drm/i915: Fix DDC on some systems by clearing BIOS GMBUS setup.

This is a sync of a fix I made in the old UMS code.  If the BIOS uses
the GMBUS and doesn't clear that setup, then our bit-banging I2C can
fail, leading to monitors not being detected.

Signed-off-by: Eric Anholt <eric@anholt.net>

1 files changed, 19 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index c7eab724c418..b94acc4cc05f 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -118,6 +118,23 @@ static void set_data(void *data, int state_high)
         udelay(I2C_RISEFALL_TIME); /* wait for the line to change state */
 }
 
+/* Clears the GMBUS setup.  Our driver doesn't make use of the GMBUS I2C
+ * engine, but if the BIOS leaves it enabled, then that can break our use
+ * of the bit-banging I2C interfaces.  This is notably the case with the
+ * Mac Mini in EFI mode.
+ */
+void
+intel_i2c_reset_gmbus(struct drm_device *dev)
+{
+        struct drm_i915_private *dev_priv = dev->dev_private;
+
+        if (IS_IGDNG(dev)) {
+                I915_WRITE(PCH_GMBUS0, 0);
+        } else {
+                I915_WRITE(GMBUS0, 0);
+        }
+}
+
 /**
  * intel_i2c_create - instantiate an Intel i2c bus using the specified GPIO reg
  * @dev: DRM device
@@ -168,6 +185,8 @@ struct i2c_adapter *intel_i2c_create(struct drm_device *dev, const u32 reg,
         if(i2c_bit_add_bus(&chan->adapter))
                 goto out_free;
 
+        intel_i2c_reset_gmbus(dev);
+
         /* JJJ:  raise SCL and SDA? */
         intel_i2c_quirk_set(dev, true);
         set_data(chan, 1);