doc/README.autoboot

SPDX-License-Identifier: GPL-2.0+
/*
 * (C) Copyright 2001
 * Dave Ellis, SIXNET, dge@sixnetio.com
 *
 */

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_KEYED_CTRLC
  CONFIG_AUTOBOOT_PROMPT
  CONFIG_AUTOBOOT_DELAY_STR
  CONFIG_AUTOBOOT_STOP_STR

  "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

		#define CONFIG_AUTOBOOT_PROMPT \
			"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
	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.

	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"
	strings.

  CONFIG_AUTOBOOT_ENCRYPTION

  "bootstopkeysha256"	environment variable

	- Hash value of the input which unlocks the device and
	  stops autoboot.

	This option allows a string to be entered into U-Boot to stop the
	autoboot. The string itself is hashed and compared against the hash
	in the environment variable 'bootstopkeysha256'. If it matches then
	boot stops and a command-line prompt is presented.

	This provides a way to ship a secure production device which can also
	be accessed at the U-Boot command line.

  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.

  CONFIG_AUTOBOOT_USE_MENUKEY
  CONFIG_AUTOBOOT_MENUKEY

	If this key is pressed to stop autoboot, then the commands in the
	environment variable 'menucmd' will be executed before boot starts.
	For example, 33 means "!" in ASCII, so pressing ! at boot would take
	this action.