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Message-ID: <1532526073.4879.6.camel@pengutronix.de>
Date: Wed, 25 Jul 2018 15:41:13 +0200
From: Philipp Zabel <p.zabel@...gutronix.de>
To: Tomasz Figa <tfiga@...omium.org>, linux-media@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
Stanimir Varbanov <stanimir.varbanov@...aro.org>,
Mauro Carvalho Chehab <mchehab@...nel.org>,
Hans Verkuil <hverkuil@...all.nl>,
Pawel Osciak <posciak@...omium.org>,
Alexandre Courbot <acourbot@...omium.org>, kamil@...as.org,
a.hajda@...sung.com, Kyungmin Park <kyungmin.park@...sung.com>,
jtp.park@...sung.com,
Tiffany Lin (林慧珊)
<tiffany.lin@...iatek.com>,
Andrew-CT Chen (陳智迪)
<andrew-ct.chen@...iatek.com>, todor.tomov@...aro.org,
nicolas@...fresne.ca,
Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
Laurent Pinchart <laurent.pinchart@...asonboard.com>,
Dave Stevenson <dave.stevenson@...pberrypi.org>,
ezequiel@...labora.com
Subject: Re: [PATCH 2/2] media: docs-rst: Document memory-to-memory video
encoder interface
On Tue, 2018-07-24 at 23:06 +0900, Tomasz Figa wrote:
> 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@...omium.org>
Thanks a lot for the update,
> ---
> Documentation/media/uapi/v4l/dev-encoder.rst | 550 +++++++++++++++++++
> Documentation/media/uapi/v4l/devices.rst | 1 +
> Documentation/media/uapi/v4l/v4l2.rst | 2 +
> 3 files changed, 553 insertions(+)
> create mode 100644 Documentation/media/uapi/v4l/dev-encoder.rst
>
> diff --git a/Documentation/media/uapi/v4l/dev-encoder.rst b/Documentation/media/uapi/v4l/dev-encoder.rst
> new file mode 100644
> index 000000000000..28be1698e99c
> --- /dev/null
> +++ b/Documentation/media/uapi/v4l/dev-encoder.rst
> @@ -0,0 +1,550 @@
> +.. -*- coding: utf-8; mode: rst -*-
> +
> +.. _encoder:
> +
> +****************************************
> +Memory-to-memory Video Encoder Interface
> +****************************************
> +
> +Input data to a video encoder are raw video frames in display order
> +to be encoded into the output bitstream. Output data are complete chunks of
> +valid bitstream, including all metadata, headers, etc. The resulting stream
> +must not need 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 Stateless Video Encoder Interface (in
> +development) is strongly advised.
> +
[...]
> +
> +Commit points
> +=============
> +
> +Setting formats and allocating buffers triggers changes in the behavior
> +of the driver.
> +
> +1. Setting format on ``CAPTURE`` queue may change the set of formats
> + supported/advertised on the ``OUTPUT`` queue. In particular, it also
> + means that ``OUTPUT`` format may be reset and the client must not
> + rely on the previously set format being preserved.
Since the only property of the CAPTURE format that can be set directly
via S_FMT is the pixelformat, should this be made explicit?
1. Setting pixelformat on ``CAPTURE`` queue may change the set of
formats supported/advertised on the ``OUTPUT`` queue. In particular,
it also means that ``OUTPUT`` format may be reset and the client
must not rely on the previously set format being preserved.
?
> +2. Enumerating formats on ``OUTPUT`` queue must only return formats
> + supported for the ``CAPTURE`` format currently set.
Same here, as it usually is the codec selected by CAPTURE pixelformat
that determines the supported OUTPUT pixelformats and resolutions.
2. Enumerating formats on ``OUTPUT`` queue must only return formats
supported for the ``CAPTURE`` pixelformat currently set.
This could prevent the possible misconception that the CAPTURE
width/height might in any form limit the OUTPUT format, when in fact it
is determined by it.
> +3. Setting/changing format on ``OUTPUT`` queue does not change formats
> + available on ``CAPTURE`` queue.
3. Setting/changing format on the ``OUTPUT`` queue does not change
pixelformats available on the ``CAPTURE`` queue.
?
Because setting OUTPUT width/height or CROP SELECTION very much limits
the possible values of CAPTURE width/height.
Maybe 'available' in this context should be specified somewhere to mean
'returned by ENUM_FMT and allowed by S_FMT/TRY_FMT'.
> + An attempt to set ``OUTPUT`` format that
> + is not supported for the currently selected ``CAPTURE`` format must
> + result in the driver adjusting the requested format to an acceptable
> + one.
An attempt to set ``OUTPUT`` format that is not supported for the
currently selected ``CAPTURE`` pixelformat must result in the driver
adjusting the requested format to an acceptable one.
> +4. Enumerating formats on ``CAPTURE`` queue always returns the full set of
> + supported coded formats, irrespective of the current ``OUTPUT``
> + format.
> +
> +5. After allocating buffers on the ``CAPTURE`` queue, it is not possible to
> + change format on it.
> +
> +To summarize, setting formats and allocation must always start with the
> +``CAPTURE`` queue and the ``CAPTURE`` queue is the master that governs the
> +set of supported formats for the ``OUTPUT`` queue.
To summarize, setting formats and allocation must always start with
setting the encoded pixelformat on the ``CAPTURE`` queue. The
``CAPTURE`` queue is the master that governs the set of supported
formats for the ``OUTPUT`` queue.
Or is this too verbose?
regards
Philipp
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