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Network booting on systems using the U-Boot firmware consists of three steps: a) configuring the network, b) loading the images (kernel/initial ramdisk/dtb) into memory and c) actually executing the previosly loaded code.
First you have to configure the network, either automatically via DHCP by running
setenv autoload no dhcp
or manually by setting several environment variables
setenv ipaddr <ip address of the client> setenv netmask <netmask>
setenv serverip <ip address of the tftp server> setenv dnsip <ip address of the nameserver>
setenv gatewayip <ip address of the default gateway>
If you prefer, you can make these settings permanent by running
saveenv
Afterwards you need to load the images (kernel/initial ramdisk/dtb) into memory. This is done with the tftpboot command, which has to be provided with the address at which the image shall be stored in memory. Unfortunately the memory map can vary from system to system, so there is no general rule which addresses can be used for this.
On some systems, U-Boot predefines a set of environment variables with suitable load addresses: kernel_addr_r, ramdisk_addr_r and fdt_addr_r. You can check whether they are defined by running
printenv kernel_addr_r ramdisk_addr_r fdt_addr_r
If they are not defined, you have to check your system’s documentation for appropriate values and set them manually. For systems based on Allwinner SunXi SOCs (e.g. the Allwinner A10, architecture name “sun4i” or the Allwinner A20, architecture name “sun7i”), you can e.g. use the follwing values:
setenv kernel_addr_r 0x46000000 setenv fdt_addr_r 0x47000000 setenv ramdisk_addr_r 0x48000000
When the load addresses are defined, you can load the images into memory from the previously defined tftp server with
tftpboot ${kernel_addr_r} <filename of the kernel image> tftpboot ${fdt_addr_r} <filename of the dtb>
tftpboot ${ramdisk_addr_r} <filename of the initial ramdisk image>
The third part is setting the kernel commandline and actually executing the loaded code. U-Boot passes the content of the “bootargs” environment variable as commandline to the kernel, so any pa- rameters for the kernel and the installer - such as the console device (see Section 5.3.1) or preseeding options (see Section 5.3.2 and Appendix B) - can be set with a command like
setenv bootargs console=ttyS0,115200 rootwait panic=10
The exact command to execute the previously loaded code depends on the image format used. With uImage/uInitrd, the command is
bootm ${kernel_addr_r} ${ramdisk_addr_r} ${fdt_addr_r}
and with native Linux images it is
bootz ${kernel_addr_r} ${ramdisk_addr_r}:${filesize} ${fdt_addr_r}
Note: When booting standard linux images, it is important to load the initial ramdisk image after the kernel and the dtb as U-Boot sets the filesize variable to the size of the last file loaded and the bootz command requires the size of the ramdisk image to work correctly. In case of booting a platform- specific kernel, i.e. a kernel without device-tree, simply omit the ${fdt_addr_r} parameter.