doc/README.autoboot

   1 /*
   2  * (C) Copyright 2001
   3  * Dave Ellis, SIXNET, dge@sixnetio.com
   4  *
   5  * SPDX-License-Identifier:     GPL-2.0+
   6  */
   7
   8 Using autoboot configuration options
   9 ====================================
  10
  11 The basic autoboot configuration options are documented in the main
  12 U-Boot README. See it for details. They are:
  13
  14   bootdelay
  15   bootcmd
  16   CONFIG_BOOTDELAY
  17   CONFIG_BOOTCOMMAND
  18
  19 Some additional options that make autoboot safer in a production
  20 product are documented here.
  21
  22 Why use them?
  23 -------------
  24
  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.
  31
  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
  38 why it isn't working.
  39
  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.
  44
  45 These additional configuration options can help provide a system that
  46 boots when it should, but still allows access to U-Boot.
  47
  48 What they do
  49 ------------
  50
  51   CONFIG_BOOT_RETRY_TIME
  52   CONFIG_BOOT_RETRY_MIN
  53
  54   "bootretry" environment variable
  55
  56         These options determine what happens after autoboot is
  57         stopped and U-Boot is waiting for commands.
  58
  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.
  64
  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
  70         timeout.
  71
  72         If CONFIG_BOOT_RETRY_TIME < 0 the feature is there, but
  73         doesn't do anything unless the environment variable
  74         "bootretry" is >= 0.
  75
  76   CONFIG_AUTOBOOT_KEYED
  77   CONFIG_AUTOBOOT_KEYED_CTRLC
  78   CONFIG_AUTOBOOT_PROMPT
  79   CONFIG_AUTOBOOT_DELAY_STR
  80   CONFIG_AUTOBOOT_STOP_STR
  81
  82   "bootdelaykey"  environment variable
  83   "bootstopkey"   environment variable
  84
  85         These options give more control over stopping autoboot. When
  86         they are used a specific character or string is required to
  87         stop or delay autoboot.
  88
  89         Define CONFIG_AUTOBOOT_KEYED (no value required) to enable
  90         this group of options.  CONFIG_AUTOBOOT_DELAY_STR,
  91         CONFIG_AUTOBOOT_STOP_STR or both should be specified (or
  92         specified by the corresponding environment variable),
  93         otherwise there is no way to stop autoboot.
  94
  95         CONFIG_AUTOBOOT_PROMPT is displayed before the boot delay
  96         selected by CONFIG_BOOTDELAY starts. If it is not defined
  97         there is no output indicating that autoboot is in progress.
  98
  99         Note that CONFIG_AUTOBOOT_PROMPT is used as the (only)
 100         argument to a printf() call, so it may contain '%' format
 101         specifications, provided that it also includes, sepearated by
 102         commas exactly like in a printf statement, the required
 103         arguments. It is the responsibility of the user to select only
 104         such arguments that are valid in the given context. A
 105         reasonable prompt could be defined as
 106
 107                 #define CONFIG_AUTOBOOT_PROMPT \
 108                         "autoboot in %d seconds\n",bootdelay
 109
 110         If CONFIG_AUTOBOOT_DELAY_STR or "bootdelaykey" is specified
 111         and this string is received from console input before
 112         autoboot starts booting, U-Boot gives a command prompt. The
 113         U-Boot prompt will time out if CONFIG_BOOT_RETRY_TIME is
 114         used, otherwise it never times out.
 115
 116         If CONFIG_AUTOBOOT_STOP_STR or "bootstopkey" is specified and
 117         this string is received from console input before autoboot
 118         starts booting, U-Boot gives a command prompt. The U-Boot
 119         prompt never times out, even if CONFIG_BOOT_RETRY_TIME is
 120         used.
 121
 122         The string recognition is not very sophisticated. If a
 123         partial match is detected, the first non-matching character
 124         is checked to see if starts a new match. There is no check
 125         for a shorter partial match, so it's best if the first
 126         character of a key string does not appear in the rest of the
 127         string.
 128
 129         The CONFIG_AUTOBOOT_KEYED_CTRLC #define allows for the boot
 130         sequence to be interrupted by ctrl-c, in addition to the
 131         "bootdelaykey" and "bootstopkey". Setting this variable
 132         provides an escape sequence from the limited "password"
 133         strings.
 134
 135   CONFIG_RESET_TO_RETRY
 136
 137         (Only effective when CONFIG_BOOT_RETRY_TIME is also set)
 138         After the countdown timed out, the board will be reset to restart
 139         again.