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From:   Daniel Vetter <daniel@ffwll.ch>
Date:   Mon, 17 Jan 2022 17:38:25 +0100
Message-ID: <CAKMK7uECQU2ALkLHuFfPCjDcH456R6yJ4N_yKbHJ6PiFZ6fiow@mail.gmail.com>
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer
To:     Helge Deller <deller@gmx.de>
Cc:     Thomas Zimmermann <tzimmermann@suse.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "airlied@gmail.com" <airlied@gmail.com>,
        linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org,
        Javier Martinez Canillas <javierm@redhat.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>
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On Mon, Jan 17, 2022 at 5:22 PM Helge Deller <deller@gmx.de> wrote:
>
> On 1/17/22 16:58, Thomas Zimmermann wrote:
> > Hi
> >
> > Am 17.01.22 um 16:42 schrieb Helge Deller:
> >> [...]
> >>>> c) reintroduce the state where fbcon is fast on fbdev. This is important for non-DRM machines,
> >>>>     either when run on native hardware or in an emulator.
> >>>> d) not break DRM development
> >>>>
> >>>> Especially regarding c) I complained in [1] and got no feedback. I really would like to
> >>>> understand where the actual problems were and what's necessary to fix them.
> >>>>
> >>>> Helge
> >>>>
> >>>> [1] https://lore.kernel.org/r/feea8303-2b83-fc36-972c-4fc8ad723bde@gmx.de
> >
> > Seems like few people read linux-fbdev these days.
> > I suggest to partly revert the patch to the point were performance
> > gets better again.
> Yes, *please*!
> That would solve my biggest concern.
>
> As far as I can see that's only 2 commits to be reverted:
> b3ec8cdf457e - "fbdev: Garbage collect fbdev scrolling acceleration, part 1 (from TODO list)"
> 39aead8373b3 - "fbcon: Disable accelerated scrolling"for-next-next
>
> I think both were not related to any 0-day bug reports (but again, I might be wrong).

syzbot, not 0day, and there's like a sea of them unfortunately.
There's all kinds of funny races going on when resizing consoles (due
to bad locking design) which then blow up, especially in less tested
code. For the sw rendering we've merged a bunch of patches, but you
pretty much have to assume that it's all fairly broken code until it's
rewritten and fully covered with tests. Shadowfb + memcpy is probably
much faster for restoring scrolling performance than anything else
really.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch