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SubjectRe: [RFC PATCH 1/2] media: docs-rst: Add decoder UAPI specification to Codec Interfaces
On Wed, Jun 6, 2018 at 6:04 PM Tomasz Figa <tfiga@chromium.org> wrote:
>
> Hi Dave,
>
> Thanks for review! Please see my replies inline.
>
> On Tue, Jun 5, 2018 at 10:10 PM Dave Stevenson
> <dave.stevenson@raspberrypi.org> wrote:
> >
> > Hi Tomasz.
> >
> > Thanks for formalising this.
> > I'm working on a stateful V4L2 codec driver on the Raspberry Pi and
> > was having to deduce various implementation details from other
> > drivers. I know how much we all tend to hate having to write
> > documentation, but it is useful to have.
>
> Agreed. Piles of other work showing up out of nowhere don't help either. :(
>
> A lot of credits go to Pawel, who wrote down most of details discussed
> earlier into a document that we used internally to implement Chrome OS
> video stack and drivers. He unfortunately got flooded with loads of
> other work and ran out of time to finalize it and produce something
> usable as kernel documentation (time was needed especially in the old
> DocBook xml days).
>
> >
> > On 5 June 2018 at 11:33, Tomasz Figa <tfiga@chromium.org> wrote:
> > > 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).
> >
> > I know this isn't part of the changes, but raises a question in
> > v4l2-compliance (so probably one for Hans).
> > testUnlimitedOpens tries opening the device 100 times. On a normal
> > device this isn't a significant overhead, but when you're allocating
> > resources on a per instance basis it quickly adds up.
> > Internally I have state that has a limit of 64 codec instances (either
> > encode or decode), so either I allocate at start_streaming and fail on
> > the 65th one, or I fail on open. I generally take the view that
> > failing early is a good thing.
> > Opinions? Is 100 instances of an M2M device really sensible?
>
> I don't think we can guarantee opening an arbitrary number of
> instances. To add to your point about resource usage, this is
> something that can be limited already on hardware or firmware level.
> Another aspect is that the hardware is often rated to decode N streams
> at resolution X by Y at Z fps, so it might not even make practical
> sense to use it to decode M > N streams.
>
> >
> > > +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
> > > +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.
> >
> > This intersects with the question I asked on the list back in April
> > but got no reply [1].
> > Is there a requirement or expectation for the encoded data to be
> > framed as a single encoded frame per buffer, or is feeding in full
> > buffer sized chunks from a ES valid? It's not stated for the
> > description of V4L2_PIX_FMT_H264 etc either.
> > If not framed then anything assuming one-in one-out fails badly, but
> > it's likely to fail anyway if the stream has reference frames.
>
> I believe we agreed on the data to be framed. The details are
> explained in "Decoding" session, but I guess it could actually belong
> to the definition of each specific pixel format.
>
> >
> > This description is also exclusive to video decode, whereas the top
> > section states "A V4L2 codec can compress, decompress, transform, or
> > otherwise convert video data". Should it be in the decoder section
> > below?
>
> Yeah, looks like it should be moved indeed.
>
> >
> > Have I missed a statement of what the Stream API is and how it differs
> > from any other API?
>
> This is a leftover that I should have removed, since this document
> continues to call this interface "Codec Interface".
>
> The other API is the "Stateless Codec Interface" mentioned below. As
> opposed to the regular (stateful) Codec Interface, it would target the
> hardware that do not store any decoding state for its own use, but
> rather expects the software to provide necessary data for each chunk
> of framed bitstream, such as headers parsed into predefined structures
> (as per codec standard) or reference frame lists. With stateless API,
> userspace would have to explicitly manage which buffers are used as
> reference frames, reordering to display order and so on. It's a WiP
> and is partially blocked by Request API, since it needs extra data to
> be given in a per-buffer manner.
>
> >
> > [1] https://www.spinics.net/lists/linux-media/msg133102.html
> >
> > > +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;
> > > +
> > > +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
> > > +
> > > +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.
> > > +
> > > +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``.
> > > +
> > > + 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.
> >
> > So in both these cases you expect index=0 to return a response with
> > the type V4L2_FRMSIZE_TYPE_DISCRETE, and the maximum resolution?
> > -EINVAL on any other index value?
> > And I assume you mean maximum coded resolution, not visible resolution.
> > Or is V4L2_FRMSIZE_TYPE_STEPWISE more appropriate? In which case the
> > minimum is presumably a single macroblock, max is the max coded
> > resolution, and step size is the macroblock size, at least on the
> > CAPTURE side.
>
> Codec size seems to make the most sense here, since that's what
> corresponds to the amount of data the decoder needs to process. Let's
> have it stated more explicitly.
>
> My understanding is that VIDIOC_ENUM_FRAMESIZES maintains its regular
> semantics here and which type of range is used would depend on the
> hardware capabilities. This actually matches to what we have
> implemented in Chromium video stack [1]. Let's state it more
> explicitly as well.
>
> [1] https://cs.chromium.org/chromium/src/media/gpu/v4l2/v4l2_device.cc?q=VIDIOC_ENUM_FRAMESIZES&sq=package:chromium&g=0&l=279
>
> >
> > > + 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.
> > > +
> > > +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
> > > +
> > > + 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;
> > > +
> > > + 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).
> >
> > I can't find V4L2_PIX_FMT_H264_SLICE in mainline. From trying to build
> > Chromium I believe it's a Rockchip special. Is it being upstreamed?
>
> This is a part of the stateless Codec Interface being in development.
> We used to call it "Slice API" internally and so the name. It is not
> specific to Rockchip, but rather the whole class of stateless codecs,
> as I explained by the way of your another comment.
>
> Any mention of it should be removed from the document for now.
>
> > Or use V4L2_PIX_FMT_H264 vs V4L2_PIX_FMT_H264_NO_SC as the example?
> > (I've just noticed I missed an instance of this further up as well).
>
> Yeah, sounds like it would be a better example.
>
> >
> > > +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).
> > > +
> > > + 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.
> > > +
> > > + .. 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.
> > > +
> > > + 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.
> >
> > I think you've just broken FFMpeg and Gstreamer with that statement.
> >
> > Gstreamer certainly doesn't subscribe to V4L2_EVENT_SOURCE_CHANGE but
> > has already parsed the stream and set the output format to the correct
> > resolution via S_FMT. IIRC it expects the driver to copy that across
> > from output to capture which was an interesting niggle to find.
> > FFMpeg does subscribe to V4L2_EVENT_SOURCE_CHANGE, although it seems
> > to currently have a bug around coded resolution != visible resolution
> > when it gets the event.
> >
> > One has to assume that these have been working quite happily against
> > various hardware platforms, so it seems a little unfair to just break
> > them.
>
> That's certainly not what existing drivers do and the examples would be:
>
> - s5p-mfc (the first codec driver in upstream) and mtk-vcodec (merged
> quite recently)
> It just ignores width/height and OUTPUT queue
> https://elixir.bootlin.com/linux/latest/source/drivers/media/platform/s5p-mfc/s5p_mfc_dec.c#L443
> and reports what the hardware parses from bitstream on CAPTURE:
> https://elixir.bootlin.com/linux/latest/source/drivers/media/platform/s5p-mfc/s5p_mfc_dec.c#L352
>
> - mtk-vcodec (merged quite recently):
> It indeed accepts whatever is set on OUTPUT as some kind of defaults,
> https://elixir.bootlin.com/linux/latest/source/drivers/media/platform/mtk-vcodec/mtk_vcodec_dec.c#L856
> but those are overridden as soon as the headers are parsed
> https://elixir.bootlin.com/linux/latest/source/drivers/media/platform/mtk-vcodec/mtk_vcodec_dec.c#L989
>
> However, the above probably doesn't prevent Gstreamer from working,
> because both drivers would allow REQBUFS(CAPTURE) before the parsing
> is done and luckily the resolution would match later after parsing.
>
> > So I guess my question is what is the reasoning for rejecting these
> > calls? If you know the resolution ahead of time, allocate buffers, and
> > start CAPTURE streaming before the event then should you be wrong
> > you're just going through the dynamic resolution change path described
> > later. If you're correct then you've saved some setup time. It also
> > avoids having to have a special startup case in the driver.
>
> We might need Pawel or Hans to comment on this, as I believe it has
> been decided to be like this in earlier Media Workshops.

I also don't see any hard reason to not let user-space configure the
CAPTURE queue itself if it has parsed the stream and decided to go
that way. I think of it also as a guaranteed to work, fallback
solution for devices that may not support the source change event - do
we know for sure that *all* stateful devices support this?

Supporting both flows would complicate the initialization protocol
quite a bit. There is a rather large and complex state machine that
all drivers need to maintain here. Maybe we could come with a "codec
framework" that would take care of this, with specific callbacks to be
implemented by drivers à la M2M?

>
> I personally don't see what would go wrong if we allow that and handle
> a fallback using the dynamic resolution change flow. Maybe except the
> need to rework the s5p-mfc driver.
>
> >
> > > +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
> >
> > So back to my earlier question, we're supporting tiny fragments of
> > frames here? Or is the thought that you can pick up anywhere in a
> > stream and the decoder will wait for the required resume point?
>
> I think this is precisely about the hardware/driver discarding
> bitstream frames until a frame containing resolution data is found. So
> that would be the latter, I believe.
>
> >
> > > + 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,
> >
> > s/Te/The
>
> Ack.
>
> >
> > > + 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.
> >
> > There are some funny cases here then.
> > Whilst memory bandwidth may limit the resolution that can be decoded
> > in real-time, for a transcode use case you haven't got a real-time
> > requirement. Enforcing this means you can never transcode that
> > resolution to RGB.
>
> I think the above is not about performance, but the general hardware
> ability to decode into such format. The bandwidth might be just not
> enough to even process one frame leading to some bus timeouts for
> example. The history of hardware design knows a lot of funny cases. :)
>
> > Actually I can't see any information related to frame rates being
> > passed in other than timestamps, therefore the driver hasn't got
> > sufficient information to make a sensible call based on memory
> > bandwidth.
>
> Again, I believe this is not about frame rate, but rather one-shot
> bandwidth needed to fetch 1 frame data without breaking things.
>
> > Perhaps it's just that the example of memory bandwidth being the
> > limitation is a bad one.
>
> Yeah, it might just be a not very good example. It could as well be
> just a fixed size static memory inside the codec hardware, which would
> obviously be capable of holding less pixels for 32-bit RGBx than
> 12-bit (in average) YUV420.
>
> >
> > > + 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).
> > > +
> > > + 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.
> >
> > Only recommended sounds like a great case for clients to treat codecs
> > as one-in one-out, and then fall over if you get extra header byte
> > frames in the stream.
>
> I think the meaning of "operating the queues independently" is a bit
> different here, e.g. from separate threads.
>
> But agreed that we need to make sure that the documentation explicitly
> says that there is neither one-in one-out guarantee nor 1:1 relation
> between OUT and CAP buffers, if it doesn't say it already.
>
> >
> > > +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;
> >
> > This appears to be answering my earlier question, but doesn't it
> > belong in the definition of V4L2_PIX_FMT_H264 rather than buried in
> > the codec description?
> > I'm OK with that choice, but you are closing off the use case of
> > effectively cat'ing an ES into the codec to be decoded.
>
> I think it would indeed make sense to make this behavior a part of the
> pixel format. Pawel, what do you think?
>
> >
> > There's the other niggle of how to specify sizeimage in the
> > pixelformat for compressed data. I have never seen a satisfactory
> > answer in most of the APIs I've encountered (*). How big can an
> > I-frame be in a random stream? It may be a very badly coded stream,
> > but if other decoders can cope, then it's the decoder that can't which
> > will be seen to be buggy.
>
> That's a very good question. I think we just empirically came up with
> some values that seem to work in Chromium:
> https://cs.chromium.org/chromium/src/media/gpu/v4l2/v4l2_slice_video_decode_accelerator.h?rcl=eed597a7f14cb03cd7db9d9722820dddd86b4c41&l=102
> https://cs.chromium.org/chromium/src/media/gpu/v4l2/v4l2_video_decode_accelerator.cc?rcl=eed597a7f14cb03cd7db9d9722820dddd86b4c41&l=2241
>
> Pawel, any background behind those?
>
> >
> > (* ) OpenMAX IL is the exception as you can pass partial frames with
> > appropriate values in nFlags. Not many other positives one can say
> > about IL though.
> >
> > > +- 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)
> > > +
> > > +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.
> > > +
> > > +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.
> > > +
> > > + - 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.
> > > +
> > > + - 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
> >
> > s/not/no
>
> Ack.
>
> >
> > > +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.
> >
> > How does the driver ensure the last buffer gets that flag? You may not
> > have had the new header bytes queued to the OUTPUT queue before the
> > previous frame has been decoded and dequeued on the CAPTURE queue.
> > Empty buffer with the flag set?
>
> Yes, an empty buffer. I think that was explained by the way of the
> general flush sequence later. We should state it here as well.
>
> >
> > > + .. 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.
> > > +
> > > +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.
> > > +
> > > +Flush
> > > +-----
> > > +
> > > +Flush is the process of draining the CAPTURE queue of any remaining
> > > +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
> > > + set ``V4L2_BUF_FLAG_LAST`` in :c:type:`v4l2_buffer` ``flags`` field on the
> > > + 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`.
> >
> > I guess that answers my earlier question on resolution change when
> > there are no CAPTURE buffers left to be delivered.
> >
> > > + 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``.
> > > +
> > > +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`.
> > > +
> > > +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.
> > > --
> > > 2.17.1.1185.g55be947832-goog
> >
> > Related to an earlier comment, whilst the driver has to support
> > multiple instances, there is no arbitration over the overall decode
> > rate with regard real-time performance.
> > I know our hardware is capable of 1080P60, but there's no easy way to
> > stop someone trying to decode 2 1080P60 streams simultaneously. From a
> > software perspective it'll do it, but not in real-time. I'd assume
> > most other platforms will give the similar behaviour.
> > Is it worth adding a note that real-time performance is not guaranteed
> > should multiple instances be running simultaneously, or a comment made
> > somewhere about expected performance? Or enforce it by knowing the max
> > data rates and analysing the level of each stream (please no)?
>
> This is a very interesting problem in general.
>
> I believe we don't really do anything like the latter in Chromium and
> if someone tries to play too many videos, they would just start
> dropping frames. (Pawel, correct me if I'm wrong.) It's actually
> exactly what would happen if one starts too many videos with software
> decoder running on CPU (and possibly with less instances).
>
> Best regards,
> Tomasz

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