xref: /openbmc/u-boot/doc/README.autoboot (revision cc1c8a136f9813d0a9cb9a94d2f12206873ddc4e)

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Using autoboot configuration options
====================================

The basic autoboot configuration options are documented in the main
U-Boot README. See it for details. They are:

  bootdelay
  bootcmd
  CONFIG_BOOTDELAY
  CONFIG_BOOTCOMMAND

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
------------

  CONFIG_BOOT_RETRY_TIME
  CONFIG_BOOT_RETRY_MIN

  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
        CONFIG_BOOT_RETRY_TIME. CONFIG_BOOT_RETRY_MIN is optional and
        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
        timeout.

        If CONFIG_BOOT_RETRY_TIME < 0 the feature is there, but
        doesn't do anything unless the environment variable
        'bootretry' is >= 0.

  CONFIG_AUTOBOOT_KEYED
  CONFIG_AUTOBOOT_PROMPT
  CONFIG_AUTOBOOT_DELAY_STR
  CONFIG_AUTOBOOT_STOP_STR
  CONFIG_AUTOBOOT_DELAY_STR2
  CONFIG_AUTOBOOT_STOP_STR2

  bootdelaykey	environment variable
  bootstopkey	environment variable
  bootdelaykey2	environment variable
  bootstopkey2	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.
        If "%d" is included, it is replaced by the number of seconds
        remaining before autoboot will start, but it does not count
        down the seconds. "autoboot in %d seconds\n" is a reasonable
        prompt.

        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
        used.

        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
        string.

        Using the CONFIG_AUTOBOOT_DELAY_STR2 /  bootdelaykey2  and/or
        CONFIG_AUTOBOOT_STOP_STR2   /   bootstopkey  #defines  and/or
        environment variables you can  specify  a  second,  alternate
        string (which allows you to haw two "password" strings).

  CONFIG_ZERO_BOOTDELAY_CHECK

        If this option is defined, you can stop the autoboot process
        by hitting a key even in that case when "bootdelay" has been
        set to 0. You can set "bootdelay" to a negative value to
        prevent the check for console input.

  CONFIG_RESET_TO_RETRY

        (Only effective when CONFIG_BOOT_RETRY_TIME is also set)
        After the countdown timed out, the board will be reset to restart
        again.