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84 lines
3.9 KiB
84 lines
3.9 KiB
4 months ago
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.. _module-pw_cpu_exception:
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----------------
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pw_cpu_exception
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----------------
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Pigweed's exception module provides a consistent interface for entering an
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application's CPU exception handler. While the actual exception handling
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behavior is left to an application to implement, this module deals with any
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architecture-specific actions required before calling the application exception
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handler. More specifically, the exception module collects CPU state that may
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otherwise be clobbered by an application's exception handler.
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Setup
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=====
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An application using this module **must** connect ``pw_cpu_exception_Entry()`` to
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the platform's CPU exception handler interrupt so ``pw_cpu_exception_Entry()`` is
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called immediately upon a CPU exception. For specifics on how this may be done,
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see the backend documentation for your architecture.
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Applications must also provide an implementation for
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``pw_cpu_exception_DefaultHandler()``. The behavior of this functions is entirely
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up to the application/project, but some examples are provided below:
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* Enter an infinite loop so the device can be debugged by JTAG.
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* Reset the device.
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* Attempt to handle the exception so execution can continue.
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* Capture and record additional device state and save to flash for a crash
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report.
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* A combination of the above, using logic that fits the needs of your project.
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Module Usage
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============
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Basic usage of this module entails applications supplying a definition for
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``pw_cpu_exception_DefaultHandler()``. ``pw_cpu_exception_DefaultHandler()`` should
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contain any logic to determine if a exception can be recovered from, as well as
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necessary actions to properly recover. If the device cannot recover from the
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exception, the function should **not** return.
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``pw_cpu_exception_DefaultHandler()`` is called indirectly, and may be overridden
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at runtime via ``pw_cpu_exception_SetHandler()``. The handler can also be reset to
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point to ``pw_cpu_exception_DefaultHandler()`` by calling
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``pw_cpu_exception_RestoreDefaultHandler()``.
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When writing an exception handler, prefer to use the functions provided by this
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interface rather than relying on the backend implementation of
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``pw_cpu_exception_State``. This allows better code portability as it helps
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prevent an application fault handler from being tied to a single backend.
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For example; when logging or dumping CPU state, prefer ``ToString()`` or
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``RawFaultingCpuState()`` over directly accessing members of a
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``pw_cpu_exception_State`` object.
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Some exception handling behavior may require architecture-specific CPU state to
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attempt to correct a fault. In this situation, the application's exception
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handler will be tied to the backend implementation of the CPU exception module.
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Backend Expectations
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====================
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CPU exception backends do not provide an exception handler, but instead provide
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mechanisms to capture CPU state for use by an application's exception handler,
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and allow recovery from CPU exceptions when possible.
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* A backend should provide a definition for the ``pw_cpu_exception_State``
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struct that provides suitable means to access and modify any captured CPU
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state.
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* If an application's exception handler modifies the captured CPU state, the
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state should be treated as though it were the original state of the CPU when
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the exception occurred. The backend may need to manually restore some of the
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modified state to ensure this on exception handler return.
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* A backend should implement the ``pw_cpu_exception_Entry()`` function that will
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call ``pw_cpu_exception_HandleException()`` after performing any necessary
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actions prior to handing control to the application's exception handler
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(e.g. capturing necessary CPU state).
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Compatibility
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=============
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Most of the pw_cpu_exception module is C-compatible. The exception to this is
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the "support" facade and library, which requires C++.
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Dependencies
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============
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* ``pw_span``
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* ``pw_preprocessor``
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