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

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Date:   Tue, 1 Feb 2022 10:49:03 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Simon Ser <contact@...rsion.fr>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Mark Brown <broonie@...nel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Noralf Trønnes <noralf@...nnes.org>,
        Maxime Ripard <maxime@...no.tech>,
        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

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)
2) Rx (single-channel for grayscale)
3) RxGxBxAx (4-channel fake 32-bpp truecolor)

The format preference will be in that order, so if user-space is able
to use Dx then there won't be a need for any conversion and just the
native format will be used.

If using Rx then only a Rx -> Dx conversion will happen and the last
format will require the less performant RxGxBxAx -> Rx -> Dx path.

But we still need RxGxBxAx as a fallback for compatibility with the
existing user-space, so all this could be done as a follow-up as an
optimization and shouldn't block monochromatic panel drivers IMO.

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat