Re: [PATCH] drm/i915/vlv: Fix off-by-1 error in calculating num_levels.

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Jul 19, 2016 at 04:50:49PM +0100, Chris Wilson wrote:
> On Tue, Jul 19, 2016 at 06:25:42PM +0300, Ville Syrjälä wrote:
> > On Tue, Jul 19, 2016 at 05:14:23PM +0200, Maarten Lankhorst wrote:
> > > num_levels should be level+1, not level, else num_levels - 1 becomes
> > > negative. This resulted in bogus watermarks being written to the first
> > > 255 levels like below:
> > > 
> > > [drm] Setting FIFO watermarks - C: plane=0, cursor=0, sprite0=0, sprite1=0, SR: plane=0, cursor=0 level=255 cxsr=0
> > > [drm:chv_set_memory_dvfs [i915]] *ERROR* timed out waiting for Punit DDR DVFS request
> > > [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe C FIFO underrun
> > > [drm:chv_set_memory_dvfs [i915]] *ERROR* timed out waiting for Punit DDR DVFS request
> > > 
> > > Testcase: kms_atomic_transition
> > > Fixes: 262cd2e154c2 ("drm/i915: CHV DDR DVFS support and another watermark rewrite")
> > > Cc: stable@vger.kernel.org
> > > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > > ---
> > > Urgent fix for watermark support. This is definitely a pre-requisite for this series.
> > > With this I've noticed that patch "[RFC 3/8] drm/i915/vlv: Move fifo_size from
> > > intel_plane_wm_parameters to vlv_wm_state" introduces a regression with invalid FIFO split.
> > > 
> > > I need to find out what's going wrong in that patch before this series can be applied.
> > > 
> > >  drivers/gpu/drm/i915/intel_pm.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> > > index 376c60b98515..8defdcc54529 100644
> > > --- a/drivers/gpu/drm/i915/intel_pm.c
> > > +++ b/drivers/gpu/drm/i915/intel_pm.c
> > > @@ -1148,7 +1148,7 @@ static void vlv_compute_wm(struct intel_crtc *crtc)
> > >  			}
> > >  		}
> > >  
> > > -		wm_state->num_levels = level;
> > > +		wm_state->num_levels = level + 1;
> > 
> > Nope. The loop above breaks when the current level is bad, hence level-1
> > is actually the higher usable level.
> 
> Without knowing the limits of plane->wm.fifo_size, it looks like it can
> break on level == 0 though.

Hmm. That shouldn't be possible. So looks like a bug snuck in.

> Might as well set that hack to paranoid levels:
> 
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index 630b116988f6..e8c2874b8629 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -1124,11 +1124,13 @@ static void vlv_compute_wm(struct intel_crtc *crtc)
>                 /* normal watermarks */
>                 for (level = 0; level < wm_state->num_levels; level++) {
>                         int wm = vlv_compute_wm_level(plane, crtc, state, level);
> -                       int max_wm = plane->base.type == DRM_PLANE_TYPE_CURSOR ? 63 : 511;
> -
>                         /* hack */
> -                       if (WARN_ON(level == 0 && wm > max_wm))
> -                               wm = max_wm;

Actually this should just be 

if (WARN_ON(level == 0 && wm > fifo_size))
	wm = fifo_size;

assuming we want to keep the hack around for now.

Eventually we'll want to make it just return an error though.

> +                       if (level == 0) {
> +                               int plane_wm = plane->base.type == DRM_PLANE_TYPE_CURSOR ? 63 : 511;
> +                               int max_wm = min(plane->wm.fifo_size, plane_wm);
> +                               if (WARN_ON(wm > max_wm))
> +                                       wm = max_wm;
> +                       }
>  
>                         if (wm > plane->wm.fifo_size)
>                                 break;
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre

-- 
Ville Syrjälä
Intel OTC
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