[Nouveau] [PATCH] fbcon: Disable accelerated scrolling

[Sam Ravnborg]

Hi Daniel et al.

On Wed, Oct 28, 2020 at 05:06:00PM +0100, Daniel Vetter wrote:
> So ever since syzbot discovered fbcon, we have solid proof that it's
> full of bugs. And often the solution is to just delete code and remove
> features, e.g.  50145474f6ef ("fbcon: remove soft scrollback code").
> 
> Now the problem is that most modern-ish drivers really only treat
> fbcon as an dumb kernel console until userspace takes over, and Oops
> printer for some emergencies. Looking at drm drivers and the basic
> vesa/efi fbdev drivers shows that only 3 drivers support any kind of
> acceleration:
> 
> - nouveau, seems to be enabled by default
> - omapdrm, when a DMM remapper exists using remapper rewriting for
>   y/xpanning
> - gma500, but that is getting deleted now for the GTT remapper trick,
>   and the accelerated copyarea never set the FBINFO_HWACCEL_COPYAREA
>   flag, so unused (and could be deleted already I think).
> 
> No other driver supportes accelerated fbcon. And fbcon is the only
> user of this accel code (it's not exposed as uapi through ioctls),
> which means we could garbage collect fairly enormous amounts of code
> if we kill this.
> 
> Plus because syzbot only runs on virtual hardware, and none of the
> drivers for that have acceleration, we'd remove a huge gap in testing.
> And there's no other even remotely comprehensive testing aside from
> syzbot.
> 
> This patch here just disables the acceleration code by always
> redrawing when scrolling.

So far I follow you - and agree.
Acked-by: Sam Ravnborg <sam at ravnborg.org>

> The plan is that once this has been merged
> for well over a year in released kernels, we can start to go around
> and delete a lot of code.

Why wait one year? We deleted the scrollback code without any prior
warning - which was fine. And acceleration support has less users
so there should be no reason to wait.

So unless there are good arguments that I miss then we should just
delete the acceleration code outright.

	Sam