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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Mon, 17 Jan 2022 15:10:00 +0100
Message-ID: <CAMuHMdW5M=zEuGEnQQc3JytDhoxCKRiq0QFw+HOPp0YMORzidw@mail.gmail.com>
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer
To:     Thomas Zimmermann <tzimmermann@suse.de>
Cc:     Gerd Hoffmann <kraxel@redhat.com>, Helge Deller <deller@gmx.de>,
        Daniel Vetter <daniel@ffwll.ch>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "airlied@gmail.com" <airlied@gmail.com>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Javier Martinez Canillas <javierm@redhat.com>
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Hi Thomas,

On Mon, Jan 17, 2022 at 2:51 PM Thomas Zimmermann <tzimmermann@suse.de> wrote:
> Am 17.01.22 um 14:29 schrieb Geert Uytterhoeven:
> > On Mon, Jan 17, 2022 at 1:57 PM Gerd Hoffmann <kraxel@redhat.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.

Using an XRGB32 intermediate would kill the user experience on old
machines, due to both increased memory usage and copy overhead.

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

Supporting monochrome, 16 colors, and 256 colors would be nice.

> > 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.

Which uses an DRM_FORMAT_XRGB8888 intermediate, and
drm_fb_xrgb8888_to_gray8() and repaper_gray8_to_mono_reversed()
to convert from truecolor to monochrome.  I guess that would work,
as this is a slow e-ink display.  Have fun as a text console ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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