[PATCH 1/8] drm/i915: Keep TRANS_VBLANK.vblank_start==0 on ADL+ even when doing LRR updates

[Ville Syrjälä]

On Mon, Jan 20, 2025 at 06:48:31PM +0200, Jani Nikula wrote:
> On Thu, 16 Jan 2025, Ville Syrjala <ville.syrjala at linux.intel.com> wrote:
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> >
> > intel_set_transcoder_timings() will set TRANS_VBLANK.vblank_start to 0
> > for clarity on ADL+ (non-DSI) because the hardware no longer uses that
> > value. So the same in intel_set_transcoder_timings_lrr() to make sure
> > the registers stay constitent even when doing LRR timing updates.
> >
> > Cc: Paz Zcharya <pazz at chromium.org>
> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Hmm, so how come this doesn't warrant a change in readout?
> 
> Apparently because intel_get_transcoder_timings() overwrites the read
> value for display 13+ and non-dsi. Hrmh.

Yeah, it's a bit dodgy. I did originally ponder about adding some
kind of sanity check to readout too, but the GOP might still put
something into TRANS_VBLANK.vblank_start which would cause a false
positive.

> 
> Reviewed-by: Jani Nikula <jani.nikula at intel.com>
> 
> 
> > ---
> >  drivers/gpu/drm/i915/display/intel_display.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> > index f5d2eacce119..5ba3b2d658e5 100644
> > --- a/drivers/gpu/drm/i915/display/intel_display.c
> > +++ b/drivers/gpu/drm/i915/display/intel_display.c
> > @@ -2923,6 +2923,14 @@ static void intel_set_transcoder_timings_lrr(const struct intel_crtc_state *crtc
> >  	crtc_vblank_start = adjusted_mode->crtc_vblank_start;
> >  	crtc_vblank_end = adjusted_mode->crtc_vblank_end;
> >  
> > +	if (DISPLAY_VER(dev_priv) >= 13) {
> > +		/*
> > +		 * VBLANK_START not used by hw, just clear it
> > +		 * to make it stand out in register dumps.
> > +		 */
> > +		crtc_vblank_start = 1;
> > +	}
> > +
> >  	drm_WARN_ON(&dev_priv->drm, adjusted_mode->flags & DRM_MODE_FLAG_INTERLACE);
> >  
> >  	/*
> 
> -- 
> Jani Nikula, Intel

-- 
Ville Syrjälä
Intel