[PATCH 1/2] drm/i915/hdmi: Poll for 200 msec for TMDS_Scrambler_Status

[Jerome Tollet <jerome.tollet@gmail.com>]

As per HDMI 2.0 specification, after scrambled video transmission begins,
the source must poll the TMDS_Scrambler_Status bit until it reads 1 or
until a timeout of 200 ms.

Add a polling step after enabling the HDMI port to verify scrambling
status, following the spec requirement.

Without the wait for the scrambling bit to set, some HDMI 2.0 monitors fail
to decode the signal at 4K@60Hz (594 MHz) when SCDC scrambling is not yet
fully configured by the sink.

v2:
 - Instead of the fixed delay, poll for TMDS scramble status for 200 msec
   as per the HDMI spec. (Ankit)

Reported-by: Jerome Tollet <jerome.tollet@gmail.com>
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6868
Link: https://lore.kernel.org/dri-devel/20251230091037.5603-1-jerome.tollet@gmail.com/
Signed-off-by: Jerome Tollet <jerome.tollet@gmail.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
---
 drivers/gpu/drm/i915/display/intel_ddi.c  |  2 ++
 drivers/gpu/drm/i915/display/intel_hdmi.c | 25 +++++++++++++++++++++++
 drivers/gpu/drm/i915/display/intel_hdmi.h |  2 ++
 3 files changed, 29 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_ddi.c b/drivers/gpu/drm/i915/display/intel_ddi.c
index cb91d07cdaa6..c708b713f0e8 100644
--- a/drivers/gpu/drm/i915/display/intel_ddi.c
+++ b/drivers/gpu/drm/i915/display/intel_ddi.c
@@ -3506,6 +3506,8 @@ static void intel_ddi_enable_hdmi(struct intel_atomic_state *state,
 	}

 	intel_ddi_buf_enable(encoder, buf_ctl);
+
+	intel_hdmi_poll_for_scrambling_enable(crtc_state, connector);
 }

 static void intel_ddi_enable(struct intel_atomic_state *state,
diff --git a/drivers/gpu/drm/i915/display/intel_hdmi.c b/drivers/gpu/drm/i915/display/intel_hdmi.c
index 055e68810d0d..958d939ae6ee 100644
--- a/drivers/gpu/drm/i915/display/intel_hdmi.c
+++ b/drivers/gpu/drm/i915/display/intel_hdmi.c
@@ -2694,6 +2694,31 @@ intel_hdmi_add_properties(struct intel_hdmi *intel_hdmi, struct drm_connector *_
 	drm_connector_attach_max_bpc_property(&connector->base, 8, 12);
 }

+/*
+ * As Per HDMI 2.0 spec: after scrambled video transmission begins,
+ * poll TMDS_Scrambler_Status until it reads 1, for up to 200 ms.
+ */
+void
+intel_hdmi_poll_for_scrambling_enable(const struct intel_crtc_state *crtc_state,
+				      struct drm_connector *_connector)
+{
+	struct intel_connector *connector = to_intel_connector(_connector);
+	struct intel_display *display = to_intel_display(crtc_state);
+	bool scrambling_enabled = false;
+	int ret;
+
+	if (!crtc_state->hdmi_scrambling)
+		return;
+
+	/* Poll for a max of 200 msec as per HDMI spec */
+	ret = poll_timeout_us(scrambling_enabled = drm_scdc_get_scrambling_status(&connector->base),
+			      scrambling_enabled, 1000, 200 * 1000, false);
+	if (ret)
+		drm_dbg_kms(display->drm,
+			    "[CONNECTOR:%d:%s] Timed out waiting for scrambling enable\n",
+			    connector->base.base.id, connector->base.name);
+}
+
 /*
  * intel_hdmi_handle_sink_scrambling: handle sink scrambling/clock ratio setup
  * @encoder: intel_encoder
diff --git a/drivers/gpu/drm/i915/display/intel_hdmi.h b/drivers/gpu/drm/i915/display/intel_hdmi.h
index be2fad57e4ad..0fa3661568e8 100644
--- a/drivers/gpu/drm/i915/display/intel_hdmi.h
+++ b/drivers/gpu/drm/i915/display/intel_hdmi.h
@@ -70,5 +70,7 @@ void hsw_read_infoframe(struct intel_encoder *encoder,
 			const struct intel_crtc_state *crtc_state,
 			unsigned int type,
 			void *frame, ssize_t len);
+void intel_hdmi_poll_for_scrambling_enable(const struct intel_crtc_state *crtc_state,
+					   struct drm_connector *_connector);

 #endif /* __INTEL_HDMI_H__ */
--
2.45.2