drm/i915: BDW needs D_COMP writes through MCHBAR

That's what the spec said! And HSW needs it through pcode (you can
only read it through MCHBAR), so create hsw_write_dcomp to abstract
the weirdness.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -6885,6 +6885,22 @@ static void assert_can_disable_lcpll(struct drm_i915_private *dev_priv)
 	spin_unlock_irqrestore(&dev_priv->irq_lock, irqflags);
 }
 
+static void hsw_write_dcomp(struct drm_i915_private *dev_priv, uint32_t val)
+{
+	struct drm_device *dev = dev_priv->dev;
+
+	if (IS_HASWELL(dev)) {
+		mutex_lock(&dev_priv->rps.hw_lock);
+		if (sandybridge_pcode_write(dev_priv, GEN6_PCODE_WRITE_D_COMP,
+					    val))
+			DRM_ERROR("Failed to disable D_COMP\n");
+		mutex_unlock(&dev_priv->rps.hw_lock);
+	} else {
+		I915_WRITE(D_COMP, val);
+	}
+	POSTING_READ(D_COMP);
+}
+
 /*
  * This function implements pieces of two sequences from BSpec:
  * - Sequence for display software to disable LCPLL
@@ -6922,11 +6938,7 @@ static void hsw_disable_lcpll(struct drm_i915_private *dev_priv,
 
 	val = I915_READ(D_COMP);
 	val |= D_COMP_COMP_DISABLE;
-	mutex_lock(&dev_priv->rps.hw_lock);
-	if (sandybridge_pcode_write(dev_priv, GEN6_PCODE_WRITE_D_COMP, val))
-		DRM_ERROR("Failed to disable D_COMP\n");
-	mutex_unlock(&dev_priv->rps.hw_lock);
-	POSTING_READ(D_COMP);
+	hsw_write_dcomp(dev_priv, val);
 	ndelay(100);
 
 	if (wait_for((I915_READ(D_COMP) & D_COMP_RCOMP_IN_PROGRESS) == 0, 1))
@@ -6981,11 +6993,7 @@ static void hsw_restore_lcpll(struct drm_i915_private *dev_priv)
 	val = I915_READ(D_COMP);
 	val |= D_COMP_COMP_FORCE;
 	val &= ~D_COMP_COMP_DISABLE;
-	mutex_lock(&dev_priv->rps.hw_lock);
-	if (sandybridge_pcode_write(dev_priv, GEN6_PCODE_WRITE_D_COMP, val))
-		DRM_ERROR("Failed to enable D_COMP\n");
-	mutex_unlock(&dev_priv->rps.hw_lock);
-	POSTING_READ(D_COMP);
+	hsw_write_dcomp(dev_priv, val);
 
 	val = I915_READ(LCPLL_CTL);
 	val &= ~LCPLL_PLL_DISABLE;