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Date:   Tue, 18 Jan 2022 11:44:25 +0100
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Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer
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From:   Helge Deller <deller@gmx.de>
To:     Gerd Hoffmann <kraxel@redhat.com>
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>
References: <c80ed72c-2eb4-16dd-a7ad-57e9dde59ba1@gmx.de>
 <20220117125716.yjwxsze35j2ndn2i@sirius.home.kraxel.org>
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 <57d276d3-aa12-fa40-6f90-dc19ef393679@gmx.de>
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 <20220118062947.6kfuam6ah63z5mmn@sirius.home.kraxel.org>
 <ebe9c076-e2a9-0cb7-eb6a-ae71c075975e@gmx.de>
 <20220118091615.3ih5n7vhd6tgmnku@sirius.home.kraxel.org>
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On 1/18/22 11:13, Helge Deller wrote:
> 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 frameb=
uffer
>>> is easily supported by DRM, and you can use fbcon without using lots o=
f 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 fin=
d
>> 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, wh=
ich
>>>> is probably the reason nobody stepped up fixing things and it ended u=
p
>>>> 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.  Moder=
n
>> hardware simply has no classic blitter any more.  Which is a problem
>> when it comes to keeping scroll acceleration alive, it is already a ver=
y
>> 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 re=
moving it.
>>>
>>> Reverting those commits have no effect on DRM's usage of fbcon.
>>> But reverting those commits bring back scroll acceleration for all oth=
ers.
>>> 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 issu=
es
> for SCROLL_MOVE.

I've just looked through all patches in drivers/video which were tagged
with syzbot or syzkaller back to year 2005. The vast majority fixed the
reported issues on a higher level, e.g. when screen is to be resized,
or when font size is to be changed. The few ones which touched driver
code fixed a real driver bug, e.g. by adding a check.

NONE of those patches touched either the SCROLL_MOVE or the SCROLL_REDRAW =
case.
That means, I see no reason why SCROLL_MOVE had to be ripped-out and just
SCROLL_REDRAW had to be used instead, other than simply "it's not being
used by DRM, so let's pull it out".
The patches which removed SCROLL_MOVE support simply ignored the fact
that SCROLL_MOVE is still heavily used by fbdev (non-DRM).

I don't see a reason why the two patches which removed SCROLL_MOVE
shouldn't be reverted. Or what am I missing?

Helge