Revert "drm/i915: Avoid waking the engines just to check if they are idle"

This reverts commit 0b702dca.

CI reports that this is not as reliable as it first appears, with
failures starting to sporadically occur in selftests.

Fixes: 0b702dca ("drm/i915: Avoid waking the engines just to check if they are idle")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190227204654.14907-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -1007,7 +1007,6 @@ static bool ring_is_idle(struct intel_engine_cs *engine)
 {
 	struct drm_i915_private *dev_priv = engine->i915;
 	intel_wakeref_t wakeref;
-	unsigned long flags;
 	bool idle = true;
 
 	if (I915_SELFTEST_ONLY(!engine->mmio_base))
@@ -1018,19 +1017,15 @@ static bool ring_is_idle(struct intel_engine_cs *engine)
 	if (!wakeref)
 		return true;
 
-	spin_lock_irqsave(&dev_priv->uncore.lock, flags);
+	/* First check that no commands are left in the ring */
+	if ((I915_READ_HEAD(engine) & HEAD_ADDR) !=
+	    (I915_READ_TAIL(engine) & TAIL_ADDR))
+		idle = false;
 
-	/*
-	 * Check that no commands are left in the ring.
-	 *
-	 * If the engine is not awake, both reads return 0 as we do so without
-	 * forcewake.
-	 */
-	if ((I915_READ_FW(RING_HEAD(engine->mmio_base)) & HEAD_ADDR) !=
-	    (I915_READ_FW(RING_TAIL(engine->mmio_base)) & TAIL_ADDR))
+	/* No bit for gen2, so assume the CS parser is idle */
+	if (INTEL_GEN(dev_priv) > 2 && !(I915_READ_MODE(engine) & MODE_IDLE))
 		idle = false;
 
-	spin_unlock_irqrestore(&dev_priv->uncore.lock, flags);
 	intel_runtime_pm_put(dev_priv, wakeref);
 
 	return idle;