Re: [PATCH v4 0/9] drm: Support simple-framebuffer devices and firmware fbs

Geert Uytterhoeven
2021-04-26T19:05:41Z

Hi Thomas,

On Mon, Apr 26, 2021 at 2:22 PM Thomas Zimmermann <tzimmermann@suse.de> wrote:
> Am 20.04.21 um 11:27 schrieb Geert Uytterhoeven:
> > On Tue, Apr 20, 2021 at 11:22 AM Gerd Hoffmann <kraxel@redhat.com> wrote:
> >>>>> Patches 4 to 8 add the simpledrm driver. It's build on simple DRM helpers
> >>>>> and SHMEM. It supports 16-bit, 24-bit and 32-bit RGB framebuffers. During
> >>>>
> >>>> .... if support for 8-bit frame buffers would be added?
> >>>
> >>> Is that 8-bit greyscale or 8-bit indexed with 256 entry palette? Former
> >>> shouldn't be a big thing, but the latter is only really supported by the
> >>> overall drm ecosystem in theory. Most userspace assumes that xrgb8888
> >>> works, and we keep that illusion up by emulating it in kernel for hw which
> >>> just doesn't support it. But reformatting xrgb8888 to c8 is tricky at
> >>> best.
> >>
> >> Well. cirrus converts xrgb8888 on the fly to rgb888 or rgb565
> >> (depending on display resolution). We could pull off the same trick
> >> here and convert to rgb332 (assuming we can program the palette with the
> >> color cube needed for that). Wouldn't look pretty, but would probably
> >> work better than expecting userspace know what color palettes are in
> >> 2021 ...
> >
> > Yeah, I already had a similar idea for Amiga HAM ;-)
>
> I vaguely remember that HAM mode uses some crazy format where pixel
> colors depend in the values of their neighbors. (?) How complicated is
> it to write a conversion from RGB to HAM?

Not that complicated, unless you want to do it Good & Fast ;-)

https://en.wikipedia.org/wiki/Hold-And-Modify

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