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Date:   Mon, 17 Jan 2022 18:19:17 +0100
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Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer
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To:     Daniel Vetter <daniel@ffwll.ch>
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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From:   Helge Deller <deller@gmx.de>
In-Reply-To: <CAKMK7uECQU2ALkLHuFfPCjDcH456R6yJ4N_yKbHJ6PiFZ6fiow@mail.gmail.com>
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On 1/17/22 17:38, Daniel Vetter wrote:
> 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 impo=
rtant 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 f=
ix 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, pa=
rt 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 mi=
ght 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

The patches above are not about resizing consoles.
Even if a resize should happen in between, it's better to introduce some k=
ind of big lock
instead of completely disable acceleration for est. 58 other graphic card =
drivers and slow them
down that much that it renders them to become unusable.

> 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.

That's maybe true for fast new machines with fast PCI busses.
But have you measured that on other/older hardware too?
There is a good reason why 2D acceleration was introduced.

Helge