perf ordered_events: Add first_time() method

To get the timestamp in the first event in the queue.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Dmitry Levin <ldv@altlinux.org>
Cc: Eugene Syromiatnikov <esyr@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Luis Cláudio Gonçalves <lclaudio@uudg.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/n/tip-appp27jw1ul8kgg872j43r5o@git.kernel.org
[ split from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/ordered-events.c

@@ -343,6 +343,17 @@ int ordered_events__flush_time(struct ordered_events *oe, u64 timestamp)
 	return __ordered_events__flush(oe, OE_FLUSH__TIME, timestamp);
 }
 
+u64 ordered_events__first_time(struct ordered_events *oe)
+{
+	struct ordered_event *event;
+
+	if (list_empty(&oe->events))
+		return 0;
+
+	event = list_first_entry(&oe->events, struct ordered_event, list);
+	return event->timestamp;
+}
+
 void ordered_events__init(struct ordered_events *oe, ordered_events__deliver_t deliver,
 			  void *data)
 {

tools/perf/util/ordered-events.h

@@ -61,6 +61,7 @@ void ordered_events__init(struct ordered_events *oe, ordered_events__deliver_t d
 			  void *data);
 void ordered_events__free(struct ordered_events *oe);
 void ordered_events__reinit(struct ordered_events *oe);
+u64 ordered_events__first_time(struct ordered_events *oe);
 
 static inline
 void ordered_events__set_alloc_size(struct ordered_events *oe, u64 size)