Hi Thomas,
On Mon, Jan 17, 2022 at 2:51 PM Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
> Am 17.01.22 um 14:29 schrieb Geert Uytterhoeven:
> > On Mon, Jan 17, 2022 at 1:57 PM Gerd Hoffmann <kraxel@xxxxxxxxxx> 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@xxxxxxxxxxxxxx
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
