5.1. Starta upp installationsprogrammet på ARM

5.1.1. Boot image formats

On ARM-based systems in most cases one of two formats for boot images is used: a) standard Linux zImage-format kernels (vmlinuz) in conjunction with standard Linux initial ramdisks (initrd.gz) or b) uImage-format kernels (uImage) in conjunction with corresponding initial ramdisks (uInitrd).

uImage/uInitrd are image formats designed for the u-boot firmware that is used on many ARM-based systems. Older u-boot versions can only boot files in uImage/uInitrd format, so these are often used on older armel systems. Newer u-boot versions can - besides booting uImages/uInitrds - also boot standard Linux kernels and ramdisk images, but the command syntax to do that is slightly different from that for booting uImages.

For systems using a multiplatform kernel, besides kernel and initial ramdisk a so-called device-tree file (or device-tree blob, dtb) is needed. It is specific to each supported system and contains a description of the particular hardware.

5.1.2. Booting by TFTP

Booting from the network requires that you have a network connection and a TFTP network boot server (and probably also a DHCP, RARP, or BOOTP server for automatic network configuration).

The server-side setup to support network booting is described in Avsnitt 4.3, ”Förbered filerna för nätverksuppstart via TFTP”.

5.1.2.1. TFTP-booting in u-boot

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 parameters for the kernel and the installer - such as the console device (see Avsnitt 5.3.1, ”Boot console”) or preseeding options (see Avsnitt 5.3.2, ”Parametrar för Debian Installer” and Appendix B, Automatiserad installation med förinställningar) - 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.