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"Das U-Boot" Source Tree
1.. SPDX-License-Identifier: GPL-2.0+
2
3Boot Count Limit
4================
5
6This is enabled by CONFIG_BOOTCOUNT_LIMIT.
7
8This allows to detect multiple failed attempts to boot Linux.
9
10After a power-on reset, the ``bootcount`` variable will be initialized to 1, and
11each reboot will increment the value by 1.
12
13If, after a reboot, the new value of ``bootcount`` exceeds the value of
14``bootlimit``, then instead of the standard boot action (executing the contents
15of ``bootcmd``), an alternate boot action will be performed, and the contents of
16``altbootcmd`` will be executed.
17
18If the variable ``bootlimit`` is not defined in the environment, the Boot Count
19Limit feature is disabled. If it is enabled, but ``altbootcmd`` is not defined,
20then U-Boot will drop into interactive mode and remain there.
21
22It is the responsibility of some application code (typically a Linux
23application) to reset the variable ``bootcount`` to 0 when the system booted
24successfully, thus allowing for more boot cycles.
25
26CONFIG_BOOTCOUNT_FS
27--------------------
28
29This adds support for maintaining boot count in a file on a filesystem.
30Tested filesystems are FAT and EXT. The file to use is defined by:
31
32CONFIG_SYS_BOOTCOUNT_FS_INTERFACE
33CONFIG_SYS_BOOTCOUNT_FS_DEVPART
34CONFIG_SYS_BOOTCOUNT_FS_NAME
35
36The format of the file is:
37
38.. list-table::
39 :header-rows: 1
40
41 * - type
42 - entry
43 * - u8
44 - magic
45 * - u8
46 - version
47 * - u8
48 - bootcount
49 * - u8
50 - upgrade_available
51
52To prevent unintended usage of ``altbootcmd``, the ``upgrade_available``
53variable is used.
54If ``upgrade_available`` is 0, ``bootcount`` is not saved.
55If ``upgrade_available`` is 1, ``bootcount`` is saved.
56So a userspace application should take care of setting the ``upgrade_available``
57and ``bootcount`` variables to 0, if the system boots successfully.
58This also avoids writing the ``bootcount`` information on all reboots.