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

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Date:   Tue, 1 Feb 2022 17:30:36 +0100
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Helge Deller <deller@....de>
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 Tue, Feb 1, 2022 at 3:52 PM Helge Deller <deller@....de> wrote:
>
> 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?

Sounds good, and yeah rough idea is that the maintainers + revert +
Kconfig should go in for rc3 or rc4 if we hit another bump, and the
locking stuff then in for -next (since it needs a backmerge and is
defo tricky stuff).

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch