Use Cramfs as root

["colin" <colin@xxxxxxxxxxxxxx>]

Hi all,
I had successfully used JFFS2 as root.
Then I tried to use the same way to use Cramfs as root, and it cannot work.

Use JFFS2 as root:
    mtd/util/mkfs.jffs2 -d root root.jffs2
    go 0x802b2018 rootfstype=jffs2 root=31:01
mtdparts=maltaflash:1536k(ldr),2048k(root)

Use Cramfs as root:
    /sbin/mkfs.cramfs root root.cramfs
    go 0x802b2018 rootfstype=cramfs root=31:01
mtdparts=maltaflash:1536k(ldr),2048k(root)

Error message:
    VFS: Cannot open root device "31:01" or unknown-block(31,1)
    Please append a correct "root=" boot option
    Kernel panic: VFS: Unable to mount root fs on unknown-block(31,1)


I donot know the right way to boot up Linux with cramfs root image.
There is not much information about it.
Could anyone give me a clue?

Thanks and regards,
Colin