multi-boot in embedded device

My device have U-Boot boot loader and 64MB cavium-nand flash (for "kernel" and "filesystem"). How can i support dual-boot in it???

I have decided to format the NAND flash like this:

/dev/mtd0 : 0x00000000-0x00300000 :  3 MB : "kernel1"
/dev/mtd1 : 0x00300000-0x01300000 : 16 MB : "cramfs1"

/dev/mtdblock2 : 0x01300000-0x01700000 :  4 MB : "jffs2"
/dev/mtdblock3 : 0x01700000-0x01800000 :  1 MB : "nvram"

/dev/mtd4 : 0x01800000-0x01b00000 :  3 MB : "kernel2"
/dev/mtd5 : 0x01b00000-0x02b00000 : 16 MB : "cramfs2"

And made device soft link like:

/dev/mtd-kernel     -> mtd0
/dev/mtd-rfs        -> mtd1

/dev/mtd-kernel-alt -> mtd4
/dev/mtd-rfs-alt    -> mtd5

How does U-Boot does the boot process:

1. Read kernel from NAND to RAM
2. un-compress the kernel to RAM
3. execute kernel

Ex:-
bootcmd=nand read <TO> <FROM> <SIZE>; \
        gunzip <FROM> <SIZE> <TO> <MAX SIZE>; \
        bootoctlinux <NEW FROM> root=/dev/mtdblock1 ${mtdparts}

bootcmd=nand read 0x500000 0x00000 0x300000; \
        gunzip 0x500000 0x300000 0x2800000 0xf00000; \
        bootoctlinux 0x2800000 root=/dev/mtdblock1 ${mtdparts}

Solution:

1. get the current kernel information from U-Boot parameter
2. burnel kernel image in alternate locations (mtd0, mtd5)
3. update U-Boot parameter

Ideal U-Boot parameters:

bootcmd=run boot1
altbootcmd=run boot2

boot1=namedfree __tmp_reserved_bootloader; \
      nand read 500000 00000 300000; \
      run boot11
boot2=namedfree __tmp_reserved_bootloader; \
      nand read 500000 1800000 300000; \
      run boot21

mtdparts=mtdparts=cavium-nand:3072k(kernel1),16384k(cramfs1), \ 
                              4096k(jffs2),1024k(nvram), \
                              3072k(kernel2),16384k(cramfs2),3072k()

boot11=gunzip 0x500000 300000 0x2800000 0xf00000; \
       bootoctlinux 2800000 root=/dev/mtdblock1 ${mtdparts}
boot21=gunzip 0x500000 300000 0x2800000 0xf00000; \
       bootoctlinux 2800000 root=/dev/mtdblock5 ${mtdparts}

firmware upgrade script:

#!/bin/sh

bootcmd=`bootenv -d bootcmd`
altcmd=`bootenv -d altbootcmd`

if [ "$bootcmd" == "run boot1" ];
then
    kernelimage="/dev/mtd-kernel-alt"
    rfsimage="/dev/mtd-rfs-alt"
else
    kernelimage="/dev/mtd-kernel"
    rfsimage="/dev/mtd-rfs"
fi

# Kernel in mtd-kernel
echo "Erasing the kernel image in $kernelimage"
echo -n "Flashing in the new kernel image. Please wait..."

# Root File System in mtd-rfs 
echo "Erasing the root f/s image in $rfsimage"
echo -n "Flashing in the new root f/s image. Please wait..."

if [ "$bootcmd" == "run boot1" ];
then
    echo "Setting new bootcmd as run boot2 and backup as run boot1"
    bootenv -s bootver "$upgradeVersion"
    bootenv -s bootcmd "run boot2"
    bootenv -s altbootver "$existingVersion"
    bootenv -s altbootcmd "run boot1"
else
    echo "Setting new bootcmd as run boot1 and backup as run boot2"
    bootenv -s bootver "$upgradeVersion"
    bootenv -s bootcmd "run boot1"
    bootenv -s altbootver "$existingVersion"
    bootenv -s altbootcmd "run boot2"
fi

/bin/sync
/bin/sleep 2

# Reboot
exec /sbin/init -r

Reference: