[PATCH 3.16 017/305] drm/i915: Prevent machine death on Ivybridge context switching

[Ben Hutchings]

3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>

commit e9135c4f08d9acb0f3da3ad2643b669dee3217c2 upstream.

Two concurrent writes into the same register cacheline has the chance of
killing the machine on Ivybridge and other gen7. This includes LRI
emitted from the command parser.  The MI_SET_CONTEXT itself serves as
serialising barrier and prevents the pair of register writes in the first
packet from triggering the fault.  However, if a second switch-context
immediately occurs then we may have two adjacent blocks of LRI to the
same registers which may then trigger the hang. To counteract this we
need to insert a delay after the second register write using SRM.

This is easiest to reproduce with something like
igt/gem_ctx_switch/interruptible that triggers back-to-back context
switches (with no operations in between them in the command stream,
which requires the execbuf operation to be interrupted after the
MI_SET_CONTEXT) but can be observed sporadically elsewhere when running
interruptible igt. No reports from the wild though, so it must be of low
enough frequency that no one has correlated the random machine freezes
with i915.ko

The issue was introduced with
commit 2c550183476dfa25641309ae9a28d30feed14379 [v3.19]
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date:   Tue Dec 16 10:02:27 2014 +0000

    drm/i915: Disable PSMI sleep messages on all rings around context switches

Testcase: igt/gem_ctx_switch/render-interruptible #ivb
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel@xxxxxxxx>
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-11-git-send-email-chris@xxxxxxxxxxxxxxxxxx
[bwh: Backported to 3.16:
 - Pass ring, not engine, to intel_ring_emit()
 - Register type is u32 not i915_reg_t
 - MI_STORE_REGISTER_MEM is a function-macro]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_gem_context.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -566,7 +566,7 @@ mi_set_context(struct intel_engine_cs *r
 
 	len = 4;
 	if (INTEL_INFO(ring->dev)->gen >= 7)
-		len += 2 + (num_rings ? 4*num_rings + 2 : 0);
+		len += 2 + (num_rings ? 4*num_rings + 6 : 0);
 
 	ret = intel_ring_begin(ring, len);
 	if (ret)
@@ -605,15 +605,25 @@ mi_set_context(struct intel_engine_cs *r
 	if (INTEL_INFO(ring->dev)->gen >= 7) {
 		if (num_rings) {
 			struct intel_engine_cs *signaller;
+			u32 last_reg = 0; /* keep gcc quiet */
 
 			intel_ring_emit(ring, MI_LOAD_REGISTER_IMM(num_rings));
 			for_each_ring(signaller, to_i915(ring->dev), i) {
 				if (signaller == ring)
 					continue;
 
-				intel_ring_emit(ring, RING_PSMI_CTL(signaller->mmio_base));
+				last_reg = RING_PSMI_CTL(signaller->mmio_base);
+				intel_ring_emit(ring, last_reg);
 				intel_ring_emit(ring, _MASKED_BIT_DISABLE(GEN6_PSMI_SLEEP_MSG_DISABLE));
 			}
+
+			/* Insert a delay before the next switch! */
+			intel_ring_emit(ring,
+					MI_STORE_REGISTER_MEM(1) |
+					MI_SRM_LRM_GLOBAL_GTT);
+			intel_ring_emit(ring, last_reg);
+			intel_ring_emit(ring, ring->scratch.gtt_offset);
+			intel_ring_emit(ring, MI_NOOP);
 		}
 		intel_ring_emit(ring, MI_ARB_ON_OFF | MI_ARB_ENABLE);
 	}