Booting Android completely over ethernet

When developing embedded-systems, initial development stages often involve huge number of "Modify-Build-Flash-Test" cycles. Test-Driven-Development methodology further promotes this style of development. This leads to a break in the "flow" at the Flash stage. Flashing the device with a newly built set of binaries interrupts the otherwise smooth "Modify-Build-Test" flow. Also errors tend to creep-in in the form of an older binary being copied/flashed, often causing confusion during debugging and  endless grief to the developer.

A simple way to avoid this is to have the binaries on the host-machine (a PC) and boot the embedded device directly using those binaries. In case of Android embedded system development, these binaries are the Linux-Kernel and the Android filesystem image.

Pre-requisites:

• The embedded device
• A linux PC
• Ethernet connectivity between the two

NOTE: Below listed parts 1, 2 & 3 involve setting-up the "host" Linux PC. Part 4 describes configuring the device to boot directly using the binaries present on the "host". It is assumed that a functional bootloader (u-boot) is present on the device (internal-flash/mmc-card) and that ethernet-support(either direct or over usb) is enabled.

Part1: Linux kernel over tftp

1. Install tftpd and related packages

host-PC$ sudo apt-get install xinetd tftpd tftp

2. Create /etc/xinetd.d/tftp

host-PC$ cat <<EOF | sudo tee /etc/xinetd.d/tftp
service tftp
{
    protocol        = udp
    port            = 69
    socket_type     = dgram
    wait            = yes
    user            = nobody
    server          = /usr/sbin/in.tftpd
    server_args     = /srv/tftp
    disable         = no
}
EOF

3. Make tftp-server directory

host-PC$ mkdir <tftp-server-path>

host-PC$ chmod -R 777 <tftp-server-path>

host-PC$ chown -R nobody <tftp-server-path>

4. Start tftpd through xinetd

host-PC$ sudo /etc/init.d/xinetd restart 
This concludes the tftp part of the setup process on the host.

Part2: Android fs over NFS

1. Install nfs packages

host-PC$ sudo apt-get install nfs-kernel-server nfs-common 

2. Add this line to /etc/exports

<rootfs-path> *(rw,sync,no_subtree_check,no_root_squash)

3. Restart service

host-PC$ sudo service nfs-kernel-server restart

4. Update exports for the NFS server

host-PC$ exportfs -a

5. Check NFS server

host-PC$ showmount -e

If everything went right, the <rootfs-path> will be listed in the output of showmount.

Part3: Where to put the files

1. Linux Kernel uImage

On the "host" PC,
Copy the Linux-Kernel uImage into <tftp-server-path>

2. Android rootfs

On the "host" PC,
Copy the contents of the Android rootfs into <rootfs-path>

Part4: Configuring the bootloader

1. Update bootargs

Connect the embedded device to the host-PC over ethernet (either directly or via a switch/router) and power it on. As shown below, configure the bootloader to pick-up the kernel from the host-PC over tftp and to mount the filesystem from the host-PC over NFS. As both support configuring a static-ip for the embedded-device or obtaining one dynamically using dhcp, 4 combinations are possible (2 shown below).

nfs(static-ip) and tftp(dhcp)
U-Boot# setenv bootargs 'console=ttyO0,115200n8 androidboot.console=ttyO0 mem=256M root=/dev/nfs ip=<client-device-ip> nfsroot=<nfs-server-ip>:<rootfs-path> rootdelay=2'

U-Boot# setenv serverip 'host-pc-ip'

U-Boot# bootm <Load address>

nfs(dhcp) and tftp(static-ip)
U-Boot# setenv bootargs 'console=ttyO0,115200n8 androidboot.console=ttyO0 mem=256M root=/dev/nfs ip=dhcp nfsroot=<nfs-server-ip>:<rootfs-path> rootdelay=2'

U-Boot# setenv serverip 'host-pc-ip'

U-Boot# setenv ipaddr 'client-device-ip'

U-Boot# tftp

U-Boot# bootm <Load address>

2. Boot ;-)

Linux-Kernel loaded over tftp

Filesystem mounted over NFS