[PATCH 12/16] drm/i915: Force ringbuffers to not be at offset 0

[Chris Wilson]

For reasons unknown Sandybridge GT1 (at least) will eventually hang when
it encounters a ring wraparound at offset 0. The test case that
reproduces the bug reliably forces a large number of interrupted context
switches, thereby causing very frequent ring wraparounds, but there are
similar bug reports in the wild with the same symptoms, seqno writes
stop just before the wrap and the ringbuffer at address 0. It is also
timing crucial, but adding various delays hasn't helped pinpoint where
the window lies.

Whether the fault is restricted to the ringbuffer itself or the GTT
addressing is unclear, but moving the ringbuffer fixes all the hangs I
have been able to reproduce.

References: (e.g.) https://bugs.freedesktop.org/show_bug.cgi?id=93262
Testcase: igt/gem_exec_whisper/render-contexts-interruptible #snb-gt1
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: stable at vger.kernel.org
Acked-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 0b89cf82ba4d..0d24494904ef 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -2097,10 +2097,12 @@ int intel_pin_and_map_ringbuffer_obj(struct drm_device *dev,
 	struct drm_i915_private *dev_priv = to_i915(dev);
 	struct i915_ggtt *ggtt = &dev_priv->ggtt;
 	struct drm_i915_gem_object *obj = ringbuf->obj;
+	/* Ring wraparound at offset 0 sometimes hangs. No idea why. */
+	unsigned flags = PIN_OFFSET_BIAS | 4096;
 	int ret;
 
 	if (HAS_LLC(dev_priv) && !obj->stolen) {
-		ret = i915_gem_obj_ggtt_pin(obj, PAGE_SIZE, 0);
+		ret = i915_gem_obj_ggtt_pin(obj, PAGE_SIZE, flags);
 		if (ret)
 			return ret;
 
@@ -2114,7 +2116,8 @@ int intel_pin_and_map_ringbuffer_obj(struct drm_device *dev,
 			goto err_unpin;
 		}
 	} else {
-		ret = i915_gem_obj_ggtt_pin(obj, PAGE_SIZE, PIN_MAPPABLE);
+		ret = i915_gem_obj_ggtt_pin(obj, PAGE_SIZE,
+					    flags | PIN_MAPPABLE);
 		if (ret)
 			return ret;
 
-- 
2.8.0.rc3