drm/i915: Add a tracepoint for using a semaphore

So that we can find the callers who introduce a ring stall. A single
ring stall is not too unwelcome, the right issue becomes when they start
to interlock and prevent any concurrent work. That, however, is a little
tricker to detect with a mere tracepoint!

v2: Rebrand it as a ring event, rather than an object event.
v3: Include the seqno in the tracepoint for posterity or something.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2625,6 +2625,7 @@ i915_gem_object_sync(struct drm_i915_gem_object *obj,
 	if (ret)
 		return ret;
 
+	trace_i915_gem_ring_sync_to(from, to, seqno);
 	ret = to->sync_to(to, from, seqno);
 	if (!ret)
 		/* We use last_read_seqno because sync_to()

drivers/gpu/drm/i915/i915_trace.h

@@ -248,6 +248,32 @@ TRACE_EVENT(i915_gem_evict_vm,
 	    TP_printk("dev=%d, vm=%p", __entry->vm->dev->primary->index, __entry->vm)
 );
 
+TRACE_EVENT(i915_gem_ring_sync_to,
+	    TP_PROTO(struct intel_ring_buffer *from,
+		     struct intel_ring_buffer *to,
+		     u32 seqno),
+	    TP_ARGS(from, to, seqno),
+
+	    TP_STRUCT__entry(
+			     __field(u32, dev)
+			     __field(u32, sync_from)
+			     __field(u32, sync_to)
+			     __field(u32, seqno)
+			     ),
+
+	    TP_fast_assign(
+			   __entry->dev = from->dev->primary->index;
+			   __entry->sync_from = from->id;
+			   __entry->sync_to = to->id;
+			   __entry->seqno = seqno;
+			   ),
+
+	    TP_printk("dev=%u, sync-from=%u, sync-to=%u, seqno=%u",
+		      __entry->dev,
+		      __entry->sync_from, __entry->sync_to,
+		      __entry->seqno)
+);
+
 TRACE_EVENT(i915_gem_ring_dispatch,
 	    TP_PROTO(struct intel_ring_buffer *ring, u32 seqno, u32 flags),
 	    TP_ARGS(ring, seqno, flags),