[PATCH] drm: Try to document legacy DPMS uapi a bit better

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

Laurent asked for this.

Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
 drivers/gpu/drm/drm_connector.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
index ba9f36cef68c..b458eb488128 100644
--- a/drivers/gpu/drm/drm_connector.c
+++ b/drivers/gpu/drm/drm_connector.c
@@ -720,6 +720,25 @@ DRM_ENUM_NAME_FN(drm_get_tv_subconnector_name,
  * 	callback. For atomic drivers the remapping to the "ACTIVE" property is
  * 	implemented in the DRM core.  This is the only standard connector
  * 	property that userspace can change.
+ *
+ * 	WARNING:
+ *
+ * 	For userspace also running on legacy drivers the "DPMS" semantics are a
+ * 	lot more complicated. First, userspace cannot rely on the "DPMS" value
+ * 	returned by the GETCONNECTOR actually reflecting reality, because many
+ * 	drivers fail to update it. For atomic drivers this is taken care of in
+ * 	drm_atomic_helper_update_legacy_modeset_state().
+ *
+ * 	The second issue is that the DPMS state is only relevant when the
+ * 	connector is connected to a CRTC. In atomic the DRM core enforces that
+ * 	"ACTIVE" is off in such a case, no such checks exists for "DPMS".
+ * 	Finally, when enabling an output using the legacy SETCONFIG ioctl then
+ * 	"DPMS" is forced to ON. But see above, that might not be reflected in
+ * 	the software value.
+ *
+ * 	Summarizing: Only set "DPMS" when the connector is known to be enabled,
+ * 	assume that a successful SETCONFIG call also set "DPMS" to on, and never
+ * 	read back the value of "DPMS" because it can be incorrect.
  * PATH:
  * 	Connector path property to identify how this sink is physically
  * 	connected. Used by DP MST. This should be set by calling
-- 
2.13.3

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