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Subject: Re: [RFC PATCH 2/2] media: docs-rst: Add encoder UAPI specification
 to Codec Interfaces
From:   Philipp Zabel <p.zabel@pengutronix.de>
To:     Hans Verkuil <hverkuil@xs4all.nl>, Tomasz Figa <tfiga@chromium.org>
Cc:     Pawel Osciak <posciak@chromium.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Stanimir Varbanov <stanimir.varbanov@linaro.org>,
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        Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Date:   Thu, 07 Jun 2018 12:32:23 +0200
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On Thu, 2018-06-07 at 09:27 +0200, Hans Verkuil wrote:
[...]
> > > > > I think it could be useful to enforce the same colorimetry on CAPTURE
> > > > > and OUTPUT queue if the hardware doesn't do any colorspace conversion.
> > > > 
> > > > After thinking a bit more on this, I guess it wouldn't overly
> > > > complicate things if we require that the values from OUTPUT queue are
> > > > copied to CAPTURE queue, if the stream doesn't include such
> > > > information or the hardware just can't parse them.
> > > 
> > > And for encoders it would be copied from CAPTURE queue to OUTPUT queue?
> > > 
> > 
> > I guess iy would be from OUTPUT to CAPTURE for encoders as well, since
> > the colorimetry of OUTPUT is ultimately defined by the raw frames that
> > userspace is going to be feeding to the encoder.
> 
> Correct. All mem2mem drivers should just copy the colorimetry from the
> output buffers to the capture buffers, unless the decoder hardware is able to
> extract that data from the stream, in which case it can overwrite it for
> the capture buffer.
> 
> Currently colorspace converters are not supported since the V4L2 API does
> not provide a way to let userspace define colorimetry for the capture queue.

Oh, I never realized this limitation [1] ...

 "Image colorspace, from enum v4l2_colorspace. This information
  supplements the pixelformat and must be set by the driver for capture
  streams and by the application for output streams, see Colorspaces."

[1] https://linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/pixfmt-v4l2.html

It's just a bit unintuitive that the initialization sequence requires to
set S_FMT(CAP) first and then S_FMT(OUT) but with colorspace there is
information that flows the opposite way.

> I have a patch to add a new v4l2_format flag for that since forever, but
> since we do not have any drivers that can do this in the kernel it has never
> been upstreamed.

Has this patch been posted some time? I think we could add a mem2mem
device to imx-media with support for linear transformations.

regards
Philipp