LinuxKernelArgs < DULG

%SECTION0{name=LinuxKernelArgs}% Passing Kernel Arguments

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 address %IF{ "%ARCH%" eq "powerpc"}%0x400000%ELSE% 0x%KERNEL_ADDR% %ENDIF% using
the =initrd= ramdisk image at address %IF{ "%ARCH%" eq "powerpc"}%0x600000 %ELSE% 0x%INITRD_ADDR% %ENDIF% as
root filesystem, %IF{ "%ARCH%" eq "powerpc"}% and with the flattened device tree blob at address 0x800000, %ENDIF%
%IF{ "%HAVE_FDT%" eq "true" }% and with the flattened device tree blob at address 0x%FDT_ADDR%, %ENDIF%
you can use the following commands:

<pre>
%UBOOT_PROMPT% setenv bootargs root=/dev/ram rw
%UBOOT_PROMPT% bootm %IF1{ "%ARCH%" eq "powerpc"}%0x400000 0x600000%ELSE1% 0x%KERNEL_ADDR% 0x%INITRD_ADDR% %ENDIF1% %IF2{ "%ARCH%" eq "powerpc"}% 0x800000 %ENDIF2% %IF2{ "%HAVE_FDT%" eq "true"}% 0x%FDT_ADDR% %ENDIF2%
</pre>

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 "%SERVERIP%" and exports the directory
"%ROOTPATH%" as root filesystem for the target. The target has been
assigned the IP address "%IPADDR%" and the hostname "%HOSTNAME%". A netmask
of "%NETMASK%" is used:

<pre>
%UBOOT_PROMPT% setenv bootargs root=/dev/nfs rw nfsroot=%SERVERIP%:%ROOTPATH% ip=%IPADDR%:%SERVERIP%:%GATEWAYIP%:%NETMASK%:%HOSTNAME%::off
%UBOOT_PROMPT% bootm %IF1{ "%ARCH%" eq "powerpc"}%0x400000%ELSE1% 0x%KERNEL_ADDR% %ENDIF1% %IF{ "%ARCH%" eq "powerpc"}% - 0x800000 %ENDIF% %IF{ "%HAVE_FDT%" eq "true"}% - 0x%FDT_ADDR% %ENDIF%
</pre>

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.

%X% 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:
<verbatim>
$ cat /proc/cmdline
</verbatim>
7.2. Flattened Device Tree Blob		1. Abstract		7.4. Boot Arguments Unleashed
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