Date:   Mon, 17 Jan 2022 21:17:04 +0100
From:   Helge Deller <deller@....de>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Gerd Hoffmann <kraxel@...hat.com>,
        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>,
        Sven Schnelle <svens@...ckframe.org>
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

On 1/17/22 16:03, Daniel Vetter wrote:
> On Mon, Jan 17, 2022 at 3:48 PM Helge Deller <deller@....de> wrote:
>>
>> On 1/17/22 15:10, Geert Uytterhoeven wrote:
>>> Hi Thomas,
>>>
>>> On Mon, Jan 17, 2022 at 2:51 PM Thomas Zimmermann <tzimmermann@...e.de> wrote:
>>>> Am 17.01.22 um 14:29 schrieb Geert Uytterhoeven:
>>>>> 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.
>>>
>>> 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 ?
>
> Not sure who you talked to, but we have drivers with fbdev bitblt
> accel (well, in some cases had, because driver maintainers decided
> it's just not worth it and ripped it out again or never merged it).
> Also the other discussions about some low-bit formats is pretty much
> just a question of extending a few enums and wiring through the fbdev
> emulation layer.

No, you got me wrong.

I'm not talking about making other low-bit formats available to userspace.

I'm talking about running the framebuffer natively on a lower-bit format
and to speed up text emulation (fbcon) with help of on-chip 2D bitblt.
So, similiar as it was done in fbdev for non-DRM graphic cards before two
patches were applied and which disabled this speedup for *all* existing fbdev drivers:
b3ec8cdf457e - "fbdev: Garbage collect fbdev scrolling acceleration, part 1 (from TODO list)"
39aead8373b3 - "fbcon: Disable accelerated scrolling"for-next-next

Esp. the commit message of patch 39aead8373b3 completely
ignored the acceleration of the fbdev drivers.

Please correct me if I'm wrong, but text-console emulation/scrolling on DRM is
currently unaccelerated and bound to Truecolour modes only, which is probably
one of the main reasons why most fbdev drivers can't be ported to DRM...

Helge



> So the things brought up in this thread thus far are
> actually the fairly easy items, which should take at most a handful of
> patches to rectify. There's much more nastier issues in fbdev, which
> will take serious amounts of development time to fix.
>
> Unfortunately in the past 5+ years absolutely no one stepped up with
> actual patches, which is why fbdev was marked as orphaned in
> MAINTAINERS.
> -Daniel
>
>>
>> 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@...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
>>>
>>
>
>

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