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Diffstat (limited to 'doc/README.autoboot')
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diff --git a/doc/README.autoboot b/doc/README.autoboot new file mode 100644 index 00000000000..5e9a5e1cf7f --- /dev/null +++ b/doc/README.autoboot @@ -0,0 +1,162 @@ +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. |