[PATCH 2/2] drm/crtc-helper: clamp unknown connector status in the poll work

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On some chipset we try to avoid possibly invasive output detection
methods (like load detect which can cause flickering elsewhere) in the
output poll work. Drivers could hence return unknown when a previous
full ->detect call returned a different state.

This change will generate a hotplug event, forcing userspace to do a
full scan. This in turn updates the connector->status field so that we
will _again_ get a state change when the hotplug work re-runs in 10
seconds.

To avoid this ping-pong loop detect this situation and clamp the
connector state to the old value.

Patch is inspired by a patch from Knut Peterson. Knut's patch
completely ignored connector state changes if either the old or new
status was unknown, which seemed to be a bit too agressive to me.

References: http://lists.freedesktop.org/archives/dri-devel/2012-August/025975.html
Cc: Knut Petersen <Knut_Petersen@t-online.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/drm_crtc_helper.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/gpu/drm/drm_crtc_helper.c b/drivers/gpu/drm/drm_crtc_helper.c
index 1c0c0bc..9985a17 100644
--- a/drivers/gpu/drm/drm_crtc_helper.c
+++ b/drivers/gpu/drm/drm_crtc_helper.c
@@ -1040,6 +1040,24 @@ static void output_poll_execute(struct work_struct *work)
 		if (old_status != connector->status) {
 			const char *old, *new;
 
+			/*
+			 * The poll work sets force=false when calling detect so
+			 * that drivers can avoid to do disruptive tests (e.g.
+			 * when load detect cycles could cause flickering on
+			 * other, running displays). This bears the risk that we
+			 * flip-flop between unknown here in the poll work and
+			 * the real state when userspace forces a full detect
+			 * call after receiving a hotplug event due to this
+			 * change.
+			 *
+			 * Hence clamp an unknown detect status to the old
+			 * value.
+			 */
+			if (connector->status == connector_status_unknown) {
+				connector->status = old_status;
+				continue;
+			}
+
 			old = drm_get_connector_status_name(old_status);
 			new = drm_get_connector_status_name(connector->status);
 
-- 
1.7.11.7