perf report: Add documentation for dynamic sort keys

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1451991518-25673-3-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-report.txt

@@ -117,6 +117,30 @@ OPTIONS
 	And default sort keys are changed to comm, dso_from, symbol_from, dso_to
 	and symbol_to, see '--branch-stack'.
 
+	If the data file has tracepoint event(s), following (dynamic) sort keys
+	are also available:
+	trace, trace_fields, [<event>.]<field>[/raw]
+
+	- trace: pretty printed trace output in a single column
+	- trace_fields: fields in tracepoints in separate columns
+	- <field name>: optional event and field name for a specific field
+
+	The last form consists of event and field names.  If event name is
+	omitted, it searches all events for matching field name.  The matched
+	field will be shown only for the event has the field.  The event name
+	supports substring match so user doesn't need to specify full subsystem
+	and event name everytime.  For example, 'sched:sched_switch' event can
+	be shortened to 'switch' as long as it's not ambiguous.  Also event can
+	be specified by its index (starting from 1) preceded by the '%'.
+	So '%1' is the first event, '%2' is the second, and so on.
+
+	The field name can have '/raw' suffix which disables pretty printing
+	and shows raw field value like hex numbers.  The --raw-trace option
+	has the same effect for all dynamic sort keys.
+
+	The default sort keys are changed to 'trace' if all events in the data
+	file are tracepoint.
+
 -F::
 --fields=::
 	Specify output field - multiple keys can be specified in CSV format.