Re: [PATCH v4 11/13] drm/i915/display: Evade scanline 0 as well if PSR1 or PSR2 is enabled

["Hogander, Jouni" <jouni.hogander@intel.com>]

On Fri, 2025-01-24 at 14:37 +0200, Ville Syrjälä wrote:
> On Fri, Jan 24, 2025 at 11:57:10AM +0000, Hogander, Jouni wrote:
> > On Fri, 2025-01-24 at 13:39 +0200, Ville Syrjälä wrote:
> > > On Fri, Jan 24, 2025 at 12:56:22PM +0200, Jouni Högander wrote:
> > > > PIPEDSL is reading as 0 when in SRDENT(PSR1) or
> > > > DEEP_SLEEP(PSR2).
> > > > On
> > > > wake-up scanline counting starts from vblank_start - 1. We
> > > > don't
> > > > know if
> > > > wake-up is already ongoing when evasion starts. In worst case
> > > > PIPEDSL could
> > > > start reading valid value right after checking the scanline. In
> > > > this
> > > > scenario we wouldn't have enough time to write all registers.
> > > > To
> > > > tackle
> > > > this evade scanline 0 as well. As a drawback we have 1 frame
> > > > delay
> > > > in flip
> > > > when waking up.
> > > > 
> > > > Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/display/intel_dsb.c | 12 ++++++++++++
> > > >  1 file changed, 12 insertions(+)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c
> > > > b/drivers/gpu/drm/i915/display/intel_dsb.c
> > > > index bb77ded8bf726..914f0be4491c4 100644
> > > > --- a/drivers/gpu/drm/i915/display/intel_dsb.c
> > > > +++ b/drivers/gpu/drm/i915/display/intel_dsb.c
> > > > @@ -528,6 +528,18 @@ void intel_dsb_vblank_evade(struct
> > > > intel_atomic_state *state,
> > > >  	int latency = intel_usecs_to_scanlines(&crtc_state-
> > > > > hw.adjusted_mode, 20);
> > > >  	int start, end;
> > > >  
> > > > +	/*
> > > > +	 * PIPEDSL is reading as 0 when in SRDENT(PSR1) or
> > > > DEEP_SLEEP(PSR2). On
> > > > +	 * wake-up scanline counting starts from vblank_start
> > > > - 1.
> > > > We don't know
> > > > +	 * if wake-up is already ongoing when evasion starts.
> > > > In
> > > > worst case
> > > > +	 * PIPEDSL could start reading valid value right after
> > > > checking the
> > > > +	 * scanline. In this scenario we wouldn't have enough
> > > > time
> > > > to write all
> > > > +	 * registers. To tackle this evade scanline 0 as well.
> > > > As
> > > > a drawback we
> > > > +	 * have 1 frame delay in flip when waking up.
> > > > +	 */
> > > > +	if (crtc_state->has_psr && !crtc_state-
> > > > >has_panel_replay)
> > > 
> > > What's the deal with panel replay here?
> > 
> > I couldn't see PIPEDSL returning 0 when using Panel Replay. Even on
> > same setup with PSR I saw it, but with PR I couldn't see.
> 
> Hmm. Are you sure it's reaching DC5/6?
> 
> Though I suppose it's possible that panel replay (unlike PSR)
> does pretty much everything from the DMC's DC state handler,
> so as soon as you read the register it restores things fully
> enough that you get the vblank_start-1 response...

Maybe we just add that evasion without checking panel replay. It
shouldn't be too expensive anyways. What do you think?

BR,

Jouni Högander
>