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* (C) Copyright 2001
* Dave Ellis, SIXNET,
* SPDX-License-Identifier: GPL-2.0+
Using autoboot configuration options
The basic autoboot configuration options are documented in the main
U-Boot README. See it for details. They are:
Some additional options that make autoboot safer in a production
product are documented here.
Why use them?
The basic autoboot feature allows a system to automatically boot to
the real application (such as Linux) without a user having to enter
any commands. If any key is pressed before the boot delay time
expires, U-Boot stops the autoboot process, gives a U-Boot prompt
and waits forever for a command. That's a good thing if you pressed a
key because you wanted to get the prompt.
It's not so good if the key press was a stray character on the
console serial port, say because a user who knows nothing about
U-Boot pressed a key before the system had time to boot. It's even
worse on an embedded product that doesn't have a console during
normal use. The modem plugged into that console port sends a
character at the wrong time and the system hangs, with no clue as to
why it isn't working.
You might want the system to autoboot to recover after an external
configuration program stops autoboot. If the configuration program
dies or loses its connection (modems can disconnect at the worst
time) U-Boot will patiently wait forever for it to finish.
These additional configuration options can help provide a system that
boots when it should, but still allows access to U-Boot.
What they do
"bootretry" environment variable
These options determine what happens after autoboot is
stopped and U-Boot is waiting for commands.
CONFIG_BOOT_RETRY_TIME must be defined to enable the boot
retry feature. If the environment variable "bootretry" is
found then its value is used, otherwise the retry timeout is
defaults to CONFIG_BOOT_RETRY_TIME. All times are in seconds.
If the retry timeout is negative, the U-Boot command prompt
never times out. Otherwise it is forced to be at least
CONFIG_BOOT_RETRY_MIN seconds. If no valid U-Boot command is
entered before the specified time the boot delay sequence is
restarted. Each command that U-Boot executes restarts the
If CONFIG_BOOT_RETRY_TIME < 0 the feature is there, but
doesn't do anything unless the environment variable
"bootretry" is >= 0.
"bootdelaykey" environment variable
"bootstopkey" environment variable
These options give more control over stopping autoboot. When
they are used a specific character or string is required to
stop or delay autoboot.
Define CONFIG_AUTOBOOT_KEYED (no value required) to enable
this group of options. CONFIG_AUTOBOOT_DELAY_STR,
CONFIG_AUTOBOOT_STOP_STR or both should be specified (or
specified by the corresponding environment variable),
otherwise there is no way to stop autoboot.
CONFIG_AUTOBOOT_PROMPT is displayed before the boot delay
selected by CONFIG_BOOTDELAY starts. If it is not defined
there is no output indicating that autoboot is in progress.
Note that CONFIG_AUTOBOOT_PROMPT is used as the (only)
argument to a printf() call, so it may contain '%' format
specifications, provided that it also includes, sepearated by
commas exactly like in a printf statement, the required
arguments. It is the responsibility of the user to select only
such arguments that are valid in the given context. A
reasonable prompt could be defined as
"autoboot in %d seconds\n",bootdelay
If CONFIG_AUTOBOOT_DELAY_STR or "bootdelaykey" is specified
and this string is received from console input before
autoboot starts booting, U-Boot gives a command prompt. The
U-Boot prompt will time out if CONFIG_BOOT_RETRY_TIME is
used, otherwise it never times out.
If CONFIG_AUTOBOOT_STOP_STR or "bootstopkey" is specified and
this string is received from console input before autoboot
starts booting, U-Boot gives a command prompt. The U-Boot
prompt never times out, even if CONFIG_BOOT_RETRY_TIME is
The string recognition is not very sophisticated. If a
partial match is detected, the first non-matching character
is checked to see if starts a new match. There is no check
for a shorter partial match, so it's best if the first
character of a key string does not appear in the rest of the
The CONFIG_AUTOBOOT_KEYED_CTRLC #define allows for the boot
sequence to be interrupted by ctrl-c, in addition to the
"bootdelaykey" and "bootstopkey". Setting this variable
provides an escape sequence from the limited "password"
(Only effective when CONFIG_BOOT_RETRY_TIME is also set)
After the countdown timed out, the board will be reset to restart