UBI: Add ro-mode sysfs attribute

On serious situations, UBI may detect serious device corruption,
and switch to read-only mode to protect the data and allow debugging.
This commit exposes this ro-mode on sysfs, so it can be obtained
by userspace tools.
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Richard Weinberger <richard@nod.at>

Documentation/ABI/stable/sysfs-class-ubi

@@ -107,6 +107,15 @@ Contact:	Artem Bityutskiy <dedekind@infradead.org>
 Description:
 		Number of physical eraseblocks reserved for bad block handling.
 
+What:		/sys/class/ubi/ubiX/ro_mode
+Date:		April 2016
+KernelVersion:	4.7
+Contact:	linux-mtd@lists.infradead.org
+Description:
+		Contains ASCII "1\n" if the read-only flag is set on this
+		device, and "0\n" if it is cleared. UBI devices mark themselves
+		as read-only when they detect an unrecoverable error.
+
 What:		/sys/class/ubi/ubiX/total_eraseblocks
 Date:		July 2006
 KernelVersion:	2.6.22

drivers/mtd/ubi/build.c

@@ -149,6 +149,8 @@ static struct device_attribute dev_bgt_enabled =
 	__ATTR(bgt_enabled, S_IRUGO, dev_attribute_show, NULL);
 static struct device_attribute dev_mtd_num =
 	__ATTR(mtd_num, S_IRUGO, dev_attribute_show, NULL);
+static struct device_attribute dev_ro_mode =
+	__ATTR(ro_mode, S_IRUGO, dev_attribute_show, NULL);
 
 /**
  * ubi_volume_notify - send a volume change notification.
@@ -385,6 +387,8 @@ static ssize_t dev_attribute_show(struct device *dev,
 		ret = sprintf(buf, "%d\n", ubi->thread_enabled);
 	else if (attr == &dev_mtd_num)
 		ret = sprintf(buf, "%d\n", ubi->mtd->index);
+	else if (attr == &dev_ro_mode)
+		ret = sprintf(buf, "%d\n", ubi->ro_mode);
 	else
 		ret = -EINVAL;
 
@@ -404,6 +408,7 @@ static struct attribute *ubi_dev_attrs[] = {
 	&dev_min_io_size.attr,
 	&dev_bgt_enabled.attr,
 	&dev_mtd_num.attr,
+	&dev_ro_mode.attr,
 	NULL
 };
 ATTRIBUTE_GROUPS(ubi_dev);