drm/i915: write D_COMP using the mailbox

You can't write it using the MCHBAR mirror, the write will just get
dropped.

This should make us BSpec-compliant, but there's no real bug I could
reproduce that is fixed by this patch.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Fix spelling mistake in the comment that Damien spotted.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -1441,6 +1441,8 @@
  * device 0 function 0's pci config register 0x44 or 0x48 and matches it in
  * every way.  It is not accessible from the CP register read instructions.
  *
+ * Starting from Haswell, you can't write registers using the MCHBAR mirror,
+ * just read.
  */
 #define MCHBAR_MIRROR_BASE	0x10000
 
@@ -4724,6 +4726,8 @@
 #define   GEN6_PCODE_READ_MIN_FREQ_TABLE	0x9
 #define	  GEN6_PCODE_WRITE_RC6VIDS		0x4
 #define	  GEN6_PCODE_READ_RC6VIDS		0x5
+#define   GEN6_PCODE_READ_D_COMP		0x10
+#define   GEN6_PCODE_WRITE_D_COMP		0x11
 #define   GEN6_ENCODE_RC6_VID(mv)		(((mv) - 245) / 5)
 #define   GEN6_DECODE_RC6_VID(vids)		(((vids) * 5) + 245)
 #define GEN6_PCODE_DATA				0x138128

drivers/gpu/drm/i915/intel_display.c

@@ -6140,7 +6140,10 @@ void hsw_disable_lcpll(struct drm_i915_private *dev_priv,
 
 	val = I915_READ(D_COMP);
 	val |= D_COMP_COMP_DISABLE;
-	I915_WRITE(D_COMP, val);
+	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);
 	ndelay(100);
 
@@ -6182,7 +6185,10 @@ 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;
-	I915_WRITE(D_COMP, val);
+	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);
 
 	val = I915_READ(LCPLL_CTL);