mtd: part: Create the master device node when partitioned

For many use cases, it helps to have a device node for the entire
MTD device as well as device nodes for the individual partitions.
For example, this allows querying the entire device's properties.
A common idiom is to create an additional partition which spans
over the whole device.

This patch makes a config option, CONFIG_MTD_PARTITIONED_MASTER,
which makes the master partition present even when the device is
partitioned. This isn't turned on by default since it presents
a backwards-incompatible device numbering.

The patch also makes the parent of a partition device be the master,
if the config flag is set, now that the master is a full device.

Signed-off-by: Dan Ehrenberg <dehrenberg@chromium.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

1 files changed, 13 insertions, 5 deletions

diff --git a/drivers/mtd/mtdpart.c b/drivers/mtd/mtdpart.c
index e779de315ade..a19ec5a4e409 100644
--- a/drivers/mtd/mtdpart.c
+++ b/drivers/mtd/mtdpart.c
@@ -30,6 +30,7 @@
 #include <linux/mtd/mtd.h>
 #include <linux/mtd/partitions.h>
 #include <linux/err.h>
+#include <linux/kconfig.h>
 
 #include "mtdcore.h"
 
@@ -379,10 +380,17 @@ static struct mtd_part *allocate_partition(struct mtd_info *master,
 	slave->mtd.name = name;
 	slave->mtd.owner = master->owner;
 
-	/* NOTE:  we don't arrange MTDs as a tree; it'd be error-prone
-	 * to have the same data be in two different partitions.
+	/* NOTE: Historically, we didn't arrange MTDs as a tree out of
+	 * concern for showing the same data in multiple partitions.
+	 * However, it is very useful to have the master node present,
+	 * so the MTD_PARTITIONED_MASTER option allows that. The master
+	 * will have device nodes etc only if this is set, so make the
+	 * parent conditional on that option. Note, this is a way to
+	 * distinguish between the master and the partition in sysfs.
 	 */
-	slave->mtd.dev.parent = master->dev.parent;
+	slave->mtd.dev.parent = IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER) ?
+				&master->dev :
+				master->dev.parent;
 
 	slave->mtd._read = part_read;
 	slave->mtd._write = part_write;
@@ -631,8 +639,8 @@ EXPORT_SYMBOL_GPL(mtd_del_partition);
  * and registers slave MTD objects which are bound to the master according to
  * the partition definitions.
  *
- * We don't register the master, or expect the caller to have done so,
- * for reasons of data integrity.
+ * For historical reasons, this function's caller only registers the master
+ * if the MTD_PARTITIONED_MASTER config option is set.
  */
 
 int add_mtd_partitions(struct mtd_info *master,