Re: [PATCH] drm/i915/ltphy: Return true for TBT scenario during lt_phy_state compare

[Imre Deak <imre.deak@intel.com>]

On Tue, Feb 03, 2026 at 10:40:33AM +0200, Kandpal, Suraj wrote:
> > Subject: Re: [PATCH] drm/i915/ltphy: Return true for TBT scenario during
> > lt_phy_state compare
> > 
> > On Tue, Feb 03, 2026 at 08:11:41AM +0530, Suraj Kandpal wrote:
> > > TBT PHY is enablement/disablement is handled by its own TBT module.
> > > We do not play a big part in it's state management that being take
> > > care by it's own TBT modeule. With that in mind comparing the state
> > > would be wrong since we really don't touch it.
> > > Simple return true when we are in tbt mode.
> > >
> > > Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/display/intel_lt_phy.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/i915/display/intel_lt_phy.c
> > > b/drivers/gpu/drm/i915/display/intel_lt_phy.c
> > > index 04f63bdd0b87..27ad8407606b 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_lt_phy.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_lt_phy.c
> > > @@ -2158,6 +2158,9 @@ bool
> > >  intel_lt_phy_pll_compare_hw_state(const struct intel_lt_phy_pll_state *a,
> > >  				  const struct intel_lt_phy_pll_state *b)  {
> > > +	if (a->tbt_mode || b->tbt_mode)
> > > +		return true;
> > 
> > It is a relevant state whether the PHY is in TBT mode or in a non-TBT mode,
> > even if the PHY is programmed by the TBT HW block in TBT mode. So it's not ok
> > to ignore a difference in this state. Please provide the actual context for why
> > the change was added.
> 
> Bspec : 74492
>
> We do not write the PLL registers when it comes to TBT Mode that is
> done by the TBT dock.  Since the VDR register are shadow registers our
> shadow registers will never have those values. Which Means we will
> always have a state mismatch.

Reading out the HW state of the actual PLL dividers and configuration is
not read out in TBT mode for the above reason but the verification is
skipped in TBT mode for another reason:

The PHY/PLL TypeC mode (TBT, DP-alt) can change after the PLL state was
computed for a modeset, so the state verification after the modeset
sequence would indicate a mismatch in case the mode changed from DP-alt
to TBT, or from TBT to DP-alt mode. To avoid such a mismatch error the
verification is skipped if the mode for either the read-out or the
computed state is TBT (where that TBT PLL state doesn't reflect anyway
the PLL's actual HW state).

Could you please amend the commit long along the above lines as a
rationale for the change?

> This has always been the case since SNPS PHY.  Check
> intel_cx0pll_compare_hw_state int intel_cx0_phy.c too
> 
> Regards,
> Suraj Kandpal
> 
> > 
> > > +
> > >  	/*
> > >  	 * With LT PHY values other than VDR0_CONFIG and VDR2_CONFIG are
> > >  	 * unreliable. They cannot always be read back since internally
> > > --
> > > 2.34.1
> > >