linux-kernel - Re: [PATCH v6 2/3] drm/st7571-i2c: add support for Sitronix ST7571 LCD controller

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Message-ID: <aBBukAqH3SKV9_Gl@gmail.com>
Date: Tue, 29 Apr 2025 08:15:44 +0200
From: Marcus Folkesson <marcus.folkesson@...il.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, dri-devel@...ts.freedesktop.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Thomas Zimmermann <tzimmrmann@...e.de>,
	Javier Martinez Canillas <javierm@...hat.com>
Subject: Re: [PATCH v6 2/3] drm/st7571-i2c: add support for Sitronix ST7571
 LCD controller

Hello Geert,

On Thu, Apr 24, 2025 at 10:38:33AM +0200, Geert Uytterhoeven wrote:

[...]

> > +                       /*
> > +                        * As the display supports grayscale, all pixels must be written as two bits
> > +                        * even if the format is monochrome.
> > +                        *
> > +                        * The bit values maps to the following grayscale:
> > +                        * 0 0 = White
> > +                        * 0 1 = Light gray
> > +                        * 1 0 = Dark gray
> > +                        * 1 1 = Black
> 
> That is not R2, but D2?
> include/uapi/drm/drm_fourcc.h:
> 
>     /* 2 bpp Red (direct relationship between channel value and brightness) */
>     #define DRM_FORMAT_R2             fourcc_code('R', '2', ' ', ' ')
> /* [7:0] R0:R1:R2:R3 2:2:2:2 four pixels/byte */
> 
>     /* 2 bpp Darkness (inverse relationship between channel value and
> brightness) */
>     #define DRM_FORMAT_D2             fourcc_code('D', '2', ' ', ' ')
> /* [7:0] D0:D1:D2:D3 2:2:2:2 four pixels/byte */
> 
> So the driver actually supports D1 and D2, and XRGB8888 should be
> inverted while converting to monochrome (and grayscale, which is not
> yet implemented).

The display supports "reverse" grayscale, so the mapping becomes
1 1 = White
1 0 = Light gray
0 1 = Dark gray
0 0 = Black
instead.

So I will probably add support for D1 and D2 formats and invert the
pixels for the R1, R2 and XRGB8888 formats.

Could that work or are there any side effects that I should be aware of?

Best regards,
Marcus Folkesson

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