Patch "drm/i915: Force ringbuffers to not be at offset 0" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

drm/i915: Force ringbuffers to not be at offset 0

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     drm-i915-force-ringbuffers-to-not-be-at-offset-0.patch

and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Chris Wilson <[email protected]>
Date: Wed, 13 Apr 2016 17:35:11 +0100
Subject: drm/i915: Force ringbuffers to not be at offset 0

From: Chris Wilson <[email protected]>

commit 1b3e885a05d4f0a35dde035724e7c6453d2cbe71 upstream.

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

---
 drivers/gpu/drm/i915/intel_ringbuffer.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

@@ -2031,10 +2031,12 @@ int intel_pin_and_map_ringbuffer_obj(str
 {
 	struct drm_i915_private *dev_priv = to_i915(dev);
 	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;
 

@@ -2050,7 +2052,8 @@ int intel_pin_and_map_ringbuffer_obj(str
 			return -ENOMEM;
 		}
 	} 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;
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/drm-i915-force-ringbuffers-to-not-be-at-offset-0.patch queue-4.5/drm-i915-fixup-the-free-space-logic-in-ring_prepare.patch queue-4.5/drm-i915-exit-cherryview_irq_handler-after-one-pass.patch queue-4.5/drm-i915-cleanup-phys-status-page-too.patch