Re: [PATCH v5 14/15] drm/i915/alpm: Compute and program switch to active latency

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

On Tue, 2026-08-25 at 11:15 +0530, Animesh Manna wrote:
> Program the ALPM switch to active latency into ALPM_CTL2. For UHBR
> link
> rates the latency is derived from the link symbol clock and converted
> to
> scanlines, for non-UHBR rates it is zero.
> 
> Cc: Jouni Högander <jouni.hogander@intel.com>
> Signed-off-by: Animesh Manna <animesh.manna@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_alpm.c     | 32
> +++++++++++++++++--
>  .../drm/i915/display/intel_display_types.h    |  1 +
>  2 files changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_alpm.c
> b/drivers/gpu/drm/i915/display/intel_alpm.c
> index 8c17da9883d6..5d2ac7561567 100644
> --- a/drivers/gpu/drm/i915/display/intel_alpm.c
> +++ b/drivers/gpu/drm/i915/display/intel_alpm.c
> @@ -270,6 +270,31 @@ static int get_establishment_period(struct
> intel_dp *intel_dp,
>  	return establishment_period;
>  }
>  
> +
> +static int get_switch_to_active(const struct intel_crtc_state
> *crtc_state)
> +{
> +	int port_clock = crtc_state->port_clock;
> +	int switch_to_active;
> +
> +	if (intel_dp_is_uhbr(crtc_state)) {
> +		/* symbol_clock (fLink) in MHz */
> +		int symbol_clock = port_clock /
> intel_dp_link_symbol_size(port_clock) / 100;
> +
> +		/*
> +		 * tSwitch to Active = 32 * (ML_PHY_LOCK Length + 3
> + 64) / fLink
> +		 *
> +		 * The "+ 3" term is the trailing zero padding after
> the
> +		 * POST_LT_SCRAMBLER_RESET, the "+ 64" term
> represents the MTP
> +		 * time slots. The result is in microseconds.
> +		 */
> +		switch_to_active = 32 * ((396 + 3 + 64) /
> symbol_clock);

You have added ML_PHY_LOCK_LEN_UHBR in your earlier patch. I think 32
in this formula is same as intel_dp_link_symbol_size. So you could use
that.

BR,
Jouni Högander
> +	} else {
> +		switch_to_active = 0;
> +	}
> +
> +	return switch_to_active;
> +}
> +
>  /*
>   * AUX-Less Wake Time = CEILING( ((PHY P2 to P0) + tLFPS_Period,
> Max+
>   * tSilence, Max+ tPHY Establishment + tCDS) / tline)
> @@ -329,7 +354,8 @@ _lnl_compute_aux_less_alpm_params(struct intel_dp
> *intel_dp,
>  	crtc_state->alpm_state.aux_less_wake_lines =
> aux_less_wake_lines;
>  	crtc_state->alpm_state.silence_period_sym_clocks =
> silence_period;
>  	crtc_state->alpm_state.lfps_half_cycle_num_of_syms =
> lfps_half_cycle;
> -
> +	crtc_state->alpm_state.switch_to_active =
> intel_usecs_to_scanlines(&crtc_state->hw.adjusted_mode,
> +								
> 	   get_switch_to_active(crtc_state));
>  	return true;
>  }
>  
> @@ -619,8 +645,10 @@ static void lnl_alpm_configure(struct intel_dp
> *intel_dp,
>  	lttpr_count = drm_dp_lttpr_count(intel_dp-
> >lttpr_common_caps);
>  
>  	alpm_ctl2 = intel_de_read(display, ALPM_CTL2(display,
> cpu_transcoder));
> -	alpm_ctl2 &= ~ALPM_CTL2_NUMBER_OF_LTTPR_MASK;
> +	alpm_ctl2 &= ~(ALPM_CTL2_NUMBER_OF_LTTPR_MASK |
> +		       ALPM_CTL2_SWITCH_TO_ACTIVE_LATENCY_MASK);
>  	alpm_ctl2 |= ALPM_CTL2_NUMBER_OF_LTTPR(lttpr_count);
> +	alpm_ctl2 |= ALPM_CTL2_SWITCH_TO_ACTIVE_LATENCY(crtc_state-
> >alpm_state.switch_to_active);
>  
>  	intel_de_write(display, ALPM_CTL2(display, cpu_transcoder),
> alpm_ctl2);
>  	intel_de_write(display, ALPM_CTL(display, cpu_transcoder),
> alpm_ctl);
> diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h
> b/drivers/gpu/drm/i915/display/intel_display_types.h
> index f8ac98ff8a87..4cbf56a9195f 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_types.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_types.h
> @@ -1441,6 +1441,7 @@ struct intel_crtc_state {
>  		u8 aux_less_wake_lines;
>  		u8 silence_period_sym_clocks;
>  		u8 lfps_half_cycle_num_of_syms;
> +		int switch_to_active;
>  	} alpm_state;
>  	bool has_alpm;
>