drm/i915/execlists: Document runtime pm for intel_lrc_irq_handler()

We indirectly hold the runtime-pm for the intel_lrc_irq_handler() by
virtue of dev_priv->gt.awake keeping a wakeref whilst the requests are
busy. As this is not obvious from the code, add a comment.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170411175850.2470-1-chris@chris-wilson.co.ukReviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/intel_lrc.c

@@ -515,6 +515,15 @@ static void intel_lrc_irq_handler(unsigned long data)
 	struct execlist_port *port = engine->execlist_port;
 	struct drm_i915_private *dev_priv = engine->i915;
 
+	/* We can skip acquiring intel_runtime_pm_get() here as it was taken
+	 * on our behalf by the request (see i915_gem_mark_busy()) and it will
+	 * not be relinquished until the device is idle (see
+	 * i915_gem_idle_work_handler()). As a precaution, we make sure
+	 * that all ELSP are drained i.e. we have processed the CSB,
+	 * before allowing ourselves to idle and calling intel_runtime_pm_put().
+	 */
+	GEM_BUG_ON(!dev_priv->gt.awake);
+
 	intel_uncore_forcewake_get(dev_priv, engine->fw_domains);
 
 	/* Prefer doing test_and_clear_bit() as a two stage operation to avoid