Re: [PATCH] drm/fb-helper: Don't call dirty callback for untouched clips

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Oct 20, 2016 at 05:00:35PM +0200, Takashi Iwai wrote:
> On Thu, 20 Oct 2016 16:56:04 +0200,
> Ville Syrjälä wrote:
> > 
> > On Thu, Oct 20, 2016 at 04:39:52PM +0200, Takashi Iwai wrote:
> > > Since 4.7 kernel, we've seen the error messages like
> > > 
> > >  kernel: [TTM] Buffer eviction failed
> > >  kernel: qxl 0000:00:02.0: object_init failed for (4026540032, 0x00000001)
> > >  kernel: [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to allocate VRAM BO
> > > 
> > > on QXL when switching and accessing on VT.  The culprit was the
> > > generic deferred_io code (qxl driver switched to it since 4.7).
> > > There is a race between the dirty clip update and the call of
> > > callback.
> > > 
> > > In drm_fb_helper_dirty(), the dirty clip is updated in the spinlock,
> > > while it kicks off the update worker outside the spinlock.  Meanwhile
> > > the update worker clears the dirty clip in the spinlock, too.  Thus,
> > > when drm_fb_helper_dirty() is called concurrently, schedule_work() is
> > > called after the clip is cleared in the first worker call.
> > > 
> > > This patch addresses it by validating the clip before calling the
> > > dirty fb callback.
> > > 
> > > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98322
> > > Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1003298
> > > Fixes: eaa434defaca ('drm/fb-helper: Add fb_deferred_io support')
> > > Cc: <stable@vger.kernel.org>
> > > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > > ---
> > >  drivers/gpu/drm/drm_fb_helper.c | 13 +++++++++----
> > >  1 file changed, 9 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> > > index 03414bde1f15..d790d205129e 100644
> > > --- a/drivers/gpu/drm/drm_fb_helper.c
> > > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > > @@ -636,15 +636,20 @@ static void drm_fb_helper_dirty_work(struct work_struct *work)
> > >  						    dirty_work);
> > >  	struct drm_clip_rect *clip = &helper->dirty_clip;
> > >  	struct drm_clip_rect clip_copy;
> > > +	bool dirty;
> > >  	unsigned long flags;
> > >  
> > >  	spin_lock_irqsave(&helper->dirty_lock, flags);
> > > -	clip_copy = *clip;
> > > -	clip->x1 = clip->y1 = ~0;
> > > -	clip->x2 = clip->y2 = 0;
> > > +	dirty = (clip->x1 < clip->x2 && clip->y1 < clip->y2);
> > > +	if (dirty) {
> > > +		clip_copy = *clip;
> > > +		clip->x1 = clip->y1 = ~0;
> > > +		clip->x2 = clip->y2 = 0;
> > > +	}
> > >  	spin_unlock_irqrestore(&helper->dirty_lock, flags);
> > >  
> > > -	helper->fb->funcs->dirty(helper->fb, NULL, 0, 0, &clip_copy, 1);
> > > +	if (dirty)
> > 
> > Could do it the other way too, ie. just make the copy, and then check the
> > copy (can be done after dropping the lock even). Would avoid having to
> > add the 'dirty' variable.
> 
> Sounds good.  Let me try...

Another thing: How do we prevent userspace from doing the same, i.e.
submitting an empty rectangle? Do we need to patch up
drm_mode_dirtyfb_ioctl too? Not much point if we fix this bug in the fb
helpers and leave the barn door wide open for userspace to oops drivers
;-)
-Daniel

> 
> 
> Takashi

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