mtd: block2mtd: add support for an optional custom MTD label

This patch adds support for an optional MTD label for mtd2block emulated
MTD devices.  Useful when, e.g., testing device images using Qemu.

The following line in /etc/fstab can then be used to mount a file system
regardless if running on an embedded system, or emulated with block2mtd:

    mtd:Config  /mnt    jffs2   noatime,nodiratime      0    0

Kernel command line syntax in the emulated case:

    block2mtd.block2mtd=/dev/sda,,Config

Notice the ',,' it is the optional erase_size, which like before this
patch, defaults to PAGE_SIZE when omitted.  Hence the strlen() check.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211009060955.552636-3-troglobit@gmail.com

drivers/mtd/devices/block2mtd.c

@@ -32,7 +32,7 @@
 #include <linux/major.h>
 
 /* Maximum number of comma-separated items in the 'block2mtd=' parameter */
-#define BLOCK2MTD_PARAM_MAX_COUNT 2
+#define BLOCK2MTD_PARAM_MAX_COUNT 3
 
 /* Info for the block device */
 struct block2mtd_dev {
@@ -217,7 +217,7 @@ static void block2mtd_free_device(struct block2mtd_dev *dev)
 
 
 static struct block2mtd_dev *add_device(char *devname, int erase_size,
-		int timeout)
+		char *label, int timeout)
 {
 #ifndef MODULE
 	int i;
@@ -281,7 +281,10 @@ static struct block2mtd_dev *add_device(char *devname, int erase_size,
 
 	/* Setup the MTD structure */
 	/* make the name contain the block device in */
-	name = kasprintf(GFP_KERNEL, "block2mtd: %s", devname);
+	if (!label)
+		name = kasprintf(GFP_KERNEL, "block2mtd: %s", devname);
+	else
+		name = kstrdup(label, GFP_KERNEL);
 	if (!name)
 		goto err_destroy_mutex;
 
@@ -308,7 +311,7 @@ static struct block2mtd_dev *add_device(char *devname, int erase_size,
 	list_add(&dev->list, &blkmtd_device_list);
 	pr_info("mtd%d: [%s] erase_size = %dKiB [%d]\n",
 		dev->mtd.index,
-		dev->mtd.name + strlen("block2mtd: "),
+		label ? label : dev->mtd.name + strlen("block2mtd: "),
 		dev->mtd.erasesize >> 10, dev->mtd.erasesize);
 	return dev;
 
@@ -386,6 +389,7 @@ static int block2mtd_setup2(const char *val)
 	char *str = buf;
 	char *token[BLOCK2MTD_PARAM_MAX_COUNT];
 	char *name;
+	char *label = NULL;
 	size_t erase_size = PAGE_SIZE;
 	unsigned long timeout = MTD_DEFAULT_TIMEOUT;
 	int i, ret;
@@ -417,7 +421,8 @@ static int block2mtd_setup2(const char *val)
 		return 0;
 	}
 
-	if (token[1]) {
+	/* Optional argument when custom label is used */
+	if (token[1] && strlen(token[1])) {
 		ret = parse_num(&erase_size, token[1]);
 		if (ret) {
 			pr_err("illegal erase size\n");
@@ -425,7 +430,12 @@ static int block2mtd_setup2(const char *val)
 		}
 	}
 
-	add_device(name, erase_size, timeout);
+	if (token[2]) {
+		label = token[2];
+		pr_info("Using custom MTD label '%s' for dev %s\n", label, name);
+	}
+
+	add_device(name, erase_size, label, timeout);
 
 	return 0;
 }
@@ -459,7 +469,7 @@ static int block2mtd_setup(const char *val, const struct kernel_param *kp)
 
 
 module_param_call(block2mtd, block2mtd_setup, NULL, NULL, 0200);
-MODULE_PARM_DESC(block2mtd, "Device to use. \"block2mtd=<dev>[,<erasesize>]\"");
+MODULE_PARM_DESC(block2mtd, "Device to use. \"block2mtd=<dev>[,[<erasesize>][,<label>]]\"");
 
 static int __init block2mtd_init(void)
 {