drm: Try to document legacy DPMS uapi a bit better

Due to inconsistency of how various legacy drivers implemented DPMS
the DPMS uabi has a lot of quirks. Atomic standardizes this, but
drivers using the DPMS support can't rely on that since legacy drivers
still exist.

Laurent asked for this.

v2:
Improve commit message and explain that DPMS doesn't really exist for
the atomic ioctl (it's "ACTIVE" on the CRTC instead).

Text polish from Eric's review.

Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170920225957.16278-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_connector.c

@@ -719,6 +719,29 @@ 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.
+ *
+ * 	Note that this property cannot be set through the MODE_ATOMIC ioctl,
+ * 	userspace must use "ACTIVE" on the CRTC instead.
+ *
+ * 	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 well-defined 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 on legacy drivers.
+ *
+ * 	Summarizing: Only set "DPMS" when the connector is known to be enabled,
+ * 	assume that a successful SETCONFIG call also sets "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