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 <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

drivers/mtd/Kconfig

···
 	  The driver provides wear leveling by storing erase counter into the
 	  OOB.
 
+config MTD_PARTITIONED_MASTER
+	bool "Retain master device when partitioned"
+	default n
+	depends on MTD
+	help
+	  For historical reasons, by default, either a master is present or
+	  several partitions are present, but not both. The concern was that
+	  data listed in multiple partitions was dangerous; however, SCSI does
+	  this and it is frequently useful for applications. This config option
+	  leaves the master in even if the device is partitioned. It also makes
+	  the parent of the partition device be the master device, rather than
+	  what lies behind the master.
+
 source "drivers/mtd/chips/Kconfig"
 
 source "drivers/mtd/maps/Kconfig"

drivers/mtd/mtdcore.c

···
 #include <linux/gfp.h>
 #include <linux/slab.h>
 #include <linux/reboot.h>
+#include <linux/kconfig.h>
 
 #include <linux/mtd/mtd.h>
 #include <linux/mtd/partitions.h>
···
 	return ret;
 }
 
+static int mtd_add_device_partitions(struct mtd_info *mtd,
+				     struct mtd_partition *real_parts,
+				     int nbparts)
+{
+	int ret;
+
+	if (nbparts == 0 || IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER)) {
+		ret = add_mtd_device(mtd);
+		if (ret == 1)
+			return -ENODEV;
+	}
+
+	if (nbparts > 0) {
+		ret = add_mtd_partitions(mtd, real_parts, nbparts);
+		if (ret && IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER))
+			del_mtd_device(mtd);
+		return ret;
+	}
+
+	return 0;
+}
+
+
 /**
  * mtd_device_parse_register - parse partitions and register an MTD device.
  *
···
  *   found this functions tries to fallback to information specified in
  *   @parts/@nr_parts.
  * * If any partitioning info was found, this function registers the found
- *   partitions.
+ *   partitions. If the MTD_PARTITIONED_MASTER option is set, then the device
+ *   as a whole is registered first.
  * * If no partitions were found this function just registers the MTD device
  *   @mtd and exits.
  *
···
 			      const struct mtd_partition *parts,
 			      int nr_parts)
 {
-	int err;
-	struct mtd_partition *real_parts;
+	int ret;
+	struct mtd_partition *real_parts = NULL;
 
-	err = parse_mtd_partitions(mtd, types, &real_parts, parser_data);
-	if (err <= 0 && nr_parts && parts) {
+	ret = parse_mtd_partitions(mtd, types, &real_parts, parser_data);
+	if (ret <= 0 && nr_parts && parts) {
 		real_parts = kmemdup(parts, sizeof(*parts) * nr_parts,
 				     GFP_KERNEL);
 		if (!real_parts)
-			err = -ENOMEM;
+			ret = -ENOMEM;
 		else
-			err = nr_parts;
+			ret = nr_parts;
 	}
 
-	if (err > 0) {
-		err = add_mtd_partitions(mtd, real_parts, err);
-		kfree(real_parts);
-	} else if (err == 0) {
-		err = add_mtd_device(mtd);
-		if (err == 1)
-			err = -ENODEV;
-	}
+	if (ret >= 0)
+		ret = mtd_add_device_partitions(mtd, real_parts, ret);
 
 	/*
 	 * FIXME: some drivers unfortunately call this function more than once.
···
 		register_reboot_notifier(&mtd->reboot_notifier);
 	}
 
-	return err;
+	kfree(real_parts);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(mtd_device_parse_register);
 

drivers/mtd/mtdpart.c

···
 #include <linux/mtd/mtd.h>
 #include <linux/mtd/partitions.h>
 #include <linux/err.h>
+#include <linux/kconfig.h>
 
 #include "mtdcore.h"
 
···
 	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;
···
  * 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,