linux-kernel - Re: [PATCH 0/4] drm/tiny: Add driver for Solomon SSD1307 OLED displays

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Date:   Tue, 1 Feb 2022 12:07:07 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Pekka Paalanen <ppaalanen@...il.com>
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Simon Ser <contact@...rsion.fr>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Noralf Trønnes <noralf@...nnes.org>,
        David Airlie <airlied@...ux.ie>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Maxime Ripard <maxime@...no.tech>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Lee Jones <lee.jones@...aro.org>
Subject: Re: [PATCH 0/4] drm/tiny: Add driver for Solomon SSD1307 OLED displays

Hi Pekka,

On Tue, Feb 1, 2022 at 11:42 AM Pekka Paalanen <ppaalanen@...il.com> wrote:
> On Tue, 1 Feb 2022 10:49:03 +0100
> Javier Martinez Canillas <javierm@...hat.com> wrote:
> > On 2/1/22 09:38, Daniel Vetter wrote:
> > > On Tue, Feb 1, 2022 at 9:34 AM Simon Ser <contact@...rsion.fr> wrote:
> > >> On Tuesday, February 1st, 2022 at 09:26, Geert Uytterhoeven <geert@...ux-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
> >
> > To sump up the conversation in the #dri-devel channel, these drivers
> > should support the following formats:
> >
> > 1) Dx (Daniel suggested that for darkness, but inverted mono)
>
> Did you consider format C1 instead?

That would be a 2-color display, which is not necessarily black
and white. Cfr. Amiga or Atari bit planes with bpp=1.
That's why fbdev has separate visuals for monochrome.

> I have no idea how this would map to fbdev API though.

    #define FB_VISUAL_MONO01                0       /* Monochr.
1=Black 0=White */
    #define FB_VISUAL_MONO10                1       /* Monochr.
1=White 0=Black */
    #define FB_VISUAL_TRUECOLOR             2       /* True color   */

The above is RGB (or grayscale, see below).

    #define FB_VISUAL_PSEUDOCOLOR           3       /* Pseudo color
(like atari) */

Palette

    #define FB_VISUAL_DIRECTCOLOR           4       /* Direct color */

Usually used as RGB with gamma correction, but the actual hardware
is more flexible.

    #define FB_VISUAL_STATIC_PSEUDOCOLOR    5       /* Pseudo color readonly */

Fixed palette

And:

    struct fb_var_screeninfo {
            ...
            __u32 grayscale;                /* 0 = color, 1 = grayscale,    */

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds