Re: [PATCH] fbcon: Disable accelerated scrolling

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Wed, Oct 28, 2020 at 8:02 PM Thomas Zimmermann <tzimmermann@suse.de> wrote:
>
> Hi
>
> Am 28.10.20 um 17:06 schrieb Daniel Vetter:
> > 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.
>
> Why wait a year? I'd say delete early, delete often. ;)
>
> >
> > Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Ben Skeggs <bskeggs@redhat.com>
> > Cc: nouveau@lists.freedesktop.org
> > Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Cc: Jiri Slaby <jirislaby@kernel.org>
> > Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> > Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > Cc: Peilin Ye <yepeilin.cs@gmail.com>
> > Cc: George Kennedy <george.kennedy@oracle.com>
> > Cc: Nathan Chancellor <natechancellor@gmail.com>
> > Cc: Peter Rosin <peda@axentia.se>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > ---
> >  drivers/video/fbdev/core/fbcon.c | 38 ++++++--------------------------
> >  1 file changed, 7 insertions(+), 31 deletions(-)
> >
> > diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> > index cef437817b0d..d74ccbbb29bb 100644
> > --- a/drivers/video/fbdev/core/fbcon.c
> > +++ b/drivers/video/fbdev/core/fbcon.c
> > @@ -1147,11 +1147,13 @@ static void fbcon_init(struct vc_data *vc, int init)
> >
> >       ops->graphics = 0;
> >
> > -     if ((cap & FBINFO_HWACCEL_COPYAREA) &&
> > -         !(cap & FBINFO_HWACCEL_DISABLED))
> > -             p->scrollmode = SCROLL_MOVE;
> > -     else /* default to something safe */
> > -             p->scrollmode = SCROLL_REDRAW;
> > +     /*
> > +      * No more hw acceleration for fbcon.
> > +      *
> > +      * FIXME: Garabge collect all the now dead code after sufficient time
> > +      * has passed.
> > +      */
> > +     p->scrollmode = SCROLL_REDRAW;
>
> I just grepped for scrollmode and there aren't many places that use it.
> Could you remove it as well?

Removing scrollmode will start the delete feast. In fbcon alone I
think we can drop half the code.
-Daniel

>
> In any case
>
> Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
>
> Best regards
> Thomas
>
> >
> >       /*
> >        *  ++guenther: console.c:vc_allocate() relies on initializing
> > @@ -1961,7 +1963,6 @@ static void updatescrollmode(struct fbcon_display *p,
> >  {
> >       struct fbcon_ops *ops = info->fbcon_par;
> >       int fh = vc->vc_font.height;
> > -     int cap = info->flags;
> >       u16 t = 0;
> >       int ypan = FBCON_SWAP(ops->rotate, info->fix.ypanstep,
> >                                 info->fix.xpanstep);
> > @@ -1969,37 +1970,12 @@ static void updatescrollmode(struct fbcon_display *p,
> >       int yres = FBCON_SWAP(ops->rotate, info->var.yres, info->var.xres);
> >       int vyres = FBCON_SWAP(ops->rotate, info->var.yres_virtual,
> >                                  info->var.xres_virtual);
> > -     int good_pan = (cap & FBINFO_HWACCEL_YPAN) &&
> > -             divides(ypan, vc->vc_font.height) && vyres > yres;
> > -     int good_wrap = (cap & FBINFO_HWACCEL_YWRAP) &&
> > -             divides(ywrap, vc->vc_font.height) &&
> > -             divides(vc->vc_font.height, vyres) &&
> > -             divides(vc->vc_font.height, yres);
> > -     int reading_fast = cap & FBINFO_READS_FAST;
> > -     int fast_copyarea = (cap & FBINFO_HWACCEL_COPYAREA) &&
> > -             !(cap & FBINFO_HWACCEL_DISABLED);
> > -     int fast_imageblit = (cap & FBINFO_HWACCEL_IMAGEBLIT) &&
> > -             !(cap & FBINFO_HWACCEL_DISABLED);
> >
> >       p->vrows = vyres/fh;
> >       if (yres > (fh * (vc->vc_rows + 1)))
> >               p->vrows -= (yres - (fh * vc->vc_rows)) / fh;
> >       if ((yres % fh) && (vyres % fh < yres % fh))
> >               p->vrows--;
> > -
> > -     if (good_wrap || good_pan) {
> > -             if (reading_fast || fast_copyarea)
> > -                     p->scrollmode = good_wrap ?
> > -                             SCROLL_WRAP_MOVE : SCROLL_PAN_MOVE;
> > -             else
> > -                     p->scrollmode = good_wrap ? SCROLL_REDRAW :
> > -                             SCROLL_PAN_REDRAW;
> > -     } else {
> > -             if (reading_fast || (fast_copyarea && !fast_imageblit))
> > -                     p->scrollmode = SCROLL_MOVE;
> > -             else
> > -                     p->scrollmode = SCROLL_REDRAW;
> > -     }
> >  }
> >
> >  #define PITCH(w) (((w) + 7) >> 3)
> >
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer



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