Re: [RFC/RFT PATCH 4/4] drm/bridge: dw-hdmi: Take input format from plat_data

[Hans Verkuil]

On 01/19/17 16:21, Jose Abreu wrote:
> Hi Laurent,
>
>
> On 18-01-2017 20:49, Laurent Pinchart wrote:
> > Ideally the bridge mode set operation should be extended to take format and
> > colorspace information (or another bridge operation should be created for that
> > purpose, I'm still undecided). As that's quite a big change, I'm fine if you
> > start by passing a fixed format and colorspace through platform data. I would
> > however like the format to use the media bus format codes defined in
> > include/uapi/linux/media-bus-format.h.
> >
> > As far as I'm aware we have no colorspace definitions in DRM, so we would need
> > to fix that. V4L2 handles colorspaces, and the API went through several
> > iterations before we got it working, with stupid mistakes that we don't want
> > to repeat here.
> >
> > Jose pointed to https://urldefense.proofpoint.com/v2/url?u=https-3A__patchwork.kernel.org_patch_9495153_&d=DwICAg&c=DPL6_X_6JkXFx7AXWqB0tg&r=WHDsc6kcWAl4i96Vm5hJ_19IJiuxx_p_Rzo2g-uHDKw&m=BkyajeUCra1JNwll2fmexnvnkIDglOZPocC1ibkiNCQ&s=V21eO0BUlcWrbddiKml5I6ylkiX2ALOHPHmble7kxwI&e=  as a starting
> > point, and I would like to point out the format and colorspace are two
> > different things. A colorspace is a combination of an encoding and
> > quantization and transfer functions. Hans Verkuil has researched this topic
> > for V4L2 (see https://urldefense.proofpoint.com/v2/url?u=https-3A__gstreamer.freedesktop.org_data_events_gstreamer-2Dconference_2015_Hans&d=DwICAg&c=DPL6_X_6JkXFx7AXWqB0tg&r=WHDsc6kcWAl4i96Vm5hJ_19IJiuxx_p_Rzo2g-uHDKw&m=BkyajeUCra1JNwll2fmexnvnkIDglOZPocC1ibkiNCQ&s=z9IMQn5gPepKFfsLUwUXiG8MA2s1SBB1jyQDE0JIKdk&e=  Verkuil - Colorspaces and HDMI.pdf and
> > https://urldefense.proofpoint.com/v2/url?u=https-3A__linuxtv.org_downloads_v4l-2Ddvb-2Dapis_uapi_v4l_colorspaces.html&d=DwICAg&c=DPL6_X_6JkXFx7AXWqB0tg&r=WHDsc6kcWAl4i96Vm5hJ_19IJiuxx_p_Rzo2g-uHDKw&m=BkyajeUCra1JNwll2fmexnvnkIDglOZPocC1ibkiNCQ&s=QReIV1CxgeALvbP5QObIvfwIh7hjEL8J4fxiATaMVYc&e= ), we
> > *really* want to involve him in this discussion.
> >
> > I believe colorspace information should be shared between V4L2 and DRM the
> > same way we share the media bus formats (We should have done so for 4CCs as
> > well but it's unfortunately too late for that).
>
> Hmm, I am always confusing this :/ So, what I was refering as
> "color space" is in reality the encoding (pixel encoding). In
> HDMI the pixel encoding can be RGB 4:4:4, YCbCr 4:4:4, YCbCr
> 4:2:2 and YCbCr 4:2:0. We also have color depth which can be from
> 8-bit to 16-bit. Besides this there is also colorimetry.
>
> I've used media-bus-format.h to successfully pass information
> across a V4L2 driver but I've used it in BGR24 only, which is RGB
> 4:4:4 8 bit, which in media-bust-format.h would correspond to
> MEDIA_BUS_FMT_BGR888_1X24. So, I don't know exactly what is the
> support for other encodings and what if there's support for color
> depth. If there is then great, but if not then support for this
> should also be added.
>
> DRM already parses successfully the supported encodings from EDID
> and stores them in a structure. Note that this is the output
> encoding, not the input one. We can still have RGB as input and
> then output at YCbCr using CSC. The missing point is the
> selection of encoding (either from userspace or from some sort of
> automatic mechanism by the kernel).

The list of supported encodings for video capture depends on the HW
capabilities. So the driver will list the supported formats (ENUMFMT)
and userspace then selects a format (S_FMT) from that list.

Most HDMI receivers can convert the input to various RGB/YCbCr formats.

Deep color support for HDMI has not been added (surprisingly nobody needed
it apparently), but it is pretty easy: new V4L2_PIX_FMT defines should be
added for those.

When talking about RGB/YCbCr I prefer the term 'color encoding', since
this has nothing to do with colorspaces (sRGB. AdobeRGB, BT.2020, etc.)

Regards,

	Hans