ACPI: thinkpad-acpi: add locking to brightness subdriver

The backlight class does all the locking needed for sysfs access, but
offers no API to interface to that locking without an layer violation.

Since we need to mutex-lock procfs access, implement in-driver locking for
brightness.  It will go away the day thinkpad-acpi procfs goes away, or the
backlight class gives us a way to use its locks without a layer violation.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/misc/thinkpad_acpi.c

@@ -3000,12 +3000,16 @@ static struct backlight_ops ibm_backlight_data = {
         .update_status  = brightness_update_status,
 };
 
+static struct mutex brightness_mutex;
+
 static int __init brightness_init(struct ibm_init_struct *iibm)
 {
 	int b;
 
 	vdbg_printk(TPACPI_DBG_INIT, "initializing brightness subdriver\n");
 
+	mutex_init(&brightness_mutex);
+
 	if (!brightness_mode) {
 		if (thinkpad_id.vendor == PCI_VENDOR_ID_LENOVO)
 			brightness_mode = 2;
@@ -3092,27 +3096,44 @@ static int brightness_get(struct backlight_device *bd)
 
 static int brightness_set(int value)
 {
-	int cmos_cmd, inc, i;
-	int current_value = brightness_get(NULL);
+	int cmos_cmd, inc, i, res;
+	int current_value;
 
 	if (value > 7)
 		return -EINVAL;
 
+	res = mutex_lock_interruptible(&brightness_mutex);
+	if (res < 0)
+		return res;
+
+	current_value = brightness_get(NULL);
+	if (current_value < 0) {
+		res = current_value;
+		goto errout;
+	}
+
 	cmos_cmd = value > current_value ?
 			TP_CMOS_BRIGHTNESS_UP :
 			TP_CMOS_BRIGHTNESS_DOWN;
 	inc = value > current_value ? 1 : -1;
 
+	res = 0;
 	for (i = current_value; i != value; i += inc) {
 		if ((brightness_mode & 2) &&
-		    issue_thinkpad_cmos_command(cmos_cmd))
-			return -EIO;
+		    issue_thinkpad_cmos_command(cmos_cmd)) {
+			res = -EIO;
+			goto errout;
+		}
 		if ((brightness_mode & 1) &&
-		    !acpi_ec_write(brightness_offset, i + inc))
-			return -EIO;
+		    !acpi_ec_write(brightness_offset, i + inc)) {
+			res = -EIO;
+			goto errout;;
+		}
 	}
 
-	return 0;
+errout:
+	mutex_unlock(&brightness_mutex);
+	return res;
 }
 
 static int brightness_read(char *p)