Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

[Helge Deller]

On 1/17/22 15:10, Geert Uytterhoeven wrote:
> 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.

From this conversation it seems DRM completely lacks backwards compatibility,
including a missing 2D bitblt copy.
Isn't that all what's needed and then migrating existing drivers would
be easy ?

Helge


>>> 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
>