Re: [PATCH 06/15] drm/i915/dp: Add SDP latency computation helper

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

On Thu, Sep 11, 2025 at 08:15:45AM +0530, Ankit Nautiyal wrote:
> Add a helper to compute vblank time needed for transmitting specific
> DisplayPort SDPs like PPS, GAMUT_METADATA, and VSC_EXT. Latency is
> based on line count per packet type and current line time.
> 
> Used to ensure adequate vblank when features like DSC/HDR are enabled.
> 
> Bspec: 70151
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 47 +++++++++++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_dp.h |  2 ++
>  2 files changed, 49 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 2eab591a8ef5..83c46e4680b3 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -6857,3 +6857,50 @@ void intel_dp_mst_resume(struct intel_display *display)
>  		}
>  	}
>  }
> +
> +static
> +int intel_dp_get_sdp_latency(u32 type, int linetime_us)

The "get" in the name is redundant.

> +{
> +	int lines;
> +
> +	switch (type) {
> +	case DP_SDP_VSC_EXT_VESA:
> +	case DP_SDP_VSC_EXT_CEA:
> +		lines = 10;
> +		break;
> +	case HDMI_PACKET_TYPE_GAMUT_METADATA:
> +		lines = 8;
> +		break;
> +	case DP_SDP_PPS:
> +		lines = 6;
> +		break;
> +	default:
> +		lines = 0;
> +		break;
> +	}

We should probably calculate these properly instead of
just hardcoding annoying magic numbers. But I suppose
these are supposed to be some kind of semi worst case
numbers so perhaps OK for now.

> +
> +	return lines * linetime_us;
> +}
> +
> +int intel_dp_compute_sdp_latency(const struct intel_crtc_state *crtc_state,
> +				 bool assume_all_enabled)
> +{
> +	const struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
> +	int sdp_latency = 0;
> +	int linetime_us;
> +
> +	linetime_us = DIV_ROUND_UP(adjusted_mode->crtc_htotal * 1000,
> +				   adjusted_mode->crtc_clock);
> +	if (assume_all_enabled ||
> +	    crtc_state->infoframes.enable &
> +	    intel_hdmi_infoframe_enable(HDMI_PACKET_TYPE_GAMUT_METADATA))
> +		sdp_latency = max(sdp_latency,
> +				  intel_dp_get_sdp_latency(HDMI_PACKET_TYPE_GAMUT_METADATA,
> +							   linetime_us));
> +
> +	if (assume_all_enabled || crtc_state->dsc.compression_enable)
> +		sdp_latency = max(sdp_latency,
> +				  intel_dp_get_sdp_latency(DP_SDP_PPS, linetime_us));
> +
> +	return sdp_latency;
> +}
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.h b/drivers/gpu/drm/i915/display/intel_dp.h
> index f90cfd1dbbd0..d222749b191c 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.h
> +++ b/drivers/gpu/drm/i915/display/intel_dp.h
> @@ -215,5 +215,7 @@ int intel_dp_compute_min_hblank(struct intel_crtc_state *crtc_state,
>  int intel_dp_dsc_bpp_step_x16(const struct intel_connector *connector);
>  void intel_dp_dpcd_set_probe(struct intel_dp *intel_dp, bool force_on_external);
>  bool intel_dp_in_hdr_mode(const struct drm_connector_state *conn_state);
> +int intel_dp_compute_sdp_latency(const struct intel_crtc_state *crtc_state,
> +				 bool assume_all_enabled);
>  
>  #endif /* __INTEL_DP_H__ */
> -- 
> 2.45.2

-- 
Ville Syrjälä
Intel