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52 lines
2.1 KiB
ReStructuredText
52 lines
2.1 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0+
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Environment implementation
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==========================
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See :doc:`../usage/environment` for usage information.
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Callback functions for environment variables
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--------------------------------------------
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For some environment variables, the behavior of u-boot needs to change
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when their values are changed. This functionality allows functions to
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be associated with arbitrary variables. On creation, overwrite, or
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deletion, the callback will provide the opportunity for some side
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effect to happen or for the change to be rejected.
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The callbacks are named and associated with a function using the
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U_BOOT_ENV_CALLBACK macro in your board or driver code.
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These callbacks are associated with variables in one of two ways. The
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static list can be added to by defining CONFIG_ENV_CALLBACK_LIST_STATIC
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in the board configuration to a string that defines a list of
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associations. The list must be in the following format::
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entry = variable_name[:callback_name]
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list = entry[,list]
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If the callback name is not specified, then the callback is deleted.
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Spaces are also allowed anywhere in the list.
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Callbacks can also be associated by defining the ".callbacks" variable
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with the same list format above. Any association in ".callbacks" will
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override any association in the static list. You can define
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CONFIG_ENV_CALLBACK_LIST_DEFAULT to a list (string) to define the
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".callbacks" environment variable in the default or embedded environment.
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If CONFIG_REGEX is defined, the variable_name above is evaluated as a
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regular expression. This allows multiple variables to be connected to
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the same callback without explicitly listing them all out.
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The signature of the callback functions is::
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int callback(const char *name, const char *value, enum env_op op, int flags)
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* name - changed environment variable
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* value - new value of the environment variable
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* op - operation (create, overwrite, or delete)
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* flags - attributes of the environment variable change, see flags H_* in
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include/search.h
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The return value is 0 if the variable change is accepted and 1 otherwise.
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