1.. _module-pw_cpu_exception: 2 3================ 4pw_cpu_exception 5================ 6Pigweed's exception module provides a consistent interface for entering an 7application's CPU exception handler. While the actual exception handling 8behavior is left to an application to implement, this module deals with any 9architecture-specific actions required before calling the application exception 10handler. More specifically, the exception module collects CPU state that may 11otherwise be clobbered by an application's exception handler. 12 13----- 14Setup 15----- 16This module has three facades, each of whose backends are set with a 17different GN variable. 18 19``pw_cpu_exception_ENTRY_BACKEND`` 20================================== 21This is the library that handles early exception entry and prepares any CPU 22state that must be available to the exception handler via the 23pw_cpu_exception_State object. The backend for this facade is 24architecture-specific. 25 26An application using this module **must** connect ``pw_cpu_exception_Entry()`` to 27the platform's CPU exception handler interrupt so ``pw_cpu_exception_Entry()`` is 28called immediately upon a CPU exception. For specifics on how this may be done, 29see the backend documentation for your architecture. 30 31``pw_cpu_exception_HANDLER_BACKEND`` 32==================================== 33This facade is backed by an application-specific handler that determines what to 34do when an exception is encountered. This may be capturing a crash report before 35resetting the device, or in some cases handling the exception to allow execution 36to continue. 37 38Applications must also provide an implementation for 39``pw_cpu_exception_DefaultHandler()``. The behavior of this functions is entirely 40up to the application/project, but some examples are provided below: 41 42 * Enter an infinite loop so the device can be debugged by JTAG. 43 * Reset the device. 44 * Attempt to handle the exception so execution can continue. 45 * Capture and record additional device state and save to flash for a crash 46 report. 47 * A combination of the above, using logic that fits the needs of your project. 48 49``pw_cpu_exception_SUPPORT_BACKEND`` 50==================================== 51This facade provides architecture-independent functions that may be helpful for 52dumping CPU state in various forms. This allows an application to create an 53application-specific handler that is portable across multiple architectures. 54 55Avoiding circular dependencies with ``pw_cpu_exception_ENTRY_BACKEND`` 56====================================================================== 57The entry facade is hard tied to the definition of the 58``pw_cpu_exception_State``, so spliting them into separate facades would require 59extra configurations along with extra compatibility checks to ensure they are 60never mismatched. 61 62In GN, this circular dependency is avoided by collecting the backend's full 63implementation including the entry method through the 64``pw_cpu_exception:entry_impl`` group. When ``pw_cpu_exception_ENTRY_BACKEND`` 65is set, ``$dir_pw_cpu_exception:entry_impl`` must listed in the 66``pw_build_LINK_DEPS`` variable. See :ref:`module-pw_build-link-deps`. 67 68Entry backends must provide their own ``*.impl`` target that collects their 69entry implementation. 70 71------------ 72Module Usage 73------------ 74Basic usage of this module entails applications supplying a definition for 75``pw_cpu_exception_DefaultHandler()``. ``pw_cpu_exception_DefaultHandler()`` should 76contain any logic to determine if a exception can be recovered from, as well as 77necessary actions to properly recover. If the device cannot recover from the 78exception, the function should **not** return. 79 80``pw_cpu_exception_DefaultHandler()`` is called indirectly, and may be overridden 81at runtime via ``pw_cpu_exception_SetHandler()``. The handler can also be reset to 82point to ``pw_cpu_exception_DefaultHandler()`` by calling 83``pw_cpu_exception_RestoreDefaultHandler()``. 84 85When writing an exception handler, prefer to use the functions provided by this 86interface rather than relying on the backend implementation of 87``pw_cpu_exception_State``. This allows better code portability as it helps 88prevent an application fault handler from being tied to a single backend. 89 90For example; when logging or dumping CPU state, prefer ``ToString()`` or 91``RawFaultingCpuState()`` over directly accessing members of a 92``pw_cpu_exception_State`` object. 93 94Some exception handling behavior may require architecture-specific CPU state to 95attempt to correct a fault. In this situation, the application's exception 96handler will be tied to the backend implementation of the CPU exception module. 97 98-------------------- 99Backend Expectations 100-------------------- 101CPU exception backends do not provide an exception handler, but instead provide 102mechanisms to capture CPU state for use by an application's exception handler, 103and allow recovery from CPU exceptions when possible. 104 105 * The entry backend should provide a definition for the 106 ``pw_cpu_exception_State`` object through 107 ``pw_cpu_exception_backend/state.h``. 108 * In GN, the entry backend should also provide a ``.impl`` suffixed form of 109 the entry backend target which collects the actual entry implementation to 110 avoid circular dependencies due to the state definition in the entry backend 111 target. 112 * The entry backend should implement the ``pw_cpu_exception_Entry()`` function 113 that will call ``pw_cpu_exception_HandleException()`` after performing any 114 necessary actions prior to handing control to the application's exception 115 handler (e.g. capturing necessary CPU state). 116 * If an application's exception handler backend modifies the captured CPU 117 state, the state should be treated as though it were the original state of 118 the CPU when the exception occurred. The backend may need to manually 119 restore some of the modified state to ensure this on exception handler 120 return. 121 122------------- 123Compatibility 124------------- 125Most of the pw_cpu_exception module is C-compatible. The exception to this is 126the "support" facade and library, which requires C++. 127 128------------ 129Dependencies 130------------ 131 * ``pw_span`` 132 * ``pw_preprocessor`` 133