linux-kernel - Re: [PATCH 2/2] Revert "fbcon: Disable accelerated scrolling"

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Message-ID: <408c5dbd-a915-e3fc-fafb-738f6badc91a@gmx.de>
Date:   Wed, 19 Jan 2022 17:55:15 +0100
From:   Helge Deller <deller@....de>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Sven Schnelle <svens@...ckframe.org>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Claudio Suarez <cssk@...-c.es>,
        Gerd Hoffmann <kraxel@...hat.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH 2/2] Revert "fbcon: Disable accelerated scrolling"

Hi Thomas,

On 1/19/22 16:37, Thomas Zimmermann wrote:
> Am 19.01.22 um 16:05 schrieb Sven Schnelle:
>> Daniel Vetter <daniel@...ll.ch> writes:
>>
>>> On Wed, Jan 19, 2022 at 3:01 PM Linus Torvalds
>>> <torvalds@...ux-foundation.org> wrote:
>>> Irrespective of this code being buggy or not buggy I think the bigger
>>> pictures, and really the reason I want to see as much code ditched
>>> from the fbdev/fbcon stack as we possible can, are very clear:
>>>
>>> - it's full of bugs
>>> - there's no test coverage/CI to speak of
>>> - it's very arcane code which is damn hard to understand and fix issues within
>>> - the locking is busted (largely thanks to console_lock, and the
>>> effort to make that reasonable from -rt folks has been slowly creeping
>>> forward for years).
>>>
>>> Iow this subsystem is firmly stuck in the 90s, and I think it's best
>>> to just leave it there. There's also not been anyone actually capable
>>> and willing to put in the work to change this (pretty much all actual
>>> changes/fixes have been done by drm folks anyway, like me having a
>>> small pet project to make the fbdev vs fbcon locking slightly less
>>> busted).
>>
>> Saying it's stuck in the 90ies, and actively trying to prevent
>> Helge from taking over maintainership at the same time looks odd.
>
> The issues are in the design itself. It's impossible to model today's
> hardware and constraints with fbdev. It's impossible to change
> configuration in a reliable way (i.e., what DRM calls atomic). Fbdev
> mmaps plain video ram to userspace, which is one of the reasons why
> DRM's fbdev support is hard to improve.

That's fully understood, but I think you are mixing up things here...

The fbdev userspace api is most likely not the best way forward.
I'm sure that drm can and will provide better solutions for userspace.
And userspace will surely pick up those new interfaces.
DRM folks will drive it in the right direction, I'm sure!

But in addition fbdev/fbcon is the kernel framework for nearly
all existing graphic cards which are not (yet) supported by DRM.
They need fbdev/fbcon to show their text console and maybe a simple X server.
If you break fbdev for those cards, they are completely stuck.
Hopefully those drivers will be ported to DRM, but that's currently
not easily possible (or they would be so slow that they are unuseable).

So, I don't think you should try to improve DRM's /dev/fb0 support further,
but instead work forward for a new interface which perfectly suits DRM.
That's Ok, and my goal is not to prevent that.

>> I think Helge should at least get a chance to fix the issues. If the
>> state is still the same in a year or so it should be discussed again.
>
> You cannot fix that in 10yrs.
>
>>
>>> The other side is that being a maintainer is about collaboration, and
>>> this entire fbdev maintainership takeover has been a demonstration of
>>> anything but that. MAINTAINERS entry was a bit confusing since defacto
>>> drm has been maintaining it for years.
>>
>> It was marked as 'Orphaned'. Anyone is free to send a Patch/PR to take
>> over maintainership. If you have strong opinions about that code (And you
>> obviously have reading your mail, set it to 'maintained' and care about
>> it. Everything else is just wrong in my opinion.
>
> No, it's not wrong. Helge takes fbdev over the weekend, without noteworthy experience, and ignores advice from the people that have kept it alive over the past years. This isn't going to work in the long term.
>
> Best regards
> Thomas
>
>>
>> /Sven
>