drm/i915/pmu: Reconstruct active state on starting busy-stats

We have a hole in our busy-stat accounting if the pmu is enabled during
a long running batch, the pmu will not start accumulating busy-time
until the next context switch. This then fails tests that are only
sampling a single batch.

v2: Count each active port just once (context in/out events are only on
the first and last assignment to a port).
v3: Avoid hardcoding knowledge of 2 submission ports

Fixes: 30e17b7 ("drm/i915: Engine busy time tracking")
Testcase: igt/perf_pmu/busy-start
Testcase: igt/perf_pmu/busy-double-start
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180111073031.14614-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -1951,8 +1951,22 @@ int intel_enable_engine_stats(struct intel_engine_cs *engine)
 	spin_lock_irqsave(&engine->stats.lock, flags);
 	if (engine->stats.enabled == ~0)
 		goto busy;
-	if (engine->stats.enabled++ == 0)
+	if (engine->stats.enabled++ == 0) {
+		struct intel_engine_execlists *execlists = &engine->execlists;
+		const struct execlist_port *port = execlists->port;
+		unsigned int num_ports = execlists_num_ports(execlists);
+
 		engine->stats.enabled_at = ktime_get();
+
+		/* XXX submission method oblivious? */
+		while (num_ports-- && port_isset(port)) {
+			engine->stats.active++;
+			port++;
+		}
+
+		if (engine->stats.active)
+			engine->stats.start = engine->stats.enabled_at;
+	}
 	spin_unlock_irqrestore(&engine->stats.lock, flags);
 
 	return 0;