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

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Date:   Mon, 24 Jan 2022 16:29:34 +0100
From:   Helge Deller <deller@....de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Gerd Hoffmann <kraxel@...hat.com>
Cc:     linux-fbdev@...r.kernel.org, Pavel Machek <pavel@....cz>,
        Sam Ravnborg <sam@...nborg.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sven Schnelle <svens@...ckframe.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Claudio Suarez <cssk@...-c.es>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2/2] Revert "fbcon: Disable accelerated scrolling"

On 1/24/22 12:50, Javier Martinez Canillas wrote:
> On 1/24/22 12:33, Thomas Zimmermann wrote:
>
> [snip]
>
>>> Thoughts?
>>
>> I can't say I approve keeping fbdev alive, but...
>>
>> With fbdev emulation, every DRM driver is an fbdev driver too. So
>> CONFIG_FB_DRIVER is somewhat misleading. Better add an option like
>> CONFIG_FBCON_HW_SCROLLING and have it selected by the fbdev drivers that
>> absolutely need HW acceleration. That option would then protect the rsp
>> code.

I'm not a fan of something like CONFIG_FBCON_HW_SCROLLING, but I'm not
against it either.
For me it sounds that this is not the real direction you want to go,
which is to prevent that any other drivers take the framebuffer before
you take it with simpledrm or similiar.
CONFIG_FBCON_HW_SCROLLING IMHO just disables the (from your POV) neglectable accleration part.
With an option like CONFIG_FB_DRIVER (maybe better: CONFIG_FB_LEGACY_DRIVERS)
it's an easy option for distros to disable all of the legacy drivers
from being built & shipped.

Instead of CONFIG_FBCON_HW_SCROLLING we could also choose
CONFIG_FBCON_LEGACY_ACCELERATION, because it includes fillrect() as well...

> Agreed that this option would be better and allow distros
> to disable the code that was reverted.

Yes, but IMHO it doesn't hurt either to leave it in.
It doesn't break anything at least.
Anyway...

Helge