Re: [PATCH v6 12/18] drm/i915/alpm: Program LTTPR count for DP 2.1 ALPM

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

On Tue, 2026-09-15 at 23:34 +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]
> v3:
> - Use same encoding format for lttpr_count. [Jouni]
> - Avoid reading alpm_ctl2 register. [Jouni]
> 
> Cc: Jouni Högander <jouni.hogander@intel.com>
> Signed-off-by: Animesh Manna <animesh.manna@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_alpm.c     |  7 ++++++-
>  .../drm/i915/display/intel_dp_link_training.c | 21
> +++++++++++++++++++
>  2 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_alpm.c
> b/drivers/gpu/drm/i915/display/intel_alpm.c
> index da653a407e97..06c1a59aa2c5 100644
> --- a/drivers/gpu/drm/i915/display/intel_alpm.c
> +++ b/drivers/gpu/drm/i915/display/intel_alpm.c
> @@ -726,7 +726,7 @@ 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))
> @@ -762,6 +762,11 @@ 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);

You should check the return value here.

> +
> +	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..37c8306a1fa0 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 +2265,25 @@ 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)
> +{
> +	u8 val;
> +
> +	/*
> +	 * 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) ||

This is wrong. You probably meant
intel_dp_is_edp(intel_dp)intel_dp_is_edp(intel_dp).

> +	    lttpr_count < 0 ||

No need to check this. It's already set as 0 in start_link_train if <
0.

BR,
Jouni Högander

> +	    intel_dp->lttpr_common_caps[0] <= 0x20)
> +		return;
> +
> +	val = intel_dp->lttpr_common_caps[DP_PHY_REPEATER_CNT -
> +					 
> DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV];
> +	drm_dp_dpcd_writeb(&intel_dp->aux, DP_TOTAL_LTTPR_CNT, val);
> +}
> +
>  /**
>   * intel_dp_start_link_train - start link training
>   * @state: Atomic state
> @@ -2304,6 +2323,8 @@ 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);
> +
>  	if (intel_dp_is_uhbr(crtc_state))
>  		passed = intel_dp_128b132b_link_train(intel_dp,
> crtc_state, lttpr_count);
>  	else