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

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Date:   Wed, 19 Jan 2022 13:28:06 +0100
From:   Helge Deller <deller@....de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Sven Schnelle <svens@...ckframe.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Ilia Mirkin <imirkin@...m.mit.edu>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Pavel Machek <pavel@....cz>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Sam Ravnborg <sam@...nborg.org>, Claudio Suarez <cssk@...-c.es>
Subject: Re: [PATCH 2/2] Revert "fbcon: Disable accelerated scrolling"

Hello Greg,

On 1/19/22 12:47, Geert Uytterhoeven wrote:
> On Wed, Jan 19, 2022 at 12:28 PM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
>> On Wed, Jan 19, 2022 at 12:22:55PM +0100, Greg Kroah-Hartman wrote:
>>> On Wed, Jan 19, 2022 at 12:08:39PM +0100, Helge Deller wrote:
>>>> This reverts commit 39aead8373b3c20bb5965c024dfb51a94e526151.
>>>>
>>>> Revert this patch.  This patch started to introduce the regression that
>>>> all hardware acceleration of more than 35 existing fbdev drivers were
>>>> bypassed and thus fbcon console output for those was dramatically slowed
>>>> down by factor of 10 and more.
>>>>
>>>> Reverting this commit has no impact on DRM, since none of the DRM drivers are
>>>> tagged with the acceleration flags FBINFO_HWACCEL_COPYAREA,
>>>> FBINFO_HWACCEL_FILLRECT or others.
>>>>
>>>> Signed-off-by: Helge Deller <deller@....de>
>
>>> As for "why", I think there was a number of private bugs that were
>>> reported in this code, which is why it was removed.  I do not think it
>>> can be safely added back in without addressing them first.  Let me go
>>> dig through my email to see if I can find them...
>>
>> Ah, no, that was just the soft scrollback code I was thinking of, which

Right.
That was commit 973c096f6a85 and it was about vgacon, not fbcon.

I did mentioned it in my cover letter, together with my analysis of
the reported bugs.

Maybe I should have put all the information from the cover letter into
the patch here as well. If you haven't read the cover letter yet, please do.

Helge

> So the bugs argument is moot.
>
>> was a different revert and is still gone, thankfully :)
>
> FTR, not everyone else was thankful about that one...
>
>> This one was just removed because Daniel noticed that only 3 drivers
>> used this (nouveau, omapdrm, and gma600), so this shouldn't have caused
>> any regressions in any other drivers like you are reporting here.
>>
>> So perhaps this regression is caused by something else?
>
> 1. Daniel's patch was not CCed to linux-fbdev,
> 2. When I discovered the patch, I pointed out that the premise of 3
>    drivers was not true, and that it affects 32 more fbdev drivers[1] .
>    The patch was applied regardless.
> 3. When the patch was suggested for backporting, I pointed out the
>    same[2].
>    The patch was backported regardless.
>
> [1] https://lore.kernel.org/r/alpine.DEB.2.22.394.2010311116530.379363@ramsan.of.borg/
> [2] https://lore.kernel.org/r/CAMuHMdXRgam2zahPEGcw8+76Xm-0AO-Ci9-YmVa5JpTKVHphRw@mail.gmail.com/
>
> Gr{oetje,eeting}s,
>
>                         Geert