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Date:   Tue, 18 Jan 2022 10:16:15 +0100
From:   Gerd Hoffmann <kraxel@redhat.com>
To:     Helge Deller <deller@gmx.de>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Sven Schnelle <svens@stackframe.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer
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On Tue, Jan 18, 2022 at 09:20:43AM +0100, Helge Deller wrote:
> On 1/18/22 07:29, Gerd Hoffmann wrote:
> >> Please correct me if I'm wrong, but text-console emulation/scrolling on DRM is
> >> currently unaccelerated and bound to Truecolour modes only,
> >
> > Yes.  Adding support for formats beside argb8888 to the drm fbcon
> > emulation shouldn't be that much of a problem though.
> 
> Really? Assuming a graphic card which runs with only 256 colors framebuffer
> is easily supported by DRM, and you can use fbcon without using lots of memcpy()?

Driver: programming a fixed color cube palette, then use RGB332.

fbcon/fbdev emulation: RGB332 support must be added I think.  But both
argb888 and rgb565 are supported today, so it should not be hard to find
the places where you have to add some code to handle RGB332 too.

> > Acceleration is harder.  The scroll acceleration had issues nobody
> > addressed for years, and on modern hardware it is simply not used, which
> > is probably the reason nobody stepped up fixing things and it ended up
> > being dropped.
> 
> The DRM layer doesn't use scroll acceleration.
> More than 30 other existing fbdev drivers use it.

Yes.  The world shifted from 2d acceleration to 3d acceleration.  Modern
hardware simply has no classic blitter any more.  Which is a problem
when it comes to keeping scroll acceleration alive, it is already a very
niche use case and it will only become worse ...

> > Bringing it back is much more work than just reverting the commits removing it.
> 
> Reverting those commits have no effect on DRM's usage of fbcon.
> But reverting those commits bring back scroll acceleration for all others.
> I'm trying to find out which patches did apparently fixed such issues
> for the REDRAW case. If you have a pointer it would be helpful.

IIRC the code had a bunch of races and syzkaller flagged problems.
I didn't follow very closely though.

take care,
  Gerd