linux-kernel - Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

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Date:   Mon, 17 Jan 2022 14:51:07 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Gerd Hoffmann <kraxel@...hat.com>
Cc:     Helge Deller <deller@....de>, Daniel Vetter <daniel@...ll.ch>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "airlied@...il.com" <airlied@...il.com>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Javier Martinez Canillas <javierm@...hat.com>
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

Hi

Am 17.01.22 um 14:29 schrieb Geert Uytterhoeven:
> Hi Gerd,
> 
> On Mon, Jan 17, 2022 at 1:57 PM Gerd Hoffmann <kraxel@...hat.com> wrote:
>>> b) to include new drivers (for old hardware) if they arrive (probably happens rarely but there can be).
>>>     I know of at least one driver which won't be able to support DRM....
>>
>> Hmm?  I seriously doubt that.  There is always the option to use a
>> shadow framebuffer, then convert from standard drm formats to whatever
>> esoteric pixel format your hardware expects.
>>
>> Been there, done that.  Have a look at the cirrus driver.  The physical
>> hardware was designed in the early 90-ies, almost 30 years ago.  These
>> days it exists in virtual form only (qemu emulates it).  Thanks to the
>> drm driver it runs wayland just fine even though it has a bunch of
>> constrains dictated by the hardware design.
> 
> The Cirrus DRM driver supports TrueColor (RGB565/888 and ARGB8888)
> modes only.  The Cirrus fbdev driver also supports mochrome and 256
> color modes.
> 
> There exist some DRM drivers that do support DRM_FORMAT_C8, but none of
> the "tiny" ones do. Same for DRM_FORMAT_RGB{332,233}.  Using a shadow
> frame buffer to convert from truecolor to 256 colors would be doable,
> but would give bad results. And what about less colors?
> Adding support for e.g. DRM_FORMAT_C4 is not straight-forward, as
> the DRM core assumes in many places that a pixel is at least 1 byte,
> and would crash otherwise (yes I tried).  Other modes needed are
> DRM_FORMAT_Y4 and DRM_FORMAT_{BW,WB} (monochrome).

We export XRGB32 from each driver, because userspace expects it. But 
that is not a hard requirement. Userspace can use any format. It's just 
that no one seems to have any use cases so far, so no work has been 
done. Think of XRGB32 as a fallback.

Personally, I'd much appreciate if userspace would support more of the 
native formats and not rely on XRGB32.


> This not only to support "old" hardware, but also modern small OLED
> and e-ink displays.

There's a DRM driver for Repaper e-Ink displays. So it seems doable at 
least.

Best regards
Thomas

> 
> On the positive side: DRM would force e.g. the Amiga and Atari
> bitplane formats to become internal to the kernel driver, with the
> kernel driver converting from packed pixels to bitplanes.  Hence
> userspace would no longer have to care about bitplanes.
> 
> 
> 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

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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