3 * Dave Ellis, SIXNET, dge@sixnetio.com
5 * SPDX-License-Identifier: GPL-2.0+
8 Using autoboot configuration options
9 ====================================
11 The basic autoboot configuration options are documented in the main
12 U-Boot README. See it for details. They are:
19 Some additional options that make autoboot safer in a production
20 product are documented here.
25 The basic autoboot feature allows a system to automatically boot to
26 the real application (such as Linux) without a user having to enter
27 any commands. If any key is pressed before the boot delay time
28 expires, U-Boot stops the autoboot process, gives a U-Boot prompt
29 and waits forever for a command. That's a good thing if you pressed a
30 key because you wanted to get the prompt.
32 It's not so good if the key press was a stray character on the
33 console serial port, say because a user who knows nothing about
34 U-Boot pressed a key before the system had time to boot. It's even
35 worse on an embedded product that doesn't have a console during
36 normal use. The modem plugged into that console port sends a
37 character at the wrong time and the system hangs, with no clue as to
40 You might want the system to autoboot to recover after an external
41 configuration program stops autoboot. If the configuration program
42 dies or loses its connection (modems can disconnect at the worst
43 time) U-Boot will patiently wait forever for it to finish.
45 These additional configuration options can help provide a system that
46 boots when it should, but still allows access to U-Boot.
51 CONFIG_BOOT_RETRY_TIME
54 "bootretry" environment variable
56 These options determine what happens after autoboot is
57 stopped and U-Boot is waiting for commands.
59 CONFIG_BOOT_RETRY_TIME must be defined to enable the boot
60 retry feature. If the environment variable "bootretry" is
61 found then its value is used, otherwise the retry timeout is
62 CONFIG_BOOT_RETRY_TIME. CONFIG_BOOT_RETRY_MIN is optional and
63 defaults to CONFIG_BOOT_RETRY_TIME. All times are in seconds.
65 If the retry timeout is negative, the U-Boot command prompt
66 never times out. Otherwise it is forced to be at least
67 CONFIG_BOOT_RETRY_MIN seconds. If no valid U-Boot command is
68 entered before the specified time the boot delay sequence is
69 restarted. Each command that U-Boot executes restarts the
72 If CONFIG_BOOT_RETRY_TIME < 0 the feature is there, but
73 doesn't do anything unless the environment variable
77 CONFIG_AUTOBOOT_KEYED_CTRLC
78 CONFIG_AUTOBOOT_PROMPT
79 CONFIG_AUTOBOOT_DELAY_STR
80 CONFIG_AUTOBOOT_STOP_STR
81 CONFIG_AUTOBOOT_DELAY_STR2
82 CONFIG_AUTOBOOT_STOP_STR2
84 "bootdelaykey" environment variable
85 "bootstopkey" environment variable
86 "bootdelaykey2" environment variable
87 "bootstopkey2" environment variable
89 These options give more control over stopping autoboot. When
90 they are used a specific character or string is required to
91 stop or delay autoboot.
93 Define CONFIG_AUTOBOOT_KEYED (no value required) to enable
94 this group of options. CONFIG_AUTOBOOT_DELAY_STR,
95 CONFIG_AUTOBOOT_STOP_STR or both should be specified (or
96 specified by the corresponding environment variable),
97 otherwise there is no way to stop autoboot.
99 CONFIG_AUTOBOOT_PROMPT is displayed before the boot delay
100 selected by CONFIG_BOOTDELAY starts. If it is not defined
101 there is no output indicating that autoboot is in progress.
103 Note that CONFIG_AUTOBOOT_PROMPT is used as the (only)
104 argument to a printf() call, so it may contain '%' format
105 specifications, provided that it also includes, sepearated by
106 commas exactly like in a printf statement, the required
107 arguments. It is the responsibility of the user to select only
108 such arguments that are valid in the given context. A
109 reasonable prompt could be defined as
111 #define CONFIG_AUTOBOOT_PROMPT \
112 "autoboot in %d seconds\n",bootdelay
114 If CONFIG_AUTOBOOT_DELAY_STR or "bootdelaykey" is specified
115 and this string is received from console input before
116 autoboot starts booting, U-Boot gives a command prompt. The
117 U-Boot prompt will time out if CONFIG_BOOT_RETRY_TIME is
118 used, otherwise it never times out.
120 If CONFIG_AUTOBOOT_STOP_STR or "bootstopkey" is specified and
121 this string is received from console input before autoboot
122 starts booting, U-Boot gives a command prompt. The U-Boot
123 prompt never times out, even if CONFIG_BOOT_RETRY_TIME is
126 The string recognition is not very sophisticated. If a
127 partial match is detected, the first non-matching character
128 is checked to see if starts a new match. There is no check
129 for a shorter partial match, so it's best if the first
130 character of a key string does not appear in the rest of the
133 Using the CONFIG_AUTOBOOT_DELAY_STR2 #define or the
134 "bootdelaykey2" environment variable and/or the
135 CONFIG_AUTOBOOT_STOP_STR2 #define or the "bootstopkey"
136 environment variable you can specify a second, alternate
137 string (which allows you to have two "password" strings).
139 The CONFIG_AUTOBOOT_KEYED_CTRLC #define allows for the boot
140 sequence to be interrupted by ctrl-c, in addition to the
141 "bootdelaykey" and "bootstopkey". Setting this variable
142 provides an escape sequence from the limited "password"
146 CONFIG_ZERO_BOOTDELAY_CHECK
148 If this option is defined, you can stop the autoboot process
149 by hitting a key even in that case when "bootdelay" has been
150 set to 0. You can set "bootdelay" to a negative value to
151 prevent the check for console input.
153 CONFIG_RESET_TO_RETRY
155 (Only effective when CONFIG_BOOT_RETRY_TIME is also set)
156 After the countdown timed out, the board will be reset to restart