Re: [PATCH v5 10/15] drm/i915/alpm: Program LTTPR count for DP 2.1 ALPM

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

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 <jouni.hogander@intel.com>
> Signed-off-by: Animesh Manna <animesh.manna@intel.com>
> ---
>  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?

> +	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.

> +	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?

BR,
Jouni Högander

> +
>  	if (intel_dp_is_uhbr(crtc_state))
>  		passed = intel_dp_128b132b_link_train(intel_dp,
> crtc_state, lttpr_count);
>  	else