drm/i915: Never read FORCEWAKE

The same designer from the previous patch has told us to never read
FORCEWAKE. We only do this for the POSTING_READ(), so simply change that
to something within the same cacheline (for no reason in particular
other than it sounds nice). In the _mt case we can leverage
the gtfifodbg check for the POSTING_READ.

This partially reverts
commit 6af2d180
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jul 26 16:24:50 2012 +0200

    drm/i915: fix forcewake related hangs on snb

v2: commit message, comments about posting read from (Daniel)

Note: vlv forcewake doesn't need any changes for this special
treatment since FORCEWAKE_VLV is in a totally different register
range, and the readback FORCEWAKE_ACK_VLV readback that follows is in
the same range.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Added note.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -3969,7 +3969,7 @@ static void __gen6_gt_force_wake_get(struct drm_i915_private *dev_priv)
 		DRM_ERROR("Timed out waiting for forcewake old ack to clear.\n");
 
 	I915_WRITE_NOTRACE(FORCEWAKE, 1);
-	POSTING_READ(FORCEWAKE);
+	POSTING_READ(ECOBUS); /* something from same cacheline, but !FORCEWAKE */
 
 	if (wait_for_atomic((I915_READ_NOTRACE(forcewake_ack) & 1),
 			    FORCEWAKE_ACK_TIMEOUT_MS))
@@ -3992,7 +3992,7 @@ static void __gen6_gt_force_wake_mt_get(struct drm_i915_private *dev_priv)
 		DRM_ERROR("Timed out waiting for forcewake old ack to clear.\n");
 
 	I915_WRITE_NOTRACE(FORCEWAKE_MT, _MASKED_BIT_ENABLE(1));
-	POSTING_READ(FORCEWAKE_MT);
+	POSTING_READ(ECOBUS); /* something from same cacheline, but !FORCEWAKE */
 
 	if (wait_for_atomic((I915_READ_NOTRACE(forcewake_ack) & 1),
 			    FORCEWAKE_ACK_TIMEOUT_MS))
@@ -4029,14 +4029,14 @@ void gen6_gt_check_fifodbg(struct drm_i915_private *dev_priv)
 static void __gen6_gt_force_wake_put(struct drm_i915_private *dev_priv)
 {
 	I915_WRITE_NOTRACE(FORCEWAKE, 0);
-	POSTING_READ(FORCEWAKE);
+	/* gen6_gt_check_fifodbg doubles as the POSTING_READ */
 	gen6_gt_check_fifodbg(dev_priv);
 }
 
 static void __gen6_gt_force_wake_mt_put(struct drm_i915_private *dev_priv)
 {
 	I915_WRITE_NOTRACE(FORCEWAKE_MT, _MASKED_BIT_DISABLE(1));
-	POSTING_READ(FORCEWAKE_MT);
+	/* gen6_gt_check_fifodbg doubles as the POSTING_READ */
 	gen6_gt_check_fifodbg(dev_priv);
 }