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>
diff --git a/drivers/gpu/drm/i915/intel_i2c.c b/drivers/gpu/drm/i915/intel_i2c.c
index c7eab72..b94acc4 100644
--- a/drivers/gpu/drm/i915/intel_i2c.c
+++ b/drivers/gpu/drm/i915/intel_i2c.c
@@ -118,6 +118,23 @@
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 @@
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);