Environment Variables:
======================
U-Boot supports user configuration using Environment Variables which
can be made persistent by saving to Flash memory.
Environment Variables are set using "setenv", printed using
"printenv", and saved to Flash using "saveenv". Using "setenv"
without a value can be used to delete a variable from the
environment. As long as you don't save the environment you are
working with an in-memory copy. In case the Flash area containing the
environment is erased by accident, a default environment is provided.
Some configuration options can be set using Environment Variables.
List of environment variables (most likely not complete):
baudrate - see CONFIG_BAUDRATE
bootdelay - see CONFIG_BOOTDELAY
bootcmd - see CONFIG_BOOTCOMMAND
bootargs - Boot arguments when booting an RTOS image
bootfile - Name of the image to load with TFTP
bootm_low - Memory range available for image processing in the bootm
command can be restricted. This variable is given as
a hexadecimal number and defines lowest address allowed
for use by the bootm command. See also "bootm_size"
environment variable. Address defined by "bootm_low" is
also the base of the initial memory mapping for the Linux
kernel -- see the description of CONFIG_SYS_BOOTMAPSZ and
bootm_mapsize.
bootm_mapsize - Size of the initial memory mapping for the Linux kernel.
This variable is given as a hexadecimal number and it
defines the size of the memory region starting at base
address bootm_low that is accessible by the Linux kernel
during early boot. If unset, CONFIG_SYS_BOOTMAPSZ is used
as the default value if it is defined, and bootm_size is
used otherwise.
bootm_size - Memory range available for image processing in the bootm
command can be restricted. This variable is given as
a hexadecimal number and defines the size of the region
allowed for use by the bootm command. See also "bootm_low"
environment variable.
updatefile - Location of the software update file on a TFTP server, used
by the automatic software update feature. Please refer to
autoload - if set to "no" (any string beginning with 'n'),
"bootp" will just load perform a lookup of the
configuration from the BOOTP server, but not try to
load any image using TFTP
autostart - if set to "yes", an image loaded using the "bootp",
"rarpboot", "tftpboot" or "diskboot" commands will
be automatically started (by internally calling
"bootm")
If set to "no", a standalone image passed to the
"bootm" command will be copied to the load address
(and eventually uncompressed), but NOT be started.
This can be used to load and uncompress arbitrary
data.
fdt_high - if set this restricts the maximum address that the
flattened device tree will be copied into upon boot.
For example, if you have a system with 1 GB memory
at physical address 0x10000000, while Linux kernel
only recognizes the first 704 MB as low memory, you
may need to set fdt_high as 0x3C000000 to have the
device tree blob be copied to the maximum address
of the 704 MB low memory, so that Linux kernel can
access it during the boot procedure.
If this is set to the special value 0xFFFFFFFF then
the fdt will not be copied at all on boot. For this
to work it must reside in writable memory, have
sufficient padding on the end of it for u-boot to
add the information it needs into it, and the memory
must be accessible by the kernel.
fdtcontroladdr- if set this is the address of the control flattened
device tree used by U-Boot when CONFIG_OF_CONTROL is
defined.
i2cfast - (PPC405GP|PPC405EP only)
if set to 'y' configures Linux I2C driver for fast
mode (400kHZ). This environment variable is used in
initialization code. So, for changes to be effective
it must be saved and board must be reset.
initrd_high - restrict positioning of initrd images:
If this variable is not set, initrd images will be
copied to the highest possible address in RAM; this
is usually what you want since it allows for
maximum initrd size. If for some reason you want to
make sure that the initrd image is loaded below the
CONFIG_SYS_BOOTMAPSZ limit, you can set this environment
variable to a value of "no" or "off" or "0".
Alternatively, you can set it to a maximum upper
address to use (U-Boot will still check that it
does not overwrite the U-Boot stack and data).
For instance, when you have a system with 16 MB
RAM, and want to reserve 4 MB from use by Linux,
you can do this by adding "mem=12M" to the value of
the "bootargs" variable. However, now you must make
sure that the initrd image is placed in the first
12 MB as well - this can be done with
setenv initrd_high 00c00000
If you set initrd_high to 0xFFFFFFFF, this is an
indication to U-Boot that all addresses are legal
for the Linux kernel, including addresses in flash
memory. In this case U-Boot will NOT COPY the
ramdisk at all. This may be useful to reduce the
boot time on your system, but requires that this
feature is supported by your Linux kernel.
ipaddr - IP address; needed for tftpboot command
loadaddr - Default load address for commands like "bootp",
"rarpboot", "tftpboot", "loadb" or "diskboot"
loads_echo - see CONFIG_LOADS_ECHO
serverip - TFTP server IP address; needed for tftpboot command
bootretry - see CONFIG_BOOT_RETRY_TIME
bootdelaykey - see CONFIG_AUTOBOOT_DELAY_STR
bootstopkey - see CONFIG_AUTOBOOT_STOP_STR
ethprime - controls which interface is used first.
ethact - controls which interface is currently active.
For example you can do the following
=> setenv ethact FEC
=> ping 192.168.0.1 # traffic sent on FEC
=> setenv ethact SCC
=> ping 10.0.0.1 # traffic sent on SCC
ethrotate - When set to "no" U-Boot does not go through all
available network interfaces.
It just stays at the currently selected interface.
netretry - When set to "no" each network operation will
either succeed or fail without retrying.
When set to "once" the network operation will
fail when all the available network interfaces
are tried once without success.
Useful on scripts which control the retry operation
themselves.
npe_ucode - set load address for the NPE microcode
silent_linux - If set then Linux will be told to boot silently, by
changing the console to be empty. If "yes" it will be
made silent. If "no" it will not be made silent. If
unset, then it will be made silent if the U-Boot console
is silent.
tftpsrcp - If this is set, the value is used for TFTP's
UDP source port.
tftpdstp - If this is set, the value is used for TFTP's UDP
destination port instead of the Well Know Port 69.
tftpblocksize - Block size to use for TFTP transfers; if not set,
we use the TFTP server's default block size
tftptimeout - Retransmission timeout for TFTP packets (in milli-
seconds, minimum value is 1000 = 1 second). Defines
when a packet is considered to be lost so it has to
be retransmitted. The default is 5000 = 5 seconds.
Lowering this value may make downloads succeed
faster in networks with high packet loss rates or
with unreliable TFTP servers.
tftptimeoutcountmax - maximum count of TFTP timeouts (no
unit, minimum value = 0). Defines how many timeouts
can happen during a single file transfer before that
transfer is aborted. The default is 10, and 0 means
'no timeouts allowed'. Increasing this value may help
downloads succeed with high packet loss rates, or with
unreliable TFTP servers or client hardware.
vlan - When set to a value < 4095 the traffic over
Ethernet is encapsulated/received over 802.1q
VLAN tagged frames.
bootpretryperiod - Period during which BOOTP/DHCP sends retries.
Unsigned value, in milliseconds. If not set, the period will
be either the default (28000), or a value based on
CONFIG_NET_RETRY_COUNT, if defined. This value has
precedence over the valu based on CONFIG_NET_RETRY_COUNT.
The following image location variables contain the location of images
used in booting. The "Image" column gives the role of the image and is
not an environment variable name. The other columns are environment
variable names. "File Name" gives the name of the file on a TFTP
server, "RAM Address" gives the location in RAM the image will be
loaded to, and "Flash Location" gives the image's address in NOR
flash or offset in NAND flash.
*Note* - these variables don't have to be defined for all boards, some
boards currently use other variables for these purposes, and some
boards use these variables for other purposes.
Image File Name RAM Address Flash Location
----- --------- ----------- --------------
u-boot u-boot u-boot_addr_r u-boot_addr
Linux kernel bootfile kernel_addr_r kernel_addr
device tree blob fdtfile fdt_addr_r fdt_addr
ramdisk ramdiskfile ramdisk_addr_r ramdisk_addr
The following environment variables may be used and automatically
updated by the network boot commands ("bootp" and "rarpboot"),
depending the information provided by your boot server:
bootfile - see above
dnsip - IP address of your Domain Name Server
dnsip2 - IP address of your secondary Domain Name Server
gatewayip - IP address of the Gateway (Router) to use
hostname - Target hostname
ipaddr - see above
netmask - Subnet Mask
rootpath - Pathname of the root filesystem on the NFS server
serverip - see above
There are two special Environment Variables:
serial# - contains hardware identification information such
as type string and/or serial number
ethaddr - Ethernet address
These variables can be set only once (usually during manufacturing of
the board). U-Boot refuses to delete or overwrite these variables
once they have been set once.
Further special Environment Variables:
ver - Contains the U-Boot version string as printed
with the "version" command. This variable is
readonly (see CONFIG_VERSION_VARIABLE).
Please note that changes to some configuration parameters may take
only effect after the next boot (yes, that's just like Windoze :-).
Callback functions for environment variables:
---------------------------------------------
For some environment variables, the behavior of u-boot needs to change
when their values are changed. This functionality allows functions to
be associated with arbitrary variables. On creation, overwrite, or
deletion, the callback will provide the opportunity for some side
effect to happen or for the change to be rejected.
The callbacks are named and associated with a function using the
U_BOOT_ENV_CALLBACK macro in your board or driver code.
These callbacks are associated with variables in one of two ways. The
static list can be added to by defining CONFIG_ENV_CALLBACK_LIST_STATIC
in the board configuration to a string that defines a list of
associations. The list must be in the following format:
entry = variable_name[:callback_name]
list = entry[,list]
If the callback name is not specified, then the callback is deleted.
Spaces are also allowed anywhere in the list.
Callbacks can also be associated by defining the ".callbacks" variable
with the same list format above. Any association in ".callbacks" will
override any association in the static list. You can define
CONFIG_ENV_CALLBACK_LIST_DEFAULT to a list (string) to define the
".callbacks" environment variable in the default or embedded environment.
If CONFIG_REGEX is defined, the variable_name above is evaluated as a
regular expression. This allows multiple variables to be connected to
the same callback without explicitly listing them all out.