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

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>
Cc: stable@vger.kernel.org
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 3 insertions, 2 deletions

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 4049a65a9f8c..54e82a80cf50 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -11126,14 +11126,15 @@ 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;
 }