drm/i915: Clean up multi-threaded forcewake patch

We learned that the ECOBUS register was inside the GT power well, and
so *did* need force wake to be read, so it gets removed from the list
of 'doesn't need force wake' registers.

That means the code reading ECOBUS after forcing the mt_force_wake
function to be called needs to use I915_READ_NOTRACE; it doesn't need
to do more force wake fun as it's already done it manually.

This also adds a comment explaining why the MT forcewake testing code
only needs to call mt_forcewake_get/put and not disable RC6 manually
-- the ECOBUS read will return 0 if the device is in RC6 and isn't
using MT forcewake, causing the test to work correctly.

Signed-off-by: Keith Packard <keithp@keithp.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index cb21d5bbecc..ff99fa267bf 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -8591,9 +8591,15 @@ static void intel_init_display(struct drm_device *dev)
 		if (IS_IVYBRIDGE(dev)) {
 			u32	ecobus;
 
+			/* A small trick here - if the bios hasn't configured MT forcewake,
+			 * and if the device is in RC6, then force_wake_mt_get will not wake
+			 * the device and the ECOBUS read will return zero. Which will be
+			 * (correctly) interpreted by the test below as MT forcewake being
+			 * disabled.
+			 */
 			mutex_lock(&dev->struct_mutex);
 			__gen6_gt_force_wake_mt_get(dev_priv);
-			ecobus = I915_READ(ECOBUS);
+			ecobus = I915_READ_NOTRACE(ECOBUS);
 			__gen6_gt_force_wake_mt_put(dev_priv);
 			mutex_unlock(&dev->struct_mutex);