[Intel-gfx] [PATCH 2/4] drm/i915/vrr: Fix guardband/vblank exit length calculation for adl+

[Ville Syrjälä]

On Mon, Dec 05, 2022 at 12:34:25PM -0800, Navare, Manasi wrote:
> On Fri, Dec 02, 2022 at 03:44:10PM +0200, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > 
> > We are miscalculating both the guardband value, and the resulting
> > vblank exit length on adl+. This means that our start of vblank
> > (double buffered register latch point) is incorrect, and we also
> > think that it's not where it actually is (hence vblank evasion/etc.
> > may not work properly). Fix up the calculations to match the real
> > hardware behaviour (as reverse engineered by intel_display_poller).
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_vrr.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_vrr.c b/drivers/gpu/drm/i915/display/intel_vrr.c
> > index 6655dd2c1684..753e7b211708 100644
> > --- a/drivers/gpu/drm/i915/display/intel_vrr.c
> > +++ b/drivers/gpu/drm/i915/display/intel_vrr.c
> > @@ -78,10 +78,10 @@ static int intel_vrr_vblank_exit_length(const struct intel_crtc_state *crtc_stat
> >  	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
> >  	struct drm_i915_private *i915 = to_i915(crtc->base.dev);
> >  
> > -	/* The hw imposes the extra scanline before frame start */
> >  	if (DISPLAY_VER(i915) >= 13)
> > -		return crtc_state->vrr.guardband + crtc_state->framestart_delay + 1;
> > +		return crtc_state->vrr.guardband;
> 
> This makes sense since with guardband, there is no framestart delay

framestart delay is still a thing. But it's not something that
affects how the hardware interprets the guardband value.

> 
> >  	else
> > +		/* The hw imposes the extra scanline before frame start */
> >  		return crtc_state->vrr.pipeline_full + crtc_state->framestart_delay + 1;
> >  }
> >  
> > @@ -151,7 +151,7 @@ intel_vrr_compute_config(struct intel_crtc_state *crtc_state,
> >  		 * number of scan lines. Assuming 0 for no DSB.
> >  		 */
> >  		crtc_state->vrr.guardband =
> > -			crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay;
> > +			crtc_state->vrr.vmin + 1 - adjusted_mode->crtc_vdisplay;
> 
> Why are we adding + 1 here? The bspec says guardband should be :
> Guardband = Vmin - Vactive - Window2 where in our case Window2 = 0
> If we need that + 1 to get this working, then perhaps we need to update
> Bspec?

flipline is what actaully determines the start of vblank, and
'flipline>=vmin+1' always.

> 
> I kind of want to see if this is still breaking if we dont have that +
> 1?

Without it start of vblank happens one line later than where we want it
to happen.

> 
> Manasi
> 
> >  	} else {
> >  		crtc_state->vrr.pipeline_full =
> >  			min(255, crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay -
> > -- 
> > 2.37.4
> > 

-- 
Ville Syrjälä
Intel