVENT_EOS``. The driver must also

I have never used the EOS event, and I bet many drivers don't implement
it, why is that a must ?

> + set ``V4L2_BUF_FLAG_LAST`` in :c:type:`v4l2_buffer` ``flags`` field on the

In MFC we don't know, so we use the other method, EPIPE, there will be
no FLAG_LAST on MFC, it's just not possible with that firmware. So
FLAG_LAST is preferred, EPIPE is the fallback.

> + buffer on the CAPTURE queue containing the last frame (if
> + any) produced as a result of processing the OUTPUT buffers
> + queued before ``V4L2_DEC_CMD_STOP``. If no more frames are
> + left to be returned at the point of handling
> + ``V4L2_DEC_CMD_STOP``, the driver must return an empty buffer
> + (with :c:type:`v4l2_buffer` ``bytesused`` = 0) as the last buffer with
> + ``V4L2_BUF_FLAG_LAST`` set instead.
> + Any attempts to dequeue more buffers beyond the buffer
> + marked with ``V4L2_BUF_FLAG_LAST`` will result in a -EPIPE
> + error from :c:func:`VIDIOC_DQBUF`.
> +
> + b. If the CAPTURE queue is NOT streaming, no action is necessary for
> + CAPTURE queue and the driver must send a ``V4L2_EVENT_EOS``
> + immediately after all OUTPUT buffers in question have been
> + processed.
> +
> +4. To resume, client may issue ``V4L2_DEC_CMD_START``.
> +
> +End of stream
> +-------------
> +
> +When an explicit end of stream is encountered by the driver in the
> +stream, it must send a ``V4L2_EVENT_EOS`` to the client after all frames
> +are decoded and ready to be dequeued on the CAPTURE queue, with the
> +:c:type:`v4l2_buffer` ``flags`` set to ``V4L2_BUF_FLAG_LAST``. This behavior is
> +identical to the flush sequence as if triggered by the client via
> +``V4L2_DEC_CMD_STOP``.

I never heard of such a thing as an implicit EOS, can you elaborate ?

> +
> +Commit points
> +-------------
> +
> +Setting formats and allocating buffers triggers changes in the behavior
> +of the driver.
> +
> +1. Setting format on OUTPUT queue may change the set of formats
> + supported/advertised on the CAPTURE queue. It also must change
> + the format currently selected on CAPTURE queue if it is not
> + supported by the newly selected OUTPUT format to a supported one.
> +
> +2. Enumerating formats on CAPTURE queue must only return CAPTURE formats
> + supported for the OUTPUT format currently set.
> +
> +3. Setting/changing format on CAPTURE queue does not change formats
> + available on OUTPUT queue. An attempt to set CAPTURE format that
> + is not supported for the currently selected OUTPUT format must
> + result in an error (-EINVAL) from :c:func:`VIDIOC_S_FMT`.

That's great clarification !

> +
> +4. Enumerating formats on OUTPUT queue always returns a full set of
> + supported formats, irrespective of the current format selected on
> + CAPTURE queue.
> +
> +5. After allocating buffers on the OUTPUT queue, it is not possible to
> + change format on it.
> +
> +To summarize, setting formats and allocation must always start with the
> +OUTPUT queue and the OUTPUT queue is the master that governs the set of
> +supported formats for the CAPTURE queue.
> diff --git a/Documentation/media/uapi/v4l/v4l2.rst b/Documentation/media/uapi/v4l/v4l2.rst
> index b89e5621ae69..563d5b861d1c 100644
> --- a/Documentation/media/uapi/v4l/v4l2.rst
> +++ b/Documentation/media/uapi/v4l/v4l2.rst
> @@ -53,6 +53,10 @@ Authors, in alphabetical order:
>
> - Original author of the V4L2 API and documentation.
>
> +- Figa, Tomasz <tfiga@chromium.org>
> +
> + - Documented parts of the V4L2 (stateful) Codec Interface. Migrated from Google Docs to kernel documentation.
> +
> - H Schimek, Michael <mschimek@gmx.at>
>
> - Original author of the V4L2 API and documentation.
> @@ -65,6 +69,10 @@ Authors, in alphabetical order:
>
> - Designed and documented the multi-planar API.
>
> +- Osciak, Pawel <posciak@chromium.org>
> +
> + - Documented the V4L2 (stateful) Codec Interface.
> +
> - Palosaari, Antti <crope@iki.fi>
>
> - SDR API.
> @@ -85,7 +93,7 @@ Authors, in alphabetical order:
>
> - Designed and documented the VIDIOC_LOG_STATUS ioctl, the extended control ioctls, major parts of the sliced VBI API, the MPEG encoder and decoder APIs and the DV Timings API.
>
> -**Copyright** |copy| 1999-2016: Bill Dirks, Michael H. Schimek, Hans Verkuil, Martin Rubli, Andy Walls, Muralidharan Karicheri, Mauro Carvalho Chehab, Pawel Osciak, Sakari Ailus & Antti Palosaari.
> +**Copyright** |copy| 1999-2018: Bill Dirks, Michael H. Schimek, Hans Verkuil, Martin Rubli, Andy Walls, Muralidharan Karicheri, Mauro Carvalho Chehab, Pawel Osciak, Sakari Ailus & Antti Palosaari, Tomasz Figa.
>
> Except when explicitly stated as GPL, programming examples within this
> part can be used and distributed without restrictions.
> @@ -94,6 +102,10 @@ part can be used and distributed without restrictions.
> Revision History
> ****************
>
> +:revision: TBD / TBD (*tf*)
> +
> +Add specification of V4L2 Codec Interface UAPI.
> +
> :revision: 4.10 / 2016-07-15 (*rr*)
>
> Introduce HSV formats.

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