Partially revert "drm/i915: unconditionally use mt forcewake on hsw/ivb"

This patch partially reverts commit 36ec8f8 for
IvyBridge CPUs.

The original commit results in repeated 'Timed out waiting for forcewake old
ack to clear' messages on a Supermicro C7H61 board (BIOS version 2.00 and 2.00b)
with i7-3770K CPU. It ultimately results in a hangup if the system is highly
loaded. Reverting the commit for IvyBridge CPUs fixes the issue.

Issue a warning if the CPU is IvyBridge and mt forcewake is disabled, since
this condition can result in secondary issues.

v2: Only revert patch for Ivybridge CPUs
    Issue info message if mt forcewake is disabled on Ivybridge

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=60541
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66139
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -5500,9 +5500,38 @@ void intel_gt_init(struct drm_device *dev)
 	if (IS_VALLEYVIEW(dev)) {
 		dev_priv->gt.force_wake_get = vlv_force_wake_get;
 		dev_priv->gt.force_wake_put = vlv_force_wake_put;
-	} else if (IS_IVYBRIDGE(dev) || IS_HASWELL(dev)) {
+	} else if (IS_HASWELL(dev)) {
 		dev_priv->gt.force_wake_get = __gen6_gt_force_wake_mt_get;
 		dev_priv->gt.force_wake_put = __gen6_gt_force_wake_mt_put;
+	} else if (IS_IVYBRIDGE(dev)) {
+		u32 ecobus;
+
+		/* IVB configs may use multi-threaded forcewake */
+
+		/* A small trick here - if the bios hasn't configured
+		 * MT forcewake, and if the device is in RC6, then
+		 * force_wake_mt_get will not wake the device and the
+		 * ECOBUS read will return zero. Which will be
+		 * (correctly) interpreted by the test below as MT
+		 * forcewake being disabled.
+		 */
+		mutex_lock(&dev->struct_mutex);
+		__gen6_gt_force_wake_mt_get(dev_priv);
+		ecobus = I915_READ_NOTRACE(ECOBUS);
+		__gen6_gt_force_wake_mt_put(dev_priv);
+		mutex_unlock(&dev->struct_mutex);
+
+		if (ecobus & FORCEWAKE_MT_ENABLE) {
+			dev_priv->gt.force_wake_get =
+						__gen6_gt_force_wake_mt_get;
+			dev_priv->gt.force_wake_put =
+						__gen6_gt_force_wake_mt_put;
+		} else {
+			DRM_INFO("No MT forcewake available on Ivybridge, this can result in issues\n");
+			DRM_INFO("when using vblank-synced partial screen updates.\n");
+			dev_priv->gt.force_wake_get = __gen6_gt_force_wake_get;
+			dev_priv->gt.force_wake_put = __gen6_gt_force_wake_put;
+		}
 	} else if (IS_GEN6(dev)) {
 		dev_priv->gt.force_wake_get = __gen6_gt_force_wake_get;
 		dev_priv->gt.force_wake_put = __gen6_gt_force_wake_put;