Re: [PATCH] fbcon: Disable accelerated scrolling

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 28.10.20 um 20:55 schrieb Daniel Vetter:
> 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.

That really deserves a TODO item then IMHO.

Best regards
Thomas

> -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
> 
> 
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
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