drm/i915: Reset CSB read pointer in ring init

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@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 7670a0f0f62043b70dd530541ba79e67f88cde3b..e405b61cdac559cc8fd482291d0b765402b685e5 100644 (file)
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1137,6 +1137,7 @@ static int gen8_init_common_ring(struct intel_engine_cs *ring)
                   _MASKED_BIT_DISABLE(GFX_REPLAY_MODE) |
                   _MASKED_BIT_ENABLE(GFX_RUN_LIST_ENABLE));
        POSTING_READ(RING_MODE_GEN7(ring));
+       ring->next_context_status_buffer = 0;
        DRM_DEBUG_DRIVER("Execlists enabled for %s\n", ring->name);
 
        memset(&ring->hangcheck, 0, sizeof(ring->hangcheck));
@@ -1394,7 +1395,6 @@ static int logical_ring_init(struct drm_device *dev, struct intel_engine_cs *rin
        INIT_LIST_HEAD(&ring->execlist_queue);
        INIT_LIST_HEAD(&ring->execlist_retired_req_list);
        spin_lock_init(&ring->execlist_lock);
-       ring->next_context_status_buffer = 0;
 
        ret = i915_cmd_parser_init_ring(ring);
        if (ret)