Re: [PATCH 07/15] drm/i915/psr: Add function to compute max link-wake latency

["Nautiyal, Ankit K" <ankit.k.nautiyal@intel.com>]

On 8/5/2025 2:24 AM, Jani Nikula wrote:
> On Mon, 04 Aug 2025, Ankit Nautiyal <ankit.k.nautiyal@intel.com> wrote:
>> Introduce a helper to compute the max link wake latency when using
>> Auxless/Aux wake mechanism for PSR/Panel Replay/LOBF features.
>>
>> This will be used to compute the minimum guardband so that the link wake
>> latencies are accounted and these features work smoothly for higher
>> refresh rate panels.
>>
>> Bspec: 70151, 71477
>> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
>> ---
>>   drivers/gpu/drm/i915/display/intel_psr.c | 64 ++++++++++++++++++++++++
>>   drivers/gpu/drm/i915/display/intel_psr.h |  3 ++
>>   2 files changed, 67 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/display/intel_psr.c b/drivers/gpu/drm/i915/display/intel_psr.c
>> index 6bd3454bb00e..6cdaff3ccc9f 100644
>> --- a/drivers/gpu/drm/i915/display/intel_psr.c
>> +++ b/drivers/gpu/drm/i915/display/intel_psr.c
>> @@ -33,6 +33,7 @@
>>   #include "intel_atomic.h"
>>   #include "intel_crtc.h"
>>   #include "intel_cursor_regs.h"
>> +#include "intel_cx0_phy.h"
>>   #include "intel_ddi.h"
>>   #include "intel_de.h"
>>   #include "intel_display_irq.h"
>> @@ -4249,3 +4250,66 @@ bool intel_psr_needs_alpm_aux_less(struct intel_dp *intel_dp,
>>   {
>>   	return intel_dp_is_edp(intel_dp) && crtc_state->has_panel_replay;
>>   }
>> +
>> +static
>> +int intel_psr_compute_aux_wake_latency(struct intel_dp *intel_dp,
>> +				       struct intel_crtc_state *crtc_state)
>> +{
>> +#define TFW_EXIT_LATENCY_MS		20000
>> +#define FAST_WAKE_LATENCY_MS		12000 /* Preamble: 8us; PHY wake: 4us */
>> +	struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
>> +	int aux_wake_latency_us;
>> +	int io_buffer_wake_ms;
>> +
>> +	io_buffer_wake_ms = intel_encoder_is_c10phy(encoder) ? 9790 : 14790;
>> +
>> +	aux_wake_latency_us =
>> +		DIV_ROUND_UP(io_buffer_wake_ms + TFW_EXIT_LATENCY_MS + FAST_WAKE_LATENCY_MS, 1000);
> See https://lore.kernel.org/r/eeda84457c813151a3459a46a91946b4fbbb9e44@intel.com

Oops! I think I have messed up msecs and usecs in the calculations.

I have realized, part of these calculations are already there in 
functions in intel_alpm.c which can be used here.

I will correct this and rework on the patch.

Thanks Jani, for pointing this out.

Regards,

Ankit


>
>> +
>> +	return aux_wake_latency_us;
>> +}
>> +
>> +static
>> +int intel_psr_compute_auxless_latency(struct intel_crtc_state *crtc_state)
>> +{
>> +#define PHY_ESTABLISHMENT_PERIOD_MS	50000
>> +#define LFPS_PERIOD_MS			800
>> +#define SILENCE_MAX_MS			180
>> +	int linkrate_mhz = crtc_state->port_clock / 1000;
>> +	int clock_data_switch_ms;
>> +	int auxless_latency_us;
>> +	int time_ml_phy_lock_ms;
>> +	int num_ml_phy_lock;
>> +	/*
>> +	 * TPS4 length = 252
>> +	 * tML_PHY_LOCK = TPS4 Length * ( 10 / (Link Rate in MHz) )
>> +	 * Number ML_PHY_LOCK = ( 7 + CEILING(6.5us / tML_PHY_LOCK ) + 1)
>> +	 * t2 = Number ML_PHY_LOCK * tML_PHY_LOCK
>> +	 * tCDS term  = 2 * t2
>> +	 * =>tCDS_term  = 2 * (7 * (252 * (10 /linkrate))+6.5)
>> +	 */
>> +	time_ml_phy_lock_ms = (1000 * 252 * 10) / linkrate_mhz;
>> +	num_ml_phy_lock = 7 + DIV_ROUND_UP(6500 * 1000, time_ml_phy_lock_ms) / 1000 + 1;
>> +	clock_data_switch_ms = 2 * time_ml_phy_lock_ms * num_ml_phy_lock;
>> +
>> +	auxless_latency_us = (LFPS_PERIOD_MS  + SILENCE_MAX_MS + PHY_ESTABLISHMENT_PERIOD_MS +
>> +			      clock_data_switch_ms) / 1000;
>> +
>> +	return auxless_latency_us;
>> +}
>> +
>> +int intel_psr_compute_max_link_wake_latency(struct intel_dp *intel_dp,
>> +					    struct intel_crtc_state *crtc_state,
>> +					    bool assume_all_enabled)
>> +{
>> +	int aux_wake_latency = 0;
>> +	int auxless_latency = 0;
>> +
>> +	if (assume_all_enabled || crtc_state->has_sel_update)
>> +		auxless_latency = intel_psr_compute_aux_wake_latency(intel_dp, crtc_state);
>> +
>> +	if (assume_all_enabled || crtc_state->has_panel_replay)
>> +		aux_wake_latency = intel_psr_compute_auxless_latency(crtc_state);
>> +
>> +	return max(auxless_latency, aux_wake_latency);
>> +}
>> diff --git a/drivers/gpu/drm/i915/display/intel_psr.h b/drivers/gpu/drm/i915/display/intel_psr.h
>> index 9b061a22361f..c58d29620b49 100644
>> --- a/drivers/gpu/drm/i915/display/intel_psr.h
>> +++ b/drivers/gpu/drm/i915/display/intel_psr.h
>> @@ -81,5 +81,8 @@ void intel_psr_debugfs_register(struct intel_display *display);
>>   bool intel_psr_needs_alpm(struct intel_dp *intel_dp, const struct intel_crtc_state *crtc_state);
>>   bool intel_psr_needs_alpm_aux_less(struct intel_dp *intel_dp,
>>   				   const struct intel_crtc_state *crtc_state);
>> +int intel_psr_compute_max_link_wake_latency(struct intel_dp *intel_dp,
>> +					    struct intel_crtc_state *crtc_state,
>> +					    bool assume_all_enabled);
>>   
>>   #endif /* __INTEL_PSR_H__ */