linux-kernel - Re: [PATCH 03/21] fbcon: Restore fbcon scrolling acceleration

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Message-ID: <313c4c72-364b-1d61-09c1-e4a83cbefe6a@gmx.de>
Date:   Tue, 1 Feb 2022 15:52:11 +0100
From:   Helge Deller <deller@....de>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        linux-fbdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org, Claudio Suarez <cssk@...-c.es>,
        Dave Airlie <airlied@...il.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Machek <pavel@....cz>, Sam Ravnborg <sam@...nborg.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Sven Schnelle <svens@...ckframe.org>,
        Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [PATCH 03/21] fbcon: Restore fbcon scrolling acceleration

On 2/1/22 14:45, Daniel Vetter wrote:
> On Tue, Feb 1, 2022 at 12:01 PM Helge Deller <deller@....de> wrote:
>> On 2/1/22 11:36, Daniel Vetter wrote:
>>> On Tue, Feb 1, 2022 at 11:16 AM Helge Deller <deller@....de> wrote:
>>>>
>>>> On 1/31/22 22:05, Daniel Vetter wrote:
>>>>> This functionally undoes 39aead8373b3 ("fbcon: Disable accelerated
>>>>> scrolling"), but behind the FRAMEBUFFER_CONSOLE_LEGACY_ACCELERATION
>>>>> option.
>>>>
>>>> you have two trivial copy-n-paste errors in this patch which still prevent the
>>>> console acceleration.
>>>
>>> Duh :-(
>>>
>>> But before we dig into details I think the big picture would be
>>> better. I honestly don't like the #ifdef pile here that much.
>>
>> Me neither.
>> The ifdefs give a better separation, but prevents that the compiler
>> checks the various paths when building.
>>
>>> I wonder whether your approach, also with GETVX/YRES adjusted
>>> somehow, wouldn't look cleaner?
>> I think so.
>> You wouldn't even need to touch GETVX/YRES because the compiler
>> will optimize/reduce it from
>>
>> #define GETVYRES(s,i) ({                           \
>>         (s == SCROLL_REDRAW || s == SCROLL_MOVE) ? \
>>         (i)->var.yres : (i)->var.yres_virtual; })
>>
>> to just become:
>>
>> #define GETVYRES(s,i) ((i)->var.yres)
>
> Yeah, but you need to roll out your helper to all the callsites. But
> since you #ifdef out info->scrollmode we should catch them all I
> guess.

Right. That was the only reason why I ifdef'ed it out.
Technically we don't need that ifdef.

>>> Like I said in the cover letter I got mostly distracted with fbcon
>>> locking last week, not really with this one here at all, so maybe
>>> going with your 4 (or 2 if we squash them like I did here) patches is
>>> neater?
>>
>> The benefit of my patch series was, that it could be easily backported first,
>> and then cleaned up afterwards. Even a small additional backport patch to disable
>> the fbcon acceleration for DRM in the old releases would be easy.
>> But I'm not insisting on backporting the patches, if we find good way forward.
>>
>> So, either with the 4 (or 2) patches would be OK for me (or even your approach).
>
> The idea behind the squash was that it's then impossible to backport
> without the Kconfig,

Yes, my proposal was to simply revert the 2 patches and immediatly send
the Kconfig patch to disable it again.

> and so we'll only enable this code when people
> intentionally want it. Maybe I'm too paranoid?

I think you are too paranoid :-)
If all patches incl. the Kconfig patch are backported then people shouldn't
do it wrong.

> Anyway, you feel like finishing off your approach? Or should I send
> out v2 of this with the issues fixed you spotted? Like I said either
> is fine with me.

Ok, then let me try to finish my approach until tomorrow, and then you
check if you can and want to add your locking and other patches on top of it.
In the end I leave the decision which approach to take to you.
Ok?

Helge