Re: [Intel-gfx] [PATCH 1/3] fbcon: Disable accelerated scrolling

[Thomas Zimmermann <tzimmermann@suse.de>]

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Am 29.10.20 um 11:14 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.
> 
> v2:
> - Drop a few more unused local variables, somehow I missed the
> compiler warnings (Sam)
> - Fix typo in comment (Jiri)
> - add a todo entry for the cleanup (Thomas)

Thanks :)

> 
> Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Acked-by: Sam Ravnborg <sam@ravnborg.org>
> Cc: Jiri Slaby <jirislaby@kernel.org>
> 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>
> ---
>  Documentation/gpu/todo.rst       | 18 ++++++++++++++
>  drivers/video/fbdev/core/fbcon.c | 42 ++++++--------------------------
>  2 files changed, 25 insertions(+), 35 deletions(-)
> 
> diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> index 6b224ef14455..bec99341a904 100644
> --- a/Documentation/gpu/todo.rst
> +++ b/Documentation/gpu/todo.rst
> @@ -277,6 +277,24 @@ Contact: Daniel Vetter, Noralf Tronnes
>  
>  Level: Advanced
>  
> +Garbage collect fbdev scrolling acceleration
> +--------------------------------------------
> +
> +Scroll acceleration is disabled in fbcon by hard-wiring p->scrollmode =
> +SCROLL_REDRAW. There's a ton of code this will allow us to remove:
> +- lots of code in fbcon.c
> +- a bunch of the hooks in fbcon_ops, maybe the remaining hooks could be called
> +  directly instead of the function table (with a switch on p->rotate)
> +- fb_copyarea is unused after this, and can be deleted from all drivers
> +
> +Note that not all acceleration code can be deleted, since clearing and cursor
> +support is still accelerated, which might be good candidates for further
> +deletion projects.
> +
> +Contact: Daniel Vetter
> +
> +Level: Intermediate
> +
>  idr_init_base()
>  ---------------
>  
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index cef437817b0d..a68253485244 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: Garbage collect all the now dead code after sufficient time
> +	 * has passed.
> +	 */
> +	p->scrollmode = SCROLL_REDRAW;
>  
>  	/*
>  	 *  ++guenther: console.c:vc_allocate() relies on initializing
> @@ -1961,45 +1963,15 @@ 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);
> -	int ywrap = FBCON_SWAP(ops->rotate, info->fix.ywrapstep, t);
>  	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

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