clocksource: Split out user string input

Split out the user string input for clocksource override. Preparatory
patch for unbind.

[ jstultz: Fix an off by one error ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Magnus Damm <magnus.damm@gmail.com>
Link: http://lkml.kernel.org/r/20130425143435.895851338@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/clocksource.c

@@ -174,7 +174,8 @@ clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 maxsec)
 static struct clocksource *curr_clocksource;
 static LIST_HEAD(clocksource_list);
 static DEFINE_MUTEX(clocksource_mutex);
-static char override_name[32];
+#define CS_NAME_LEN		32
+static char override_name[CS_NAME_LEN];
 static int finished_booting;
 
 #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
@@ -838,6 +839,23 @@ sysfs_show_current_clocksources(struct device *dev,
 	return count;
 }
 
+static size_t clocksource_get_uname(const char *buf, char *dst, size_t cnt)
+{
+	size_t ret = cnt;
+
+	/* strings from sysfs write are not 0 terminated! */
+	if (!cnt || cnt >= CS_NAME_LEN)
+		return -EINVAL;
+
+	/* strip of \n: */
+	if (buf[cnt-1] == '\n')
+		cnt--;
+	if (cnt > 0)
+		memcpy(dst, buf, cnt);
+	dst[cnt] = 0;
+	return ret;
+}
+
 /**
  * sysfs_override_clocksource - interface for manually overriding clocksource
  * @dev:	unused
@@ -852,22 +870,13 @@ static ssize_t sysfs_override_clocksource(struct device *dev,
 					  struct device_attribute *attr,
 					  const char *buf, size_t count)
 {
-	size_t ret = count;
-
-	/* strings from sysfs write are not 0 terminated! */
-	if (count >= sizeof(override_name))
-		return -EINVAL;
-
-	/* strip of \n: */
-	if (buf[count-1] == '\n')
-		count--;
+	size_t ret;
 
 	mutex_lock(&clocksource_mutex);
 
-	if (count > 0)
-		memcpy(override_name, buf, count);
-	override_name[count] = 0;
-	clocksource_select();
+	ret = clocksource_get_uname(buf, override_name, count);
+	if (ret >= 0)
+		clocksource_select();
 
 	mutex_unlock(&clocksource_mutex);