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

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Message-ID: <cbe8641e-9358-2ea6-235f-bd837171eaf0@gmx.de>
Date:   Mon, 17 Jan 2022 18:19:17 +0100
From:   Helge Deller <deller@....de>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "airlied@...il.com" <airlied@...il.com>,
        linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Javier Martinez Canillas <javierm@...hat.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] MAINTAINERS: Add Helge as fbdev maintainer

On 1/17/22 17:38, Daniel Vetter wrote:
> On Mon, Jan 17, 2022 at 5:22 PM Helge Deller <deller@....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

The patches above are not about resizing consoles.
Even if a resize should happen in between, it's better to introduce some kind 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