[PATCH v3] drm/i915/dp: Gate UHBR SST SDP splitting on sink capability

[Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>]

SDP splitting for 128b/132b (UHBR) SST audio must only be enabled when
the sink advertises support for it. Previously sdp_split_enable
was set for every UHBR SST stream carrying audio, regardless of sink
capability.

In MST mode SDP splitting is inherently supported, so the sink
capability check (DP_SST_SPLIT_SDP_CAP) is applied only to the SST path.

Fixes: 8853750dbad8 ("drm/i915: Enable SDP split for DP2.0")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>

--v2:
- Correct the name in the intel_dp struct. (Suraj)
- Add the Fixes tag. (Suraj)
- Explain the reasoning for guarding SST separately from MST. (Suraj)
- Add a separate helper to read capabilities. (Suraj)

--v3:
- Use drm_dp_dpcd_read instead of drm_dp_dpcd_readb. (Suraj)
- Move Assisted-by above Signed-off-by. (Suraj)
- Add the versioning diff. (Suraj)
---
 .../drm/i915/display/intel_display_types.h    |  2 +
 drivers/gpu/drm/i915/display/intel_dp.c       | 44 ++++++++++++++++---
 2 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h b/drivers/gpu/drm/i915/display/intel_display_types.h
index 20a07ea06b5e..5f0fe18c0614 100644
--- a/drivers/gpu/drm/i915/display/intel_display_types.h
+++ b/drivers/gpu/drm/i915/display/intel_display_types.h
@@ -1951,6 +1951,8 @@ struct intel_dp {
 
 	bool colorimetry_support;
 
+	bool sst_split_sdp_support;
+
 	struct {
 		enum transcoder transcoder;
 		struct mutex lock;
diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 6b60e0a1c8ee..0cd5e6b5034c 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -3409,12 +3409,22 @@ intel_dp_audio_compute_config(struct intel_encoder *encoder,
 			      struct intel_crtc_state *pipe_config,
 			      struct drm_connector_state *conn_state)
 {
+	struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
+
 	pipe_config->has_audio =
 		intel_dp_has_audio(encoder, conn_state) &&
 		intel_audio_compute_config(encoder, pipe_config, conn_state);
 
 	pipe_config->sdp_split_enable = pipe_config->has_audio &&
 					intel_dp_is_uhbr(pipe_config);
+
+	/*
+	 * SDP splitting for UHBR audio requires explicit sink capability in
+	 * SST mode, whereas in MST mode it is inherently supported.
+	 */
+	if (pipe_config->sdp_split_enable &&
+	    !intel_crtc_has_type(pipe_config, INTEL_OUTPUT_DP_MST))
+		pipe_config->sdp_split_enable = intel_dp->sst_split_sdp_support;
 }
 
 void
@@ -4456,16 +4466,25 @@ void intel_dp_configure_protocol_converter(struct intel_dp *intel_dp,
 			    str_enable_disable(tmp));
 }
 
-static bool intel_dp_get_colorimetry_status(struct intel_dp *intel_dp)
+static u8 intel_dp_read_dprx_feature_enum(struct intel_dp *intel_dp)
 {
 	u8 dprx = 0;
 
-	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_DPRX_FEATURE_ENUMERATION_LIST,
-			      &dprx) != 1)
-		return false;
+	drm_dp_dpcd_read_data(&intel_dp->aux, DP_DPRX_FEATURE_ENUMERATION_LIST,
+			      &dprx, sizeof(dprx));
+	return dprx;
+}
+
+static bool intel_dp_get_colorimetry_status(u8 dprx)
+{
 	return dprx & DP_VSC_SDP_EXT_FOR_COLORIMETRY_SUPPORTED;
 }
 
+static bool intel_dp_get_sst_split_sdp_status(u8 dprx)
+{
+	return dprx & DP_SST_SPLIT_SDP_CAP;
+}
+
 static int intel_dp_read_dsc_dpcd(struct drm_dp_aux *aux,
 				  u8 dsc_dpcd[DP_DSC_RECEIVER_CAP_SIZE])
 {
@@ -4765,6 +4784,7 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp, struct intel_connector *connector
 {
 	struct intel_display *display = to_intel_display(intel_dp);
 	int ret;
+	u8 dprx;
 
 	/* this function is meant to be called only once */
 	drm_WARN_ON(display->drm, intel_dp->dpcd[DP_DPCD_REV] != 0);
@@ -4776,8 +4796,13 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp, struct intel_connector *connector
 			 drm_dp_is_branch(intel_dp->dpcd));
 	intel_init_dpcd_quirks(intel_dp, &intel_dp->desc.ident);
 
+	dprx = intel_dp_read_dprx_feature_enum(intel_dp);
+
 	intel_dp->colorimetry_support =
-		intel_dp_get_colorimetry_status(intel_dp);
+		intel_dp_get_colorimetry_status(dprx);
+
+	intel_dp->sst_split_sdp_support =
+		intel_dp_get_sst_split_sdp_status(dprx);
 
 	/*
 	 * Read the eDP display control registers.
@@ -4868,13 +4893,20 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
 	 * the OUI/ID since we know it won't change.
 	 */
 	if (!intel_dp_is_edp(intel_dp)) {
+		u8 dprx;
+
 		drm_dp_read_desc(&intel_dp->aux, &intel_dp->desc,
 				 drm_dp_is_branch(intel_dp->dpcd));
 
 		intel_init_dpcd_quirks(intel_dp, &intel_dp->desc.ident);
 
+		dprx = intel_dp_read_dprx_feature_enum(intel_dp);
+
 		intel_dp->colorimetry_support =
-			intel_dp_get_colorimetry_status(intel_dp);
+			intel_dp_get_colorimetry_status(dprx);
+
+		intel_dp->sst_split_sdp_support =
+			intel_dp_get_sst_split_sdp_status(dprx);
 
 		intel_dp_update_sink_caps(intel_dp);
 	}
-- 
2.48.1