tracing: Use seq file for trace_clock

The buffer for the output is as small as 64 bytes, so it'll
overflow if we add more clock type. Use seq file instead.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4B1DC4FB.5030407@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

kernel/trace/trace.c

@@ -3366,21 +3366,18 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
 	return cnt;
 }
 
-static ssize_t tracing_clock_read(struct file *filp, char __user *ubuf,
-				  size_t cnt, loff_t *ppos)
+static int tracing_clock_show(struct seq_file *m, void *v)
 {
-	char buf[64];
-	int bufiter = 0;
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(trace_clocks); i++)
-		bufiter += snprintf(buf + bufiter, sizeof(buf) - bufiter,
+		seq_printf(m,
 			"%s%s%s%s", i ? " " : "",
 			i == trace_clock_id ? "[" : "", trace_clocks[i].name,
 			i == trace_clock_id ? "]" : "");
-	bufiter += snprintf(buf + bufiter, sizeof(buf) - bufiter, "\n");
+	seq_putc(m, '\n');
 
-	return simple_read_from_buffer(ubuf, cnt, ppos, buf, bufiter);
+	return 0;
 }
 
 static ssize_t tracing_clock_write(struct file *filp, const char __user *ubuf,
@@ -3422,6 +3419,13 @@ static ssize_t tracing_clock_write(struct file *filp, const char __user *ubuf,
 	return cnt;
 }
 
+static int tracing_clock_open(struct inode *inode, struct file *file)
+{
+	if (tracing_disabled)
+		return -ENODEV;
+	return single_open(file, tracing_clock_show, NULL);
+}
+
 static const struct file_operations tracing_max_lat_fops = {
 	.open		= tracing_open_generic,
 	.read		= tracing_max_lat_read,
@@ -3460,8 +3464,10 @@ static const struct file_operations tracing_mark_fops = {
 };
 
 static const struct file_operations trace_clock_fops = {
-	.open		= tracing_open_generic,
-	.read		= tracing_clock_read,
+	.open		= tracing_clock_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
 	.write		= tracing_clock_write,
 };