init: move the nfs/cifs/ram special cases out of name_to_dev_t

The nfs/cifs/ram special case only needs to be parsed once, and only in
the boot code.  Move them out of name_to_dev_t and into
prepare_namespace.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230531125535.676098-13-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/admin-guide/kernel-parameters.txt

@@ -5452,7 +5452,12 @@
 			port and the regular usb controller gets disabled.
 
 	root=		[KNL] Root filesystem
-			See name_to_dev_t comment in init/do_mounts.c.
+			Usually this a a block device specifier of some kind,
+			see the name_to_dev_t comment in init/do_mounts.c for
+			details.
+			Alternatively this can be "ram" for the legacy initial
+			ramdisk, "nfs" and "cifs" for root on a network file
+			system, or "mtd" and "ubi" for mounting from raw flash.
 
 	rootdelay=	[KNL] Delay (in seconds) to pause before attempting to
 			mount the root filesystem

init/do_mounts.c

@@ -248,7 +248,6 @@ static dev_t devt_from_devnum(const char *name)
  *
  *	1) <hex_major><hex_minor> device number in hexadecimal represents itself
  *         no leading 0x, for example b302.
- *	2) /dev/nfs represents Root_NFS (0xff)
  *	3) /dev/<disk_name> represents the device number of disk
  *	4) /dev/<disk_name><decimal> represents the device number
  *         of partition - device number of disk plus the partition number
@@ -266,7 +265,6 @@ static dev_t devt_from_devnum(const char *name)
  *	   a colon.
  *	9) PARTLABEL=<name> with name being the GPT partition label.
  *	   MSDOS partitions do not support labels!
- *	10) /dev/cifs represents Root_CIFS (0xfe)
  *
  *	If name doesn't have fall into the categories above, we return (0,0).
  *	block_class is used to check if something is a disk name. If the disk
@@ -275,12 +273,6 @@ static dev_t devt_from_devnum(const char *name)
  */
 dev_t name_to_dev_t(const char *name)
 {
-	if (strcmp(name, "/dev/nfs") == 0)
-		return Root_NFS;
-	if (strcmp(name, "/dev/cifs") == 0)
-		return Root_CIFS;
-	if (strcmp(name, "/dev/ram") == 0)
-		return Root_RAM0;
 #ifdef CONFIG_BLOCK
 	if (strncmp(name, "PARTUUID=", 9) == 0)
 		return devt_from_partuuid(name + 9);
@@ -631,6 +623,12 @@ static dev_t __init parse_root_device(char *root_device_name)
 	if (!strncmp(root_device_name, "mtd", 3) ||
 	    !strncmp(root_device_name, "ubi", 3))
 		return Root_Generic;
+	if (strcmp(root_device_name, "/dev/nfs") == 0)
+		return Root_NFS;
+	if (strcmp(root_device_name, "/dev/cifs") == 0)
+		return Root_CIFS;
+	if (strcmp(root_device_name, "/dev/ram") == 0)
+		return Root_RAM0;
 	return name_to_dev_t(root_device_name);
 }