In nearly all cases, you will want to pass additional information to
the Linux kernel; for instance, information about the root device or
network configuration.
In U-Boot, this is supported using the
bootargs
environment variable. Its contents are automatically passed to the
Linux kernel as boot arguments (or "command line" arguments). This allows the
use of the same Linux kernel image in a wide range of configurations. For
instance, by just changing the contents of the
bootargs variable you can use the very same Linux
kernel image to boot with an
initrd ramdisk image,
with a root filesystem over NFS, with a
CompactFlash disk or from a
flash filesystem.
As one example, to boot the Linux kernel image at address0x400000 using
the
initrd ramdisk image at address0x600000 as
root filesystem, and with the flattened device tree blob at address 0x800000, and with the flattened device tree blob at address 0xFC1E0000,
you can use the following commands:
=> setenv bootargs root=/dev/ram rw
=> bootm 0x400000 0x600000 0x800000 0xFC1E0000
To boot the same kernel image with a root filesystem over NFS, the
following command sequence can be used. This example assumes that your
NFS server has the IP address "192.168.1.1" and exports the directory
"/opt/eldk-4.2/ppc_4xx" as root filesystem for the target. The target has been
assigned the IP address "192.168.100.6" and the hostname "canyonlands". A netmask
of "255.255.0.0" is used:
=> setenv bootargs root=/dev/nfs rw nfsroot=192.168.1.1:/opt/eldk-4.2/ppc_4xx ip=192.168.100.6:192.168.1.1:192.168.1.1:255.255.0.0:canyonlands::off
=> bootm 0x400000 - 0x800000 - 0xFC1E0000
Please see also the files
Documentation/initrd.txt
and
Documentation/nfsroot.txt in your Linux kernel
source directory for more information about which options can be
passed to the Linux kernel.
Note: Once your system is up and running, if you have a simple shell login,
you can normally examine the boot arguments that were used by the kernel
for the most recent boot with the command:
$ cat /proc/cmdline
