drm/i915: add comments on what stage a given PM handler is called

This will hopefully make it easier to navigate the code without the need
to consult the full PM documentation.

v2:
- add a comment that the freeze handler is also called after rebooting
- add a comment that the thaw handler is also called to recover from
  errors (Ville)
- add the PM event names (PMSG_THAW etc.) for reference (Ville)
- add comments that s0ix can be handled both via system and runtime
  suspend (Ville)
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -1534,10 +1534,30 @@ static int intel_resume_prepare(struct drm_i915_private *dev_priv,
 }
 
 static const struct dev_pm_ops i915_pm_ops = {
+	/*
+	 * S0ix (via system suspend) and S3 event handlers [PMSG_SUSPEND,
+	 * PMSG_RESUME]
+	 */
 	.suspend = i915_pm_suspend,
 	.suspend_late = i915_pm_suspend_late,
 	.resume_early = i915_pm_resume_early,
 	.resume = i915_pm_resume,
+
+	/*
+	 * S4 event handlers
+	 * @freeze, @freeze_late    : called (1) before creating the
+	 *                            hibernation image [PMSG_FREEZE] and
+	 *                            (2) after rebooting, before restoring
+	 *                            the image [PMSG_QUIESCE]
+	 * @thaw, @thaw_early       : called (1) after creating the hibernation
+	 *                            image, before writing it [PMSG_THAW]
+	 *                            and (2) after failing to create or
+	 *                            restore the image [PMSG_RECOVER]
+	 * @poweroff, @poweroff_late: called after writing the hibernation
+	 *                            image, before rebooting [PMSG_HIBERNATE]
+	 * @restore, @restore_early : called after rebooting and restoring the
+	 *                            hibernation image [PMSG_RESTORE]
+	 */
 	.freeze = i915_pm_suspend,
 	.freeze_late = i915_pm_suspend_late,
 	.thaw_early = i915_pm_resume_early,
@@ -1546,6 +1566,8 @@ static const struct dev_pm_ops i915_pm_ops = {
 	.poweroff_late = i915_pm_suspend_late,
 	.restore_early = i915_pm_resume_early,
 	.restore = i915_pm_resume,
+
+	/* S0ix (via runtime suspend) event handlers */
 	.runtime_suspend = intel_runtime_suspend,
 	.runtime_resume = intel_runtime_resume,
 };