perf mmap: Use the stored data in perf_mmap__read_event()

Using the 'start', 'end' and 'overwrite' which are stored in
struct perf_mmap to replace the parameters of perf_mmap__read_event().
The parameters will be discarded later.

No functional change.
Signed-off-by: NKan Liang <kan.liang@linux.intel.com>
Suggested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: NJiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1520350567-80082-4-git-send-email-kan.liang@linux.intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/mmap.c

@@ -76,8 +76,9 @@ static union perf_event *perf_mmap__read(struct perf_mmap *map,
  * perf_mmap__read_done()
  */
 union perf_event *perf_mmap__read_event(struct perf_mmap *map,
-					bool overwrite,
-					u64 *startp, u64 end)
+					bool overwrite __maybe_unused,
+					u64 *startp __maybe_unused,
+					u64 end __maybe_unused)
 {
 	union perf_event *event;
 
@@ -87,17 +88,14 @@ union perf_event *perf_mmap__read_event(struct perf_mmap *map,
 	if (!refcount_read(&map->refcnt))
 		return NULL;
 
-	if (startp == NULL)
-		return NULL;
-
 	/* non-overwirte doesn't pause the ringbuffer */
-	if (!overwrite)
-		end = perf_mmap__read_head(map);
+	if (!map->overwrite)
+		map->end = perf_mmap__read_head(map);
 
-	event = perf_mmap__read(map, startp, end);
+	event = perf_mmap__read(map, &map->start, map->end);
 
-	if (!overwrite)
-		map->prev = *startp;
+	if (!map->overwrite)
+		map->prev = map->start;
 
 	return event;
 }