tracing: Add trace_options kernel command line parameter

Add trace_options to the kernel command line parameter to be able to
set options at early boot. For example, to enable stack dumps of
events, add the following:

trace_options=stacktrace

This along with the trace_event option, you can get not only
traces of the events but also the stack dumps with them.

Requested-by: Frederic Weisbecker <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Steven Rostedt <[e-mail]>

Documentation/kernel-parameters.txt

···
 			to facilitate early boot debugging.
 			See also Documentation/trace/events.txt
 
+	trace_options=[option-list]
+			[FTRACE] Enable or disable tracer options at boot.
+			The option-list is a comma delimited list of options
+			that can be enabled or disabled just as if you were
+			to echo the option name into
+
+			    /sys/kernel/debug/tracing/trace_options
+
+			For example, to enable stacktrace option (to dump the
+			stack trace of each event), add to the command line:
+
+			      trace_options=stacktrace
+
+			See also Documentation/trace/ftrace.txt "trace options"
+			section.
+
 	transparent_hugepage=
 			[KNL]
 			Format: [always|madvise|never]

kernel/trace/trace.c

···
 }
 __setup("ftrace_dump_on_oops", set_ftrace_dump_on_oops);
 
+
+static char trace_boot_options_buf[MAX_TRACER_SIZE] __initdata;
+static char *trace_boot_options __initdata;
+
+static int __init set_trace_boot_options(char *str)
+{
+	strncpy(trace_boot_options_buf, str, MAX_TRACER_SIZE);
+	trace_boot_options = trace_boot_options_buf;
+	return 0;
+}
+__setup("trace_options=", set_trace_boot_options);
+
 unsigned long long ns2usecs(cycle_t nsec)
 {
 	nsec += 500;
···
 		trace_printk_start_stop_comm(enabled);
 }
 
-static ssize_t
-tracing_trace_options_write(struct file *filp, const char __user *ubuf,
-			size_t cnt, loff_t *ppos)
+static int trace_set_options(char *option)
 {
-	char buf[64];
 	char *cmp;
 	int neg = 0;
-	int ret;
+	int ret = 0;
 	int i;
 
-	if (cnt >= sizeof(buf))
-		return -EINVAL;
-
-	if (copy_from_user(&buf, ubuf, cnt))
-		return -EFAULT;
-
-	buf[cnt] = 0;
-	cmp = strstrip(buf);
+	cmp = strstrip(option);
 
 	if (strncmp(cmp, "no", 2) == 0) {
 		neg = 1;
···
 		mutex_lock(&trace_types_lock);
 		ret = set_tracer_option(current_trace, cmp, neg);
 		mutex_unlock(&trace_types_lock);
-		if (ret)
-			return ret;
 	}
+
+	return ret;
+}
+
+static ssize_t
+tracing_trace_options_write(struct file *filp, const char __user *ubuf,
+			size_t cnt, loff_t *ppos)
+{
+	char buf[64];
+
+	if (cnt >= sizeof(buf))
+		return -EINVAL;
+
+	if (copy_from_user(&buf, ubuf, cnt))
+		return -EFAULT;
+
+	trace_set_options(buf);
 
 	*ppos += cnt;
 
···
 				       &trace_panic_notifier);
 
 	register_die_notifier(&trace_die_notifier);
+
+	while (trace_boot_options) {
+		char *option;
+
+		option = strsep(&trace_boot_options, ",");
+		trace_set_options(option);
+	}
 
 	return 0;