perf tools: Consolidate management of default sort orders

The perf uses different default sort orders for different use-cases,
and this was scattered throughout the code.  Add get_default_sort_
order() function to handle this and change initial value of sort_order
to NULL to distinguish it from user-given one.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1400480762-22852-10-git-send-email-namhyung@kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
diff --git a/tools/perf/builtin-diff.c b/tools/perf/builtin-diff.c
index b60c711..8bff543 100644
--- a/tools/perf/builtin-diff.c
+++ b/tools/perf/builtin-diff.c
@@ -60,7 +60,6 @@
 #define data__for_each_file(i, d) data__for_each_file_start(i, d, 0)
 #define data__for_each_file_new(i, d) data__for_each_file_start(i, d, 1)
 
-static char diff__default_sort_order[] = "dso,symbol";
 static bool force;
 static bool show_period;
 static bool show_formula;
@@ -1142,7 +1141,6 @@
 {
 	perf_config(perf_default_config, NULL);
 
-	sort_order = diff__default_sort_order;
 	argc = parse_options(argc, argv, options, diff_usage, 0);
 
 	if (symbol__init() < 0)
@@ -1153,6 +1151,8 @@
 
 	ui_init();
 
+	sort__mode = SORT_MODE__DIFF;
+
 	if (setup_sorting() < 0)
 		usage_with_options(diff_usage, options);