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Date:	Thu, 7 Aug 2014 16:29:53 +0200 (CEST)
From:	Jiri Kosina <>
To:	Daniel Vetter <>,
	Jani Nikula <>,
	David Airlie <>
Subject: [PATCH] drm/i915: read HEAD register back in init_ring_common() to
 enforce ordering

Withtout this, ring initialization fails reliabily during resume with

	[drm:init_ring_common] *ERROR* render ring initialization failed ctl 0001f001 head ffffff8804 tail 00000000 start 000e4000

This is not a complete fix, but it is verified to make the ring 
initialization failures during resume much less likely.

We were not able to root-cause this bug (likely HW-specific to Gen4 chips) 
yet. This is therefore used as a ducttape before problem is fully 
understood and proper fix created, so that people don't suffer from 
completely unusable systems in the meantime.

The discussion and debugging is happening at

Signed-off-by: Jiri Kosina <>
 drivers/gpu/drm/i915/intel_ringbuffer.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 279488a..7add7ee 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -517,6 +517,9 @@ static int init_ring_common(struct intel_engine_cs *ring)
+	/* Enforce ordering by reading HEAD register back */
+	I915_READ_HEAD(ring);
 	/* Initialize the ring. This must happen _after_ we've cleared the ring
 	 * registers with the above sequence (the readback of the HEAD registers
 	 * also enforces ordering), otherwise the hw might lose the new ring
Jiri Kosina
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