drm/i915: Add documentation to gen9_set_dc_state()

Add documentation to gen9_set_dc_state() on what enabling a given DC
state means and at what point HW/DMC actually enters/exits these states.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180417113147.25120-1-imre.deak@intel.com

drivers/gpu/drm/i915/intel_runtime_pm.c

@@ -542,6 +542,29 @@ void gen9_sanitize_dc_state(struct drm_i915_private *dev_priv)
 	dev_priv->csr.dc_state = val;
 }
 
+/**
+ * gen9_set_dc_state - set target display C power state
+ * @dev_priv: i915 device instance
+ * @state: target DC power state
+ * - DC_STATE_DISABLE
+ * - DC_STATE_EN_UPTO_DC5
+ * - DC_STATE_EN_UPTO_DC6
+ * - DC_STATE_EN_DC9
+ *
+ * Signal to DMC firmware/HW the target DC power state passed in @state.
+ * DMC/HW can turn off individual display clocks and power rails when entering
+ * a deeper DC power state (higher in number) and turns these back when exiting
+ * that state to a shallower power state (lower in number). The HW will decide
+ * when to actually enter a given state on an on-demand basis, for instance
+ * depending on the active state of display pipes. The state of display
+ * registers backed by affected power rails are saved/restored as needed.
+ *
+ * Based on the above enabling a deeper DC power state is asynchronous wrt.
+ * enabling it. Disabling a deeper power state is synchronous: for instance
+ * setting %DC_STATE_DISABLE won't complete until all HW resources are turned
+ * back on and register state is restored. This is guaranteed by the MMIO write
+ * to DC_STATE_EN blocking until the state is restored.
+ */
 static void gen9_set_dc_state(struct drm_i915_private *dev_priv, uint32_t state)
 {
 	uint32_t val;