[linux-yocto] [PATCH 2/2] drm/i915: Reset CSB read pointer in ring init

wei.tee.ng at intel.com wei.tee.ng at intel.com
Wed Jun 3 18:19:01 PDT 2015

From: Thomas Daniel <thomas.daniel at intel.com>

A previous commit enabled execlists by default:

       commit 27401d126b5b ("drm/i915/bdw: Enable execlists by default where supported")

This allowed routine testing of execlists which exposed a regression when
resuming from suspend.  The cause was tracked down the to recent changes to the
ring init sequence:

       commit 35a57ffbb108 ("drm/i915: Only init engines once")

During a suspend/resume cycle the hardware Context Status Buffer write pointer
is reset.  However since the recent changes to the init sequence the software CSB
read pointer is no longer reset.  This means that context status events are not
handled correctly and new contexts are not written to the ELSP, resulting in an
apparent GPU hang.

Pending further changes to the ring init code, just move the
ring->next_context_status_buffer initialization into gen8_init_common_ring to
fix this regression.

v2: Moved init into gen8_init_common_ring rather than context_enable after
feedback from Daniel Vetter.  Updated commit msg to reflect this and also cite
commits related to the regression.  Fixed bz link to correct bug.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88096
Cc: Paulo Zanoni <paulo.r.zanoni at intel.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Dave Gordon <david.s.gordon at intel.com>
Signed-off-by: Thomas Daniel <thomas.daniel at intel.com>
Reviewed-by: Dave Gordon <david.s.gordon at intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
(cherry picked from commit c0a03a2e4c4e954d9acffd3ce3521e64654c9dc8)
Signed-off-by: Cheah, Vincent Beng Keat <vincent.beng.keat.cheah at intel.com>
 drivers/gpu/drm/i915/intel_lrc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index fc490d1..0adb33f 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1133,6 +1133,7 @@ static int gen8_init_common_ring(struct intel_engine_cs *ring)
+	ring->next_context_status_buffer = 0;
 	DRM_DEBUG_DRIVER("Execlists enabled for %s\n", ring->name);
 	memset(&ring->hangcheck, 0, sizeof(ring->hangcheck));
@@ -1384,7 +1385,6 @@ static int logical_ring_init(struct drm_device *dev, struct intel_engine_cs *rin
-	ring->next_context_status_buffer = 0;
 	ret = i915_cmd_parser_init_ring(ring);
 	if (ret)

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