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Diffstat (limited to 'Documentation/user/bootchooser.rst')
| -rw-r--r-- | Documentation/user/bootchooser.rst | 94 |
1 files changed, 72 insertions, 22 deletions
diff --git a/Documentation/user/bootchooser.rst b/Documentation/user/bootchooser.rst index 05725d8f21..1680ea78ed 100644 --- a/Documentation/user/bootchooser.rst +++ b/Documentation/user/bootchooser.rst @@ -3,7 +3,7 @@ Barebox Bootchooser =================== -In many cases embedded systems are layed out redundantly with multiple +In many cases embedded systems are laid out redundantly with multiple kernels and multiple root file systems. The *bootchooser* framework provides the building blocks to model different use cases without the need to start from scratch over and over again. @@ -11,6 +11,22 @@ from scratch over and over again. The *bootchooser* works on abstract boot targets, each with a set of properties and implements an algorithm which selects the highest priority target to boot. +To make the *bootchooser* work requires a fixed set of configuration parameters +and a storage backend for saving status information. +Currently supported storage backends are either nv variables or the +barebox *state* framework. + +The *Bootchooser* itself is executed as a normal barebox boot target, i.e. one +can start it via:: + + boot bootchooser + +or by e.g. setting ``boot.default`` to ``bootchooser``. + +.. note:: As ``boot.default`` accepts multiple values, it can also be used to + specify a fallback boot target in case the bootchooser fails booting, e.g. + ``bootchooser recovery``. + Bootchooser Targets ------------------- @@ -56,25 +72,58 @@ When booting, *bootchooser* starts the boot target with the highest ``priority`` has a non-zero ``remaining_attempts`` counter. With every start of a boot target the ``remaining_attempts`` counter of this boot target is decremented by one. This means every boot target's ``remaining_attempts`` counter reaches zero sooner or later and -the boot target won't be booted anymore. To prevent that, the ``remaining_attempts`` -counter must be reset to its default. There are different flags in the -*bootchooser* which control resetting the ``remaining_attempts`` counter, +the boot target won't be booted anymore. +This behavior assures that one can retry booting a target a limited number of +times to handle temporary issues (such as power outage) and optionally allows +booting a fallback in case of a permanent failure. +To indicate a successful boot, one must explicitly reset the remaining +attempts counter. See `Marking a Boot as Successful`_. + +The bootchooser algorithm aborts when all enabled targets (priority > 0) have +no remaining attempts left. + +To prevent ending up in an unbootable system after a number of failed boot +attempts, there is a also a built-in mechanism to reset the counters to their defaults, controlled by the ``global.bootchooser.reset_attempts`` variable. It holds a -list of space separated flags. Possible values are: +list of space-separated flags. Possible values are: - empty: counters will never be reset -- ``power-on``: The ``remaining_attempts`` counters of all enabled boot targets are reset - after a ``power-on`` reset (``$global.system.reset="POR"``). This means after a power - cycle all boot targets will be tried again for the configured number of retries. -- ``all-zero``: The ``remaining_attempts`` counters of all enabled boot targets are - reset when none of them has any ``remaining_attempts`` left. - -Additionally the ``remaining_attempts`` counter can be reset manually using the -:ref:`bootchoser command <command_bootchooser>`. This allows for custom conditions -under which a system is marked as good. -In case only the booted system itself knows when it is in a good state, the -barebox-state tool from the dt-utils_ package can be used to reset the -``remaining_attempts`` counter from the running system. +- ``power-on``: When the bootchooser starts and a power-on reset + (``$global.system.reset="POR"``) is detected, the ``remaining_attempts`` + counters of all enabled targets are reset to their defaults. + This means after a power cycle all boot targets will be tried again for the configured number of retries. +- ``all-zero``: When the bootchooser starts and the ``remaining_attempts`` + counters of all enabled targets are zero, the ``remaining_attempts`` + counters of all enabled targets are reset to their defaults. + +If ``global.bootchooser.retry`` is enabled (set to ``1``), the bootchooser +algorithm will iterate through all valid boot targets (and decrease their +counters) until one succeeds or none is left. +If it is disabled only one attempt will be made for each bootchooser call. + +Marking a Boot as Successful +############################ + +While the bootchooser algorithm handles attempts decrementation, retries and +selection of the right boot target itself, it cannot decide if the system +booted successfully on its own. + +In case only the booted system itself knows when it is in a good state, +it can report this to the bootchooser from Linux userspace using the +*barebox-state* tool from the dt-utils_ package.:: + + barebox-state -s bootstate.<target>.remaining_attemps <reset-value> + +If instead the bootchooser can detect a failed boot itself using the +:ref:`reset reason <reset_reason>` (WDG), one can mark the boot successful +using the barebox :ref:`bootchoser command <command_bootchooser>`:: + + bootchooser -s + +to mark the last boot successful. +This will reset the ``remaining_attempts`` counter of the *last chosen* slot to +its default value (``reset_attempts``). + .. _dt-utils: https://git.pengutronix.de/cgit/tools/dt-utils @@ -97,7 +146,7 @@ options not specific to any boot target. ``global.bootchooser.reset_attempts`` Already described in :ref:`Bootchooser Algorithm <bootchooser,algorithm>` ``global.bootchooser.reset_priorities`` - A space separated list of events that cause *bootchooser* to reset the priorities of + A space-separated list of events that cause *bootchooser* to reset the priorities of all boot targets. Possible values: * empty: priorities will never be reset @@ -108,10 +157,11 @@ options not specific to any boot target. Otherwise the ``boot`` command will return with an error after the first failed boot target. ``global.bootchooser.state_prefix`` - Variable prefix when *bootchooser* is used with the *state* framework as backend - for storing run-time data, see below. + If set, this makes *bootchooser* use the *state* framework as backend for + storing run-time data and defines the name of the state instance to use, see + :ref:`below <bootchooser,state_framework>`. ``global.bootchooser.targets`` - Space separated list of boot targets that are used. For each entry in the list + Space-separated list of boot targets that are used. For each entry in the list a corresponding set of ``global.bootchooser.<targetname>.<variablename>`` variables must exist. ``global.bootchooser.last_chosen`` @@ -125,7 +175,7 @@ Setup Example We want to set up a redundant machine with two bootable systems within one shared memory, here a NAND type flash memory with a UBI partition. We have a 512 MiB NAND type flash, to be used only for the root filesystem. The devicetree doesn't -define any partition, because we want to run one UBI partition with two volumens +define any partition, because we want to run one UBI partition with two volumes for the redundant root filesystems on this flash memory. .. code-block:: text |