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

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Oct 21, 2016 at 01:57:28PM +0100, Chris Wilson wrote:
> On Fri, Oct 21, 2016 at 02:30:32PM +0200, Daniel Vetter wrote:
> > 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
> > ;-)
> 
> I think of a use for sending an empty clip: where you don't want to
> push any new pixel data, but you do want to be sure that the pipeline
> has been flushed.

What exactly should an empty rectangle flush out? It's a bit unclear, but
for speed I guess drivers should be allowed to make dirty async ...
-Daniel

> 
> The change I would suggest here would be
> 
> 	dirty = clip->x1 <= clip->x2 && clip->y1 <= clip->y2
> 
> as the bug is not the empty rectangle but the invalid one. However, that
> may be overkill, and none of the backends care about the empty rect!
> 
> But, indeed, we do not validate the incoming dirtyfb either.
> 
> diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
> index 49fd7db758e0..ada6a5517945 100644
> --- a/drivers/gpu/drm/drm_framebuffer.c
> +++ b/drivers/gpu/drm/drm_framebuffer.c
> @@ -504,10 +504,13 @@ int drm_mode_dirtyfb_ioctl(struct drm_device *dev,
>         }
>  
>         if (num_clips && clips_ptr) {
> +               int i;
> +
>                 if (num_clips < 0 || num_clips > DRM_MODE_FB_DIRTY_MAX_CLIPS) {
>                         ret = -EINVAL;
>                         goto out_err1;
>                 }
> +
>                 clips = kcalloc(num_clips, sizeof(*clips), GFP_KERNEL);
>                 if (!clips) {
>                         ret = -ENOMEM;
> @@ -520,6 +523,14 @@ int drm_mode_dirtyfb_ioctl(struct drm_device *dev,
>                         ret = -EFAULT;
>                         goto out_err2;
>                 }
> +
> +               for (i = 0; i < num_clips; i++) {
> +                       if (clips[i].x2 < clips[i].x1 ||
> +                           clips[i].y2 < clips[i].y1) {
> +                               ret = -EINVAL;
> +                               goto out_err2;
> +                       }
> +               }
>         }
>  
>         if (fb->funcs->dirty) 
> 
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch