[Intel-gfx] [PATCH] drm/i915: Avoid fluctuating to UNKNOWN connector status during forced probes

[Chris Wilson]

For old-school component TV and CRT connectors, we require a heavyweight
load-detection mechanism. This involves setting up a CRTC and sending a
signal to the output, before reading back any response. As that is quite
slow and CPU heavy, the process is only performed when the output
detection is forced by user request. As it requires a driving CRTC, we
often don't have the resources to complete the probe. This leaves us in
a quandary where the unforced path just returns the old connector
status, but the forced detection path elects to return UNKNOWN. If we
have an active connection, we likely have the resources available to
complete the probe - but if it is currently disconnected, then it
becomes unknown and triggers a hotplug event, with often quite unfortunate
userspace behaviour (e.g. one output is blanked and the spurious TV
turned on).

To reduce spurious hotplug events on older devices, we can prevent
transitions between disconnected <-> unknown.

References: https://bugs.freedesktop.org/show_bug.cgi?id=87049
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/intel_crt.c | 39 ++++++++++++++++++---------------------
 drivers/gpu/drm/i915/intel_tv.c  | 25 +++++++++----------------
 2 files changed, 27 insertions(+), 37 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_crt.c b/drivers/gpu/drm/i915/intel_crt.c
index 521af2c..70c5288 100644
--- a/drivers/gpu/drm/i915/intel_crt.c
+++ b/drivers/gpu/drm/i915/intel_crt.c
@@ -666,8 +666,6 @@ intel_crt_detect(struct drm_connector *connector, bool force)
 	struct intel_encoder *intel_encoder = &crt->base;
 	enum intel_display_power_domain power_domain;
 	enum drm_connector_status status;
-	struct intel_load_detect_pipe tmp;
-	struct drm_modeset_acquire_ctx ctx;
 
 	DRM_DEBUG_KMS("[CONNECTOR:%d:%s] force=%d\n",
 		      connector->base.id, connector->name,
@@ -703,27 +701,26 @@ intel_crt_detect(struct drm_connector *connector, bool force)
 		goto out;
 	}
 
-	if (!force) {
-		status = connector->status;
-		goto out;
-	}
-
-	drm_modeset_acquire_init(&ctx, 0);
-
-	/* for pre-945g platforms use load detect */
-	if (intel_get_load_detect_pipe(connector, NULL, &tmp, &ctx)) {
-		if (intel_crt_detect_ddc(connector))
-			status = connector_status_connected;
-		else if (INTEL_INFO(dev)->gen < 4)
-			status = intel_crt_load_detect(crt);
-		else
+	status = connector->status;
+	if (force) {
+		struct intel_load_detect_pipe tmp;
+		struct drm_modeset_acquire_ctx ctx;
+
+		drm_modeset_acquire_init(&ctx, 0);
+
+		/* for pre-945g platforms use load detect */
+		if (intel_get_load_detect_pipe(connector, NULL, &tmp, &ctx)) {
+			if (intel_crt_detect_ddc(connector))
+				status = connector_status_connected;
+			else if (INTEL_INFO(dev)->gen < 4)
+				status = intel_crt_load_detect(crt);
+			intel_release_load_detect_pipe(connector, &tmp, &ctx);
+		} else if (status == connector_status_connected)
 			status = connector_status_unknown;
-		intel_release_load_detect_pipe(connector, &tmp, &ctx);
-	} else
-		status = connector_status_unknown;
 
-	drm_modeset_drop_locks(&ctx);
-	drm_modeset_acquire_fini(&ctx);
+		drm_modeset_drop_locks(&ctx);
+		drm_modeset_acquire_fini(&ctx);
+	}
 
 out:
 	intel_display_power_put(dev_priv, power_domain);
diff --git a/drivers/gpu/drm/i915/intel_tv.c b/drivers/gpu/drm/i915/intel_tv.c
index 8b9d325..e3c0fed 100644
--- a/drivers/gpu/drm/i915/intel_tv.c
+++ b/drivers/gpu/drm/i915/intel_tv.c
@@ -1313,44 +1313,37 @@ static void intel_tv_find_better_format(struct drm_connector *connector)
 static enum drm_connector_status
 intel_tv_detect(struct drm_connector *connector, bool force)
 {
-	struct drm_display_mode mode;
 	struct intel_tv *intel_tv = intel_attached_tv(connector);
-	enum drm_connector_status status;
-	int type;
+	enum drm_connector_status status = connector->status;
 
 	DRM_DEBUG_KMS("[CONNECTOR:%d:%s] force=%d\n",
 		      connector->base.id, connector->name,
 		      force);
-
-	mode = reported_modes[0];
-
 	if (force) {
+		struct drm_display_mode mode = reported_modes[0];
 		struct intel_load_detect_pipe tmp;
 		struct drm_modeset_acquire_ctx ctx;
 
 		drm_modeset_acquire_init(&ctx, 0);
 
 		if (intel_get_load_detect_pipe(connector, &mode, &tmp, &ctx)) {
-			type = intel_tv_detect_type(intel_tv, connector);
+			int type = intel_tv_detect_type(intel_tv, connector);
 			intel_release_load_detect_pipe(connector, &tmp, &ctx);
 			status = type < 0 ?
 				connector_status_disconnected :
 				connector_status_connected;
-		} else
+			intel_tv->type = type;
+		} else if (status == connector_status_connected)
 			status = connector_status_unknown;
 
 		drm_modeset_drop_locks(&ctx);
 		drm_modeset_acquire_fini(&ctx);
-	} else
-		return connector->status;
 
-	if (status != connector_status_connected)
-		return status;
-
-	intel_tv->type = type;
-	intel_tv_find_better_format(connector);
+		if (status == connector_status_connected)
+			intel_tv_find_better_format(connector);
+	}
 
-	return connector_status_connected;
+	return status;
 }
 
 static const struct input_res {
-- 
1.9.1