drm/i915: Use the correct GMCH_CTRL register for Sandybridge+

commit a885b3cc upstream.

The GMCH_CTRL register (or MGCC in the spec) is at a different address
on Sandybridge, and the address to which we currently write to is
undefined. These stray writes appear to upset (hard hang) my Ivybridge
machine whilst it is in UEFI mode.

Note that the register is still marked as locked RO on Sandybridge, so
vgaarb is still dysfunctional.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwh: Backported to 3.2: add definition of SNB_GMCH_CTRL in i915_reg.h]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/i915/i915_reg.h

@@ -37,6 +37,7 @@
  */
 #define INTEL_GMCH_CTRL		0x52
 #define INTEL_GMCH_VGA_DISABLE  (1 << 1)
+#define SNB_GMCH_CTRL		0x50
 
 /* PCI config space */
 

drivers/gpu/drm/i915/intel_display.c

@@ -9141,14 +9141,15 @@ void intel_connector_attach_encoder(struct intel_connector *connector,
 int intel_modeset_vga_set_state(struct drm_device *dev, bool state)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
+	unsigned reg = INTEL_INFO(dev)->gen >= 6 ? SNB_GMCH_CTRL : INTEL_GMCH_CTRL;
 	u16 gmch_ctrl;
 
-	pci_read_config_word(dev_priv->bridge_dev, INTEL_GMCH_CTRL, &gmch_ctrl);
+	pci_read_config_word(dev_priv->bridge_dev, reg, &gmch_ctrl);
 	if (state)
 		gmch_ctrl &= ~INTEL_GMCH_VGA_DISABLE;
 	else
 		gmch_ctrl |= INTEL_GMCH_VGA_DISABLE;
-	pci_write_config_word(dev_priv->bridge_dev, INTEL_GMCH_CTRL, gmch_ctrl);
+	pci_write_config_word(dev_priv->bridge_dev, reg, gmch_ctrl);
 	return 0;
 }