[PATCH] RFC drm: Do not leak connector_status_unknown to userspace

[Chris Wilson]

A detect() probe may return 'unknown' to indicate that it was not able
to successfully determine the connector status. However, exposing this
to userspace just results in lots of confusion as it is uncertain
whether or not that is a valid connection. The usual result is for X to
create a mode that encompasses the unknown connector causing an abnormal
setup on the known good outputs.

This patch initialises the connector status to disconnected and
thereafter ignores indeterminate results from detect(), leaving the
status as the old value. This prevents the status from being set to
unknown, and so leaked to userspace, and so leaves the connector as
disconnected until we have a valid connection and also prevents that
valid connection being dropped due to a failed probe.

The alternative to this is to never return unknown from the drivers, but
return connector->status instead so that the value never changes on a
failed probe (and also to initialise the connector to disconnected in
the driver).
---
 drivers/gpu/drm/drm_crtc.c        |    1 +
 drivers/gpu/drm/drm_crtc_helper.c |   20 +++++++++++++-------
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 08a7aa7..22f58b9 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -495,6 +495,7 @@ int drm_connector_init(struct drm_device *dev,
 	INIT_LIST_HEAD(&connector->probed_modes);
 	INIT_LIST_HEAD(&connector->modes);
 	connector->edid_blob_ptr = NULL;
+	connector->status = connector_status_disconnected;
 
 	list_add_tail(&connector->head, &dev->mode_config.connector_list);
 	dev->mode_config.num_connector++;
diff --git a/drivers/gpu/drm/drm_crtc_helper.c b/drivers/gpu/drm/drm_crtc_helper.c
index 3252e70..143d6d5 100644
--- a/drivers/gpu/drm/drm_crtc_helper.c
+++ b/drivers/gpu/drm/drm_crtc_helper.c
@@ -108,7 +108,11 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
 		if (connector->funcs->force)
 			connector->funcs->force(connector);
 	} else {
-		connector->status = connector->funcs->detect(connector, true);
+		enum drm_connector_status status;
+
+		status = connector->funcs->detect(connector, true);
+		if (status)
+			connector->status = status;
 		drm_kms_helper_poll_enable(dev);
 	}
 
@@ -949,13 +953,15 @@ static void output_poll_execute(struct work_struct *work)
 		    !(connector->polled & DRM_CONNECTOR_POLL_HPD))
 			continue;
 
-		connector->status = connector->funcs->detect(connector, false);
-		DRM_DEBUG_KMS("[CONNECTOR:%d:%s] status updated from %d to %d\n",
-			      connector->base.id,
-			      drm_get_connector_name(connector),
-			      old_status, connector->status);
-		if (old_status != connector->status)
+		new_status = connector->funcs->detect(connector, false);
+		if (new_status && old_status != new_status) {
+			DRM_DEBUG_KMS("[CONNECTOR:%d:%s] status updated from %d to %d\n",
+				      connector->base.id,
+				      drm_get_connector_name(connector),
+				      old_status, new_status);
+			connector->status = new_status;
 			changed = true;
+		}
 	}
 
 	mutex_unlock(&dev->mode_config.mutex);
-- 
1.7.10