Re: [PATCH 16/19] drm/i915/dp: Set relevant Downspread Ctrl DPCD bits for PR + Auxless ALPM

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Mar 30, 2026 at 09:36:53AM +0530, Ankit Nautiyal wrote:
> To support Panel Replay with Auxless-ALPM, the source must transmit
> Adaptive-Sync SDPs for video timing synchronization while PR is active.
> As per the DP spec v2.1, this requires setting DPCD 0x0107[6]
> (FIXED_VTOTAL_AS_SDP_EN_IN_PR_ACTIVE). This applies whether VRR is enabled
> (AVT/FAVT) or fixed-timing mode is used.
> 
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp_link_training.c | 10 ++++++++--
>  drivers/gpu/drm/i915/display/intel_dp_link_training.h |  3 ++-
>  drivers/gpu/drm/i915/display/intel_dp_mst.c           |  2 +-
>  3 files changed, 11 insertions(+), 4 deletions(-)
> 
> 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 54c585c59b90..136cabf06fd9 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -27,6 +27,7 @@
>  #include <drm/display/drm_dp_helper.h>
>  #include <drm/drm_print.h>
>  
> +#include "intel_alpm.h"
>  #include "intel_display_core.h"
>  #include "intel_display_jiffies.h"
>  #include "intel_display_types.h"
> @@ -710,11 +711,14 @@ static bool intel_dp_link_max_vswing_reached(struct intel_dp *intel_dp,
>  	return true;
>  }
>  
> -void intel_dp_link_training_set_mode(struct intel_dp *intel_dp, int link_rate, bool is_vrr)
> +void intel_dp_link_training_set_mode(struct intel_dp *intel_dp, int link_rate,
> +				     bool is_vrr,
> +				     bool is_pr_with_link_off)

>  {
>  	u8 link_config[2];
>  
>  	link_config[0] = is_vrr ? DP_MSA_TIMING_PAR_IGNORE_EN : 0;
> +	link_config[0] |= is_pr_with_link_off ? DP_FIXED_VTOTAL_AS_SDP_EN_IN_PR_ACTIVE : 0;

I think we should only set this if we are going to enable the AS SDP.

The bit seems to be perhaps a bit misnamed in the spec because you
apparently you have to set it even if you suspend AS SDP transmission
during PR active (when you have a sink with 
DP_PANEL_REPLAY_ASYNC_VIDEO_TIMING_NOT_SUPPORTED_IN_PR == 0). In that
case it seems to just mean that the sink will grab the coasting vtotal
from the last AS SDP transmitted prior to PR active.

>  	link_config[1] = drm_dp_is_uhbr_rate(link_rate) ?
>  			 DP_SET_ANSI_128B132B : DP_SET_ANSI_8B10B;
>  	drm_dp_dpcd_write(&intel_dp->aux, DP_DOWNSPREAD_CTRL, link_config, 2);
> @@ -737,7 +741,9 @@ static void intel_dp_update_downspread_ctrl(struct intel_dp *intel_dp,
>  	  * especially on the first real commit when clearing the inherited flag.
>  	  */
>  	intel_dp_link_training_set_mode(intel_dp,
> -					crtc_state->port_clock, crtc_state->vrr.in_range);
> +					crtc_state->port_clock,
> +					crtc_state->vrr.in_range,
> +					intel_alpm_is_alpm_aux_less(intel_dp, crtc_state));
>  }
>  
>  void intel_dp_link_training_set_bw(struct intel_dp *intel_dp,
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.h b/drivers/gpu/drm/i915/display/intel_dp_link_training.h
> index 1ba22ed6db08..3591210f8ee6 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.h
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.h
> @@ -18,7 +18,8 @@ int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp);
>  bool intel_dp_lttpr_transparent_mode_enabled(struct intel_dp *intel_dp);
>  
>  void intel_dp_link_training_set_mode(struct intel_dp *intel_dp,
> -				     int link_rate, bool is_vrr);
> +				     int link_rate, bool is_vrr,
> +				     bool is_pr_with_link_off);
>  void intel_dp_link_training_set_bw(struct intel_dp *intel_dp,
>  				   int link_bw, int rate_select, int lane_count,
>  				   bool enhanced_framing);
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index 887b6de14e46..2201cf7ce015 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -2142,7 +2142,7 @@ void intel_dp_mst_prepare_probe(struct intel_dp *intel_dp)
>  
>  	intel_dp_compute_rate(intel_dp, link_rate, &link_bw, &rate_select);
>  
> -	intel_dp_link_training_set_mode(intel_dp, link_rate, false);
> +	intel_dp_link_training_set_mode(intel_dp, link_rate, false, false);
>  	intel_dp_link_training_set_bw(intel_dp, link_bw, rate_select, lane_count,
>  				      drm_dp_enhanced_frame_cap(intel_dp->dpcd));
>  
> -- 
> 2.45.2

-- 
Ville Syrjälä
Intel