Re: [Intel-gfx] [PATCH 1/4] drm/i915/vrr: Make registers latch in a consitent place on icl/tgl

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

On Mon, Dec 05, 2022 at 12:13:09PM -0800, Navare, Manasi wrote:
> On Fri, Dec 02, 2022 at 03:44:09PM +0200, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > Account for the framestart delay when calculating the "pipeline full"
> > value for icl/tgl vrr. This puts the start of vblank (ie. where the
> > double bufferd registers get latched) to a consistent place regardless
> > of what framestart delay value is used. framestart delay does not
> > change where start of vblank occurs in non-vrr mode and I can't see
> > any reason why we'd want different behaviour in vrr mode.
> > 
> > Currently framestart delay is always set to 1, and the hardcoded 4
> > scanlines in the code means we're currently delaying the start of
> > vblank by three extra lines. And with framestart delay set to 4 we'd
> > have no extra delay.
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> So now basically if we want to delay the vblank, then we will need to
> update framestart_delay to somethin other than 1.

No. framestart_delay does not affect where vblank starts. Or rather
it's not supposed to, but before this patch it was was affect when
VRR was enabled.

> Currently with framestart_delay = 1, there is no vblank delay, its just
> vrr.vmin - vdisplay so the vblank start right after?
> 
> Is this the correct understanding?
> 
> Anyway, if this logic is validated to work then should be fine.
> Basically this will only impact display <13, so as long as we dont
> regress anything on TGL then we should be good.
> 
> Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
> 
> Manasi
> 
> > ---
> >  drivers/gpu/drm/i915/display/intel_vrr.c | 13 ++-----------
> >  1 file changed, 2 insertions(+), 11 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_vrr.c b/drivers/gpu/drm/i915/display/intel_vrr.c
> > index 7b1357e82b69..6655dd2c1684 100644
> > --- a/drivers/gpu/drm/i915/display/intel_vrr.c
> > +++ b/drivers/gpu/drm/i915/display/intel_vrr.c
> > @@ -153,18 +153,9 @@ intel_vrr_compute_config(struct intel_crtc_state *crtc_state,
> >  		crtc_state->vrr.guardband =
> >  			crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay;
> >  	} else {
> > -		/*
> > -		 * FIXME: s/4/framestart_delay/ to get consistent
> > -		 * earliest/latest points for register latching regardless
> > -		 * of the framestart_delay used?
> > -		 *
> > -		 * FIXME: this really needs the extra scanline to provide consistent
> > -		 * behaviour for all framestart_delay values. Otherwise with
> > -		 * framestart_delay==4 we will end up extending the min vblank by
> > -		 * one extra line.
> > -		 */
> >  		crtc_state->vrr.pipeline_full =
> > -			min(255, crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay - 4 - 1);
> > +			min(255, crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay -
> > +			    crtc_state->framestart_delay - 1);
> >  	}
> >  
> >  	crtc_state->mode_flags |= I915_MODE_FLAG_VRR;
> > -- 
> > 2.37.4
> > 

-- 
Ville Syrjälä
Intel