[Nouveau] [PATCH] fbcon: Disable accelerated scrolling

[Greg Kroah-Hartman]

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. 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.
> 
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie at samsung.com>
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> Cc: Linus Torvalds <torvalds at linux-foundation.org>
> Cc: Ben Skeggs <bskeggs at redhat.com>
> Cc: nouveau at lists.freedesktop.org
> Cc: Tomi Valkeinen <tomi.valkeinen at ti.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Jiri Slaby <jirislaby at kernel.org>
> Cc: "Gustavo A. R. Silva" <gustavoars at kernel.org>
> Cc: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
> Cc: Peilin Ye <yepeilin.cs at gmail.com>
> Cc: George Kennedy <george.kennedy at oracle.com>
> Cc: Nathan Chancellor <natechancellor at gmail.com>
> Cc: Peter Rosin <peda at axentia.se>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> ---
>  drivers/video/fbdev/core/fbcon.c | 38 ++++++--------------------------
>  1 file changed, 7 insertions(+), 31 deletions(-)

Nice!

But I'm with Sam, delete early :)

Reviewed-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>