mtd: core: Check devicetree alias for index

Allow the MTD index to be specified via a devicetree alias, so that the
number does not just depend on probe order.  This is useful to allow
pseudo-devices like phram to be optionally used on systems, without
having this affect the numbering of the real hardware MTD devices.
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220412135302.1682890-2-vincent.whitchurch@axis.com

drivers/mtd/mtdcore.c

@@ -557,9 +557,10 @@ static int mtd_nvmem_add(struct mtd_info *mtd)
 
 int add_mtd_device(struct mtd_info *mtd)
 {
+	struct device_node *np = mtd_get_of_node(mtd);
 	struct mtd_info *master = mtd_get_master(mtd);
 	struct mtd_notifier *not;
-	int i, error;
+	int i, error, ofidx;
 
 	/*
 	 * May occur, for instance, on buggy drivers which call
@@ -598,7 +599,13 @@ int add_mtd_device(struct mtd_info *mtd)
 
 	mutex_lock(&mtd_table_mutex);
 
-	i = idr_alloc(&mtd_idr, mtd, 0, 0, GFP_KERNEL);
+	ofidx = -1;
+	if (np)
+		ofidx = of_alias_get_id(np, "mtd");
+	if (ofidx >= 0)
+		i = idr_alloc(&mtd_idr, mtd, ofidx, ofidx + 1, GFP_KERNEL);
+	else
+		i = idr_alloc(&mtd_idr, mtd, 0, 0, GFP_KERNEL);
 	if (i < 0) {
 		error = i;
 		goto fail_locked;