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

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

On Mon, Jul 25, 2016 at 02:46:01PM +0200, Maarten Lankhorst wrote:
> Op 25-07-16 om 13:51 schreef Ville Syrjälä:
> > On Mon, Jul 25, 2016 at 01:32:45PM +0200, Maarten Lankhorst wrote:
> >> Hey,
> >>
> >> Op 19-07-16 om 18:21 schreef Ville Syrjälä:
> >>> 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.
> >> Yes, that's what I came up with.
> >>
> >> But we still need to clamp further, probably to plane->wm.fifo_size.
> >> However sr_fifo_size is also clamped to 511 because of the level calculations here.
> >>
> >> Below it sets sr[level].plane = min(sr_fifo_size, wm[level].plane),
> >> which seems weird. How can this ever end up being something other than wm[level].plane?
> > There are three planes. CHV was supposed to have maxfifo with up to 2 active
> > planes. But I'm not sure that feature made it in. I never got around
> > testing it.
> I don't know, we only enable it with a single plane, plus cursor. I don't see where the other
> plane wm's would fit in?

The FIFO would be split 1:1 when two planes are enabled. But as stated, I
never actually tried it, so I never enabled it.

-- 
Ville Syrjälä
Intel OTC
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