linux-kernel - Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

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Date:   Tue, 18 Jan 2022 11:13:02 +0100
From:   Helge Deller <deller@....de>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Sven Schnelle <svens@...ckframe.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

On 1/18/22 10:16, Gerd Hoffmann wrote:
> 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.

I'd expect that that framework is provided by DRM developers if there is the wish
to get rid of old fbdev and transition existing drivers over to use DRM.

>>> 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 ...

For me it's Ok that the DRM drivers don't use 2d acceleration (as it is today
with the arguments mentioned multiple times).
But the patches broke existing fbdev acceleration which is available by
the fbdev drivers. That's a big regression from point of fbdev.

>>> 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.

That's sad.
Nevertheless I wonder if the changes which were apparently done for
the SCROLL_REDRAW case (on the higher level?) didn't also fixed the issues
for SCROLL_MOVE.

Helge