[FFmpeg-devel] [PATCH v2] doc/v4l2_m2m: Add documentation

[Andriy Gelman]

On Thu, 16. Jan 10:51, Gyan wrote:
> 
> 
> On 16-01-2020 09:44 am, Andriy Gelman wrote:
> > From: Andriy Gelman <andriy.gelman at gmail.com>
> > 
> > Signed-off-by: Andriy Gelman <andriy.gelman at gmail.com>
> > ---
> >   doc/decoders.texi | 27 +++++++++++++++++++++++++++
> >   doc/encoders.texi | 42 ++++++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 69 insertions(+)
> > 
> > diff --git a/doc/decoders.texi b/doc/decoders.texi
> > index f18226b3504..7827a055408 100644
> > --- a/doc/decoders.texi
> > +++ b/doc/decoders.texi
> > @@ -86,6 +86,33 @@ AVS2-P2/IEEE1857.4 video decoder wrapper.
> >   This decoder allows libavcodec to decode AVS2 streams with davs2 library.
> > + at section v4l2m2m
> > +
> > +libavcodec supports a set of v4l2m2m wrappers for interfacing with
> > +hardware decoders. Depending on the hardware's capabilties the following decoders may be selected:
> > +h264, hevc, mpeg1, mpeg2, mpeg4, h263, vc1, vp8, and vp9.

> 
> capabilties --> capabilities
> 
> 
> > +
> > +To use a specifix decoder append a  _v4l2m2m suffix. For example to select h264
> 
> specifix --> specific
> 

Fixed both spelling mistakes.

> > +decoder use:
> > + at example
> > +ffmpeg -codec:v h264_v4l2m2m -i INPUT OUTPUT
> > + at end example
> > +
> > +The following options are supported:
> > + at table @option
> > + at item num_output_buffers
> > +Number of memory mapped buffers to store the input packets.
> > +This value is only a suggestion to the hardware device. The device will attempt to
> > +allocate the number of buffers, but the actual value may be smaller/larger and
> > +ultimately depends on the device. The default for the option is 16, minimum is
> > +6, and any large value (representable by an int) is accepted for the max.
> > +
> > + at item num_capture_buffers
> > +Number of memory mapped buffers to store the decompressed frames. The allocation
> > +description is the same as above. Default is 20, minimum is 20, and any large
> > +value (representable by an int) is accepted for the max.
> 
> Restate the description, don't reference another option entry, since in
> theory, more may be inserted or shuffled around.

ok done.

> 
> For both options, what's the significance and tradeoff? Latency?
> 

These options don't affect latency. Having a large buffer could be useful if
there is a lot of jitter in dequeuing the decoded frames from the hardware
device. If the internal buffer is full you will end up dropping frames. 

> > + at end table
> > +
> >   @c man end VIDEO DECODERS
> >   @chapter Audio Decoders
> > diff --git a/doc/encoders.texi b/doc/encoders.texi
> > index 61e674cf968..a600e76b1b9 100644
> > --- a/doc/encoders.texi
> > +++ b/doc/encoders.texi
> > @@ -3117,6 +3117,48 @@ required to produce a stream usable with all decoders.
> >   @end table
> > + at section v4l2m2m
> > +
> > +libavcodec supports a set of v4l2m2m wrappers for interfacing with hardware encoders.
> > +Depending on the hardware's capabilities the following encoders may be selected:
> > +mpeg4, h263, h264, hevc, and vp8.
> > +
> > +To use a specific encoder append _v4l2m2m suffix. For example to select h264 use:
> > + at example
> > +ffmpeg -i INPUT [-pix_fmt pixfmt] -codec:v h264_v4l2m2m OUTPUT
> > + at end example
> > +In some cases, it may be necessary to insert a pixel format conversion with
> > + at code{-pix_fmt}. This is required if the pixel format of the input does not
> > +match the format of the encoder. If there is a mismatch, libavcodec will
> > +exit and specify the required pixfmt to use.
> > +
> > +The following options are supported:
> > + at table @option
> > + at item num_output_buffers
> > +Number of memory mapped buffers to store the input frames.
> > +This value is only a suggestion to the hardware device. The device will attempt to
> > +allocate the number of buffers, but the actual value may be smaller/larger and
> > +ultimately depends on the device. The default for the option is 16, minimum is
> > +6, and any large value (representable by an int) is accepted for the max.
> > +
> > + at item num_capture_buffers
> > +Number of memory mapped buffers to store the compressed packets. The allocation
> > +description is the same as above. Default is 4, minimum is 4, and any large
> > +value (representable by an int) is accepted for the max.
> 
> Restate the description, don't reference another option entry.
> 
>  Significance and tradeoff?
> 
> > + at end table
> > +
> > +Standard libavcodec options that can be set are:
> > + at itemize
> > + at item
> > + at option{g} / @option{gop_size}
> > + at item
> > + at option{qpel}
> > + at item
> > + at option{qmin}
> > + at item
> > + at option{qmax}

> 
> Where in the code is the lavc qmin/qmax applied?

These options are set in v4l2_prepare_encoder().

> 
> You may note that bitrate is also read.

yep, I've added bitrate.

Thanks,
-- 
Andriy