[PATCH 3/3] drm/display: hdmi: Reset sink scrambling on non-scrambled modesets

[Cristian Ciocaltea <cristian.ciocaltea@collabora.com>]

drm_connector_hdmi_disable_scrambling() only clears the sink SCDC status
when the connector is known to be connected.  That is never true when a
source driver disables scrambling at boot to clean up after the
firmware, as vc4 does from vc4_crtc_disable_at_boot(), where
connector->status is still unknown and no EDID has been read.

A sink left scrambling by the firmware thus keeps both Scrambling_Enable
and the 1/40 TMDS bit clock ratio set.  If the first modeset picks a
mode below 340 MHz, no disable happens in between, since the CRTC was
never active in atomic state, and drm_connector_hdmi_enable_scrambling()
returns early, leaving the sink trying to descramble an unscrambled
stream.

Instead of blind SCDC writes at boot towards a possibly absent sink,
reset the sink status from drm_connector_hdmi_enable_scrambling() when
no scrambling is required.  By then the sink has been probed, and HDMI
2.0 expects a scrambling capable sink to have Scrambling_Enable cleared
below 340 MHz anyway.  The reset runs regardless of source scrambler
support, since the firmware may have left the sink scrambling.

Factor the reset out into a helper also used by the disable and error
paths, gated only on drm_scdc_sink_supports_scrambling(), which suffices
because drm_display_info is zeroed until the first EDID read and reset
on disconnect, making the connector->status check redundant.  As the
helper now runs on every non-scrambled modeset, read SCDC_TMDS_CONFIG
first and only clear the bits actually set, so the common case costs a
single DDC read.

Fixes: d653818324ed ("drm/display: hdmi: Add HDMI 2.0 scrambling management helpers")
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
---
 drivers/gpu/drm/display/drm_hdmi_helper.c | 45 ++++++++++++++++++++++++-------
 1 file changed, 36 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_hdmi_helper.c b/drivers/gpu/drm/display/drm_hdmi_helper.c
index 974d5de6d380..278cfda38e18 100644
--- a/drivers/gpu/drm/display/drm_hdmi_helper.c
+++ b/drivers/gpu/drm/display/drm_hdmi_helper.c
@@ -470,6 +470,28 @@ static bool drm_scdc_sink_supports_scrambling(struct drm_connector *connector)
 	       info->hdmi.scdc.scrambling.supported;
 }
 
+static void drm_connector_hdmi_reset_sink_scrambling(struct drm_connector *connector)
+{
+	struct drm_device *dev = connector->dev;
+	u8 config;
+	int ret;
+
+	if (!connector->ddc || !drm_scdc_sink_supports_scrambling(connector))
+		return;
+
+	ret = drm_scdc_readb(connector->ddc, SCDC_TMDS_CONFIG, &config);
+	if (ret) {
+		drm_dbg_kms(dev, "Failed to read TMDS config: %d\n", ret);
+		return;
+	}
+
+	if (config & SCDC_SCRAMBLING_ENABLE)
+		drm_scdc_set_scrambling(connector, false);
+
+	if (config & SCDC_TMDS_BIT_CLOCK_RATIO_BY_40)
+		drm_scdc_set_high_tmds_clock_ratio(connector, false);
+}
+
 static int drm_connector_hdmi_try_scrambling_setup(struct drm_connector *connector)
 {
 	bool done;
@@ -522,6 +544,11 @@ static void drm_connector_hdmi_scdc_work(struct work_struct *work)
  * the sink side and retry the operation, as some displays refuse to set the
  * scrambling bit right away.
  *
+ * When @conn_state doesn't require scrambling, the sink-side SCDC status is
+ * reset instead, so that a sink left scrambling by a state the driver did not
+ * control, e.g. set up by the firmware before boot, doesn't end up out of sync
+ * with the source.
+ *
  * The work item is cancelled by drm_connector_hdmi_disable_scrambling(), hence
  * source drivers must ensure the display pipeline is shut down on unbind, e.g.
  * via drm_atomic_helper_shutdown(). Otherwise the work may outlive the driver's
@@ -541,8 +568,14 @@ int drm_connector_hdmi_enable_scrambling(struct drm_connector *connector,
 	if (!conn_state)
 		return -EINVAL;
 
-	if (!conn_state->hdmi.scrambler_needed)
+	if (!conn_state->hdmi.scrambler_needed) {
+		/*
+		 * Resync the sink here, where it is known to be connected
+		 * and its capabilities have been parsed.
+		 */
+		drm_connector_hdmi_reset_sink_scrambling(connector);
 		return 0;
+	}
 
 	if (!drm_connector_hdmi_scrambler_supported(connector)) {
 		drm_dbg_kms(dev, "Source doesn't support scrambling.\n");
@@ -581,8 +614,7 @@ int drm_connector_hdmi_enable_scrambling(struct drm_connector *connector,
 		WRITE_ONCE(hdmi->scrambler_enabled, false);
 		cancel_delayed_work_sync(&hdmi->scdc_work);
 
-		drm_scdc_set_scrambling(connector, false);
-		drm_scdc_set_high_tmds_clock_ratio(connector, false);
+		drm_connector_hdmi_reset_sink_scrambling(connector);
 	}
 
 	return ret;
@@ -619,12 +651,7 @@ int drm_connector_hdmi_disable_scrambling(struct drm_connector *connector)
 	if (hdmi->scdc_work_initialized)
 		cancel_delayed_work_sync(&hdmi->scdc_work);
 
-	if (connector->ddc &&
-	    connector->status == connector_status_connected &&
-	    drm_scdc_sink_supports_scrambling(connector)) {
-		drm_scdc_set_scrambling(connector, false);
-		drm_scdc_set_high_tmds_clock_ratio(connector, false);
-	}
+	drm_connector_hdmi_reset_sink_scrambling(connector);
 
 	return hdmi->funcs->scrambler_disable(connector);
 }

-- 
2.55.0