[PATCH v2 3/5] drm/i915: fix rc6 initialization on Ironlake

[Ben Widawsky <ben@bwidawsk.net>]

There is a race condition between setting PWRCTXA and executing
MI_SET_CONTEXT. PWRCTXA must not be set until a valid context has been
written (or else the GPU could possible go into rc6, and return to an
invalid context).

Reported-and-Tested-by: Gu Rui <chaos.proton@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=28582
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
---
 drivers/gpu/drm/i915/intel_display.c |   16 +++++++++++++---
 1 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 2e3c626..6c92980 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -6602,9 +6602,6 @@ void ironlake_enable_rc6(struct drm_device *dev)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	int ret;
 
-	/* rc6 disabled by default due to repeated reports of hanging during
-	 * boot and resume.
-	 */
 	if (!i915_enable_rc6)
 		return;
 
@@ -6638,6 +6635,19 @@ void ironlake_enable_rc6(struct drm_device *dev)
 	OUT_RING(MI_FLUSH);
 	ADVANCE_LP_RING();
 
+	/*
+	 * Wait for the command parser to advance past MI_SET_CONTEXT. The HW
+	 * does an implicit flush, combined with MI_FLUSH above, it should be
+	 * safe to assume that renderctx is valid
+	 */
+	ret = intel_wait_ring_idle(LP_RING(dev_priv));
+	if (ret) {
+		DRM_ERROR("failed to enable ironlake power power savings\n");
+		ironlake_teardown_rc6(dev);
+		mutex_unlock(&dev->struct_mutex);
+		return;
+	}
+
 	I915_WRITE(PWRCTXA, dev_priv->pwrctx->gtt_offset | PWRCTX_EN);
 	I915_WRITE(RSTDBYCTL, I915_READ(RSTDBYCTL) & ~RCX_SW_EXIT);
 	mutex_unlock(&dev->struct_mutex);
-- 
1.7.3.4