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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue, 1 Feb 2022 09:38:38 +0100
Message-ID: <CAKMK7uGPuhrDf8fdDgfuPt5rzO30Rm54T7GvWb203NRbVoVDgw@mail.gmail.com>
Subject: Re: [PATCH 0/4] drm/tiny: Add driver for Solomon SSD1307 OLED displays
To:     Simon Ser <contact@emersion.fr>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux PWM List <linux-pwm@vger.kernel.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Mark Brown <broonie@kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Liam Girdwood <lgirdwood@gmail.com>,
        =?UTF-8?Q?Noralf_Tr=C3=B8nnes?= <noralf@tronnes.org>,
        Maxime Ripard <maxime@cerno.tech>,
        =?UTF-8?Q?Uwe_Kleine=2DK=C3=B6nig?= 
        <u.kleine-koenig@pengutronix.de>,
        Thierry Reding <thierry.reding@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Lee Jones <lee.jones@linaro.org>
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On Tue, Feb 1, 2022 at 9:34 AM Simon Ser <contact@emersion.fr> wrote:
>
> On Tuesday, February 1st, 2022 at 09:26, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> > What's the story with the Rn formats?
> >
> > The comments say "n bpp Red", while this is a monochrome (even
> > inverted) display?
>
> I don't think the color matters that much. "Red" was picked just because it was
> an arbitrary color, to make the difference with e.g. C8. Or am I mistaken?

The red comes from gl, where with shaders it really doesn't matter
what meaning you attach to channels, but really just how many you
have. So 2-channel formats are called RxGx, 3-channel RxGxBx,
4-channel RxGxBxAx and single-channel Rx. And we use drm_fourcc for
interop in general, hence why these exist.

We should probably make a comment that this really isn't a red channel
when used for display it's a greyscale/intensity format. Aside from
that documentation gap I think reusing Rx formats for
greyscale/intensity for display makes perfect sense.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch