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

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

On Wed, Dec 07, 2022 at 01:05:15PM -0800, Navare, Manasi wrote:
> On Wed, Dec 07, 2022 at 05:10:54PM +0200, Ville Syrjälä wrote:
> > 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@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@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.
> 
> Flipline would be always >=vmin as per the bspec,

Not sure where in bspec you see that. All I see is >vmin,
and it even says you et an extra line if you try to set them
equal. Pretty sure I verified that behaviour on the hw on icl/tgl
since I put the extra -1 to the vmin calculation. Though I haven't
actually tested it on adl+.

> have we tried with
> that or if that definitely doesnt work then we need to have this changed
> in the bspec.
> 
> Either way if this is the only value that works then with this change
> added to bspec:
> 
> Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
> 
> Manasi
> 
> > 
> > > 
> > > 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

-- 
Ville Syrjälä
Intel