[PATCH] drm/i915/hdmi: preserve cached EDID on DDC failure when connector is forced

[Aman Sharma <amansharma6122002@gmail.com>]

When an HDMI connector is forced enabled (e.g. via video=HDMI-A-1:e for
laptop clamshell operation) and the external monitor enters DPMS standby,
some monitors (such as the BenQ GW2790Q) power down their internal DDC/I2C
controller, causing SCDC/EDID reads to fail with -ENXIO (-6).

Currently, intel_hdmi_force() unconditionally calls intel_hdmi_unset_edid()
before calling intel_hdmi_set_edid(). When drm_edid_read_ddc() returns NULL
during monitor standby, the connector's cached detect_edid is wiped out and
drm_helper_probe_single_connector_modes() falls back to the 1024x768 VESA
no-EDID mode. When the monitor wakes back up, because the connector status
remained forced to connector_status_connected, no hotplug state transition
occurs and the display remains stuck at 1024x768 until a manual mode probe
(such as toggling the laptop lid) is triggered.

Fix this by retaining the previously cached detect_edid when a DDC read
fails on a forced-on connector. When the monitor is awake or a different
monitor is plugged in, drm_edid_read_ddc() succeeds and replaces the
cached EDID normally.

Signed-off-by: Aman Sharma <amansharma6122002@gmail.com>
---
 drivers/gpu/drm/i915/display/intel_hdmi.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_hdmi.c b/drivers/gpu/drm/i915/display/intel_hdmi.c
index b6fa1a1558b8..140d98a61a9c 100644
--- a/drivers/gpu/drm/i915/display/intel_hdmi.c
+++ b/drivers/gpu/drm/i915/display/intel_hdmi.c
@@ -2570,6 +2570,18 @@ intel_hdmi_set_edid(struct drm_connector *_connector)
 		intel_gmbus_force_bit(ddc, false);
 	}
 
+	/*
+	 * If the connector is forced on (e.g. video=HDMI-A-1:e) and the sink's
+	 * DDC/I2C bus is powered down during DPMS standby, keep the previously
+	 * cached EDID rather than wiping it and falling back to 1024x768.
+	 */
+	if (!drm_edid && connector->base.force && connector->detect_edid) {
+		intel_display_power_put(display, POWER_DOMAIN_GMBUS, wakeref);
+		return true;
+	}
+
+	drm_edid_free(connector->detect_edid);
+
 	/* Below we depend on display info having been updated */
 	drm_edid_connector_update(&connector->base, drm_edid);
 
@@ -2638,10 +2650,10 @@ intel_hdmi_force(struct drm_connector *_connector)
 	if (!intel_display_driver_check_access(display))
 		return;
 
-	intel_hdmi_unset_edid(&connector->base);
-
-	if (connector->base.status != connector_status_connected)
+	if (connector->base.status != connector_status_connected) {
+		intel_hdmi_unset_edid(&connector->base);
 		return;
+	}
 
 	intel_hdmi_set_edid(&connector->base);
 }
-- 
2.43.0