> -----Original Message-----
> From: Hogander, Jouni <[email protected]>
> Sent: Wednesday, September 2, 2026 2:57 PM
> To: [email protected]; Manna, Animesh
> <[email protected]>; [email protected]
> Cc: Kandpal, Suraj <[email protected]>; Nikula, Jani
> <[email protected]>
> Subject: Re: [PATCH v5 10/15] drm/i915/alpm: Program LTTPR count for DP
> 2.1 ALPM
>
> On Tue, 2026-08-25 at 11:15 +0530, Animesh Manna wrote:
> > Issue a AUX write transaction to DPCD DP_TOTAL_LTTPR_CNT (0xf000a)
> > with total number of LTTPR before link training.
> >
> > v2: Cosmetic changes. [Suraj]
> >
> > Cc: Jouni Högander <[email protected]>
> > Signed-off-by: Animesh Manna <[email protected]>
> > ---
> > drivers/gpu/drm/i915/display/intel_alpm.c | 9 ++++++++-
> > .../drm/i915/display/intel_dp_link_training.c | 16
> > ++++++++++++++++
> > 2 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_alpm.c
> > b/drivers/gpu/drm/i915/display/intel_alpm.c
> > index 890bf2488554..d23f3bdf1339 100644
> > --- a/drivers/gpu/drm/i915/display/intel_alpm.c
> > +++ b/drivers/gpu/drm/i915/display/intel_alpm.c
> > @@ -563,7 +563,7 @@ static void lnl_alpm_configure(struct intel_dp
> > *intel_dp,
> > {
> > struct intel_display *display = to_intel_display(intel_dp);
> > enum transcoder cpu_transcoder = crtc_state->cpu_transcoder;
> > - u32 alpm_ctl;
> > + u32 alpm_ctl, alpm_ctl2, lttpr_count;
> >
> > if (DISPLAY_VER(display) < 20 ||
> > (!intel_psr_needs_alpm(intel_dp, crtc_state) &&
> > !crtc_state->has_lobf))
> > @@ -613,6 +613,13 @@ static void lnl_alpm_configure(struct intel_dp
> > *intel_dp,
> >
> > alpm_ctl |= ALPM_CTL_ALPM_ENTRY_CHECK(crtc_state-
> > >alpm_state.check_entry_lines);
> >
> > + lttpr_count = drm_dp_lttpr_count(intel_dp-
> > >lttpr_common_caps);
> > +
> > + alpm_ctl2 = intel_de_read(display, ALPM_CTL2(display,
> > cpu_transcoder));
>
> Why you are reading this. Can't we just configure alpm_ctl2 as we want and
> then write that one?
Ok, can be done.
>
> > + alpm_ctl2 &= ~ALPM_CTL2_NUMBER_OF_LTTPR_MASK;
> > + alpm_ctl2 |= ALPM_CTL2_NUMBER_OF_LTTPR(lttpr_count);
> > +
> > + intel_de_write(display, ALPM_CTL2(display, cpu_transcoder),
> > alpm_ctl2);
> > intel_de_write(display, ALPM_CTL(display, cpu_transcoder),
> > alpm_ctl);
> > mutex_unlock(&intel_dp->alpm.lock);
> > }
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > index cb92cff90614..1d5873cb64f5 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > @@ -2265,6 +2265,20 @@ intel_dp_128b132b_link_train(struct intel_dp
> > *intel_dp,
> > return passed;
> > }
> >
> > +static void intel_dp_update_lttpr_count(struct intel_dp *intel_dp,
> > + int lttpr_count)
> > +{
> > + /*
> > + * Program only for DP2.1 and return if LTTPR revison is
> > less than 0x20
> > + * where 0:3 represents minor rev and 4:7 represent major
> > rev
> > + */
> > + if (!intel_dp_is_edp(intel_dp) ||
> > + intel_dp->lttpr_common_caps[0] <= 0x20)
> > + return;
> > +
>
> "The DPTX shall issue an AUX write transaction to the DPCD register with the
> value determined from the PHY_REPEATER_CNT and same encoding defined
> for DPCD F0002h."
>
> I.e. You can't just write count as it is.
Yes, you are right.
>
> > + drm_dp_dpcd_writeb(&intel_dp->aux, DP_TOTAL_LTTPR_CNT,
> > lttpr_count);
> > +}
> > +
> > /**
> > * intel_dp_start_link_train - start link training
> > * @state: Atomic state
> > @@ -2304,6 +2318,8 @@ void intel_dp_start_link_train(struct
> > intel_atomic_state *state,
> >
> > intel_dp_prepare_link_train(intel_dp, crtc_state);
> >
> > + intel_dp_update_lttpr_count(intel_dp, lttpr_count);
>
> How about having this in intel_alpm_enable_sink?
As per DP spec we need program before link training, but
intel_alpm_enable_sink() is called after link training.
Regards,
Animesh
>
> BR,
> Jouni Högander
>
> > +
> > if (intel_dp_is_uhbr(crtc_state))
> > passed = intel_dp_128b132b_link_train(intel_dp,
> > crtc_state, lttpr_count);
> > else
