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

On 1/18/22 11:13, Helge Deller wrote:

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

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On Tue, Jan 18, 2022 at 09:20:43AM +0100, Helge Deller wrote:

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On 1/18/22 07:29, Gerd Hoffmann wrote:

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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.

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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.

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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.

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