On Wed, Jun 10, 2020 at 3:08 PM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote: > > Le mercredi 10 juin 2020 à 14:28 +0200, Tomasz Figa a écrit : > > On Fri, Jun 5, 2020 at 11:24 PM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote: > > > Le vendredi 05 juin 2020 à 10:19 +0300, Stanimir Varbanov a écrit : > > > > On 5/26/20 1:09 PM, Hans Verkuil wrote: > > > > > From: Tomasz Figa <tfiga@xxxxxxxxxxxx> > > > > > > > > > > Due to complexity of the video encoding process, the V4L2 drivers of > > > > > stateful encoder hardware require specific sequences of V4L2 API calls > > > > > to be followed. These include capability enumeration, initialization, > > > > > encoding, encode parameters change, drain and reset. > > > > > > > > > > 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 coda. > > > > > > > > > > The only thing missing was the real specification included as a part of > > > > > Linux Media documentation. Fix it now and document the encoder part of > > > > > the Codec API. > > > > > > > > > > Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx> > > > > > Signed-off-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx> > > > > > --- > > > > > .../userspace-api/media/v4l/dev-encoder.rst | 728 ++++++++++++++++++ > > > > > .../userspace-api/media/v4l/dev-mem2mem.rst | 1 + > > > > > .../userspace-api/media/v4l/pixfmt-v4l2.rst | 5 + > > > > > .../userspace-api/media/v4l/v4l2.rst | 2 + > > > > > .../media/v4l/vidioc-encoder-cmd.rst | 51 +- > > > > > 5 files changed, 767 insertions(+), 20 deletions(-) > > > > > create mode 100644 Documentation/userspace-api/media/v4l/dev-encoder.rst > > > > > > > > > > diff --git a/Documentation/userspace-api/media/v4l/dev-encoder.rst b/Documentation/userspace-api/media/v4l/dev-encoder.rst > > > > > new file mode 100644 > > > > > index 000000000000..aace7b812a9c > > > > > --- /dev/null > > > > > +++ b/Documentation/userspace-api/media/v4l/dev-encoder.rst > > > > > @@ -0,0 +1,728 @@ > > > > > +.. SPDX-License-Identifier: GPL-2.0 > > > > > + > > > > > +.. _encoder: > > > > > + > > > > > +************************************************* > > > > > +Memory-to-Memory Stateful Video Encoder Interface > > > > > +************************************************* > > > > > + > > > > > +A stateful video encoder takes raw video frames in display order and encodes > > > > > +them into a bytestream. It generates complete chunks of the bytestream, including > > > > > +all metadata, headers, etc. The resulting bytestream does not require any > > > > > +further post-processing by the client. > > > > > + > > > > > +Performing software stream processing, header generation etc. in the driver > > > > > +in order to support this interface is strongly discouraged. In case such > > > > > +operations are needed, use of the Stateless Video Encoder Interface (in > > > > > +development) is strongly advised. > > > > > + > > > > > > > > <cut> > > > > > > > > > +Encoding Parameter Changes > > > > > +========================== > > > > > + > > > > > +The client is allowed to use :c:func:`VIDIOC_S_CTRL` to change encoder > > > > > +parameters at any time. The availability of parameters is encoder-specific > > > > > +and the client must query the encoder to find the set of available controls. > > > > > + > > > > > +The ability to change each parameter during encoding is encoder-specific, as > > > > > +per the standard semantics of the V4L2 control interface. The client may > > > > > +attempt to set a control during encoding and if the operation fails with the > > > > > +-EBUSY error code, the ``CAPTURE`` queue needs to be stopped for the > > > > > +configuration change to be allowed. To do this, it may follow the `Drain` > > > > > +sequence to avoid losing the already queued/encoded frames. > > > > > + > > > > > +The timing of parameter updates is encoder-specific, as per the standard > > > > > +semantics of the V4L2 control interface. If the client needs to apply the > > > > > +parameters exactly at specific frame, using the Request API > > > > > +(:ref:`media-request-api`) should be considered, if supported by the encoder. > > > > > > > > In request-api case does the control will return EBUSY immediately when > > > > S_CTRL is called from the client? I'm asking in the context of the > > > > controls which are not dynamic (cannot set during streaming and Reset > > > > sequence is needed). > > > > > > This is all hypothetical, as nothing of that has been implemented. But > > > I suppose there should be instant validation to allow that, even if > > > that means more plumbing inside the kernel. It would be better then > > > just running the request ignoring silently that control. Ideally > > > userspace should not have to go into trial and errors, so controls that > > > are means for the should be marked. > > > > > > > There isn't much to be implemented here, as it's the same standard > > V4L2 behavior as existed for a while. If a device supports changing > > the control on the fly, a call to S_CTRL succeeds, if not, it fails > > with -EBUSY. This is regardless of whether requests are used or not. > > Are you against introducing flags to signal generically when a control > will have live effect, delayed effect or will simply be rejected (- > EBUSY) if set while streaming ? I'm particularly concern for cases > where the same control may work live for one driver, and not for > another. It would be very disappointing to get an EBUSY on a bitrate > control of an encoder as an example. So if someone foresee having > bitrate control that cannot be adapted while streaming (or that will > only be applied if a key frame is generated), I'd rather prefer to know > in advance, and likely not select this encoder for WebRTC purpose as an > example. Having to do all the steps to get the driver in streaming in > order to set the control and discover that it's not suitable for the > purpose seems quite sub-optimal. > > I'm just saying, but the "standard" that existed for a while has always > been hardcoded to the control (through control documentation, and > that's fine with me) or the HW driver being used (which isn't fine in > my opinion). I was just clarifying the behavior of S_(EXT_)CTRL(S). A way to query the behavior in advance is an orthogonal problem, which I agree that should be solved and I guess a flag returned by VIDIOC_QUERY_EXT_CTRLS could do. Best regards, Tomasz
