drm/i915: Keep the rings stopped until they have been re-initialized

Before modifying the ring register (RING_START, HEAD, TAIL, CTL) we
first make sure it is stopped (or else the hw may not resample the
registers). However, we do not need to let the hw restart until after we
have reprogrammed all the rings. This should help prevent situations
where pending operations on the ring may resume (because we are trying
to re-initialize following an unsuccessful GPU hang, i.e. from
i915_gem_unset_wedged).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103260
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171013131218.18013-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -484,11 +484,6 @@ static bool stop_ring(struct intel_engine_cs *engine)
 	I915_WRITE_HEAD(engine, 0);
 	I915_WRITE_TAIL(engine, 0);
 
-	if (INTEL_GEN(dev_priv) > 2) {
-		(void)I915_READ_CTL(engine);
-		I915_WRITE_MODE(engine, _MASKED_BIT_DISABLE(STOP_RING));
-	}
-
 	return (I915_READ_HEAD(engine) & HEAD_ADDR) == 0;
 }
 
@@ -570,6 +565,9 @@ static int init_ring_common(struct intel_engine_cs *engine)
 
 	intel_engine_init_hangcheck(engine);
 
+	if (INTEL_GEN(dev_priv) > 2)
+		I915_WRITE_MODE(engine, _MASKED_BIT_DISABLE(STOP_RING));
+
 out:
 	intel_uncore_forcewake_put(dev_priv, FORCEWAKE_ALL);