tracing: Record trace_clock and recover when reboot

Record trace_clock information in the trace_scratch area and recover
the trace_clock when boot, so that reader can docode the timestamp
correctly.
Note that since most trace_clocks records the timestamp in nano-
seconds, this is not a bug. But some trace_clock, like counter and
tsc will record the counter value. Only for those trace_clock user
needs this information.

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/174720625803.1925039.1815089037443798944.stgit@mhiramat.tok.corp.google.com
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: mhiramat

kernel/trace/trace.c

@@ -6066,6 +6066,7 @@ struct trace_mod_entry {
 };
 
 struct trace_scratch {
+	unsigned int		clock_id;
 	unsigned long		text_addr;
 	unsigned long		nr_entries;
 	struct trace_mod_entry	entries[];
@@ -6181,6 +6182,7 @@ static void update_last_data(struct trace_array *tr)
 	if (tr->scratch) {
 		struct trace_scratch *tscratch = tr->scratch;
 
+		tscratch->clock_id = tr->clock_id;
 		memset(tscratch->entries, 0,
 		       flex_array_size(tscratch, entries, tscratch->nr_entries));
 		tscratch->nr_entries = 0;
@@ -7403,6 +7405,12 @@ int tracing_set_clock(struct trace_array *tr, const char *clockstr)
 	tracing_reset_online_cpus(&tr->max_buffer);
 #endif
 
+	if (tr->scratch && !(tr->flags & TRACE_ARRAY_FL_LAST_BOOT)) {
+		struct trace_scratch *tscratch = tr->scratch;
+
+		tscratch->clock_id = i;
+	}
+
 	mutex_unlock(&trace_types_lock);
 
 	return 0;
@@ -9628,6 +9636,15 @@ static void setup_trace_scratch(struct trace_array *tr,
 
 	/* Scan modules to make text delta for modules. */
 	module_for_each_mod(make_mod_delta, tr);
+
+	/* Set trace_clock as the same of the previous boot. */
+	if (tscratch->clock_id != tr->clock_id) {
+		if (tscratch->clock_id >= ARRAY_SIZE(trace_clocks) ||
+		    tracing_set_clock(tr, trace_clocks[tscratch->clock_id].name) < 0) {
+			pr_info("the previous trace_clock info is not valid.");
+			goto reset;
+		}
+	}
 	return;
  reset:
 	/* Invalid trace modules */