pw_cpu_exception

Pigweed’s exception module provides a consistent interface for entering an application’s CPU exception handler. While the actual exception handling behavior is left to an application to implement, this module deals with any architecture-specific actions required before calling the application exception handler. More specifically, the exception module collects CPU state that may otherwise be clobbered by an application’s exception handler.

Setup

An application using this module must connect pw_cpu_exception_Entry() to the platform’s CPU exception handler interrupt so pw_cpu_exception_Entry() is called immediately upon a CPU exception. For specifics on how this may be done, see the backend documentation for your architecture.

Applications must also provide an implementation for pw_cpu_exception_DefaultHandler(). The behavior of this functions is entirely up to the application/project, but some examples are provided below:

  • Enter an infinite loop so the device can be debugged by JTAG.

  • Reset the device.

  • Attempt to handle the exception so execution can continue.

  • Capture and record additional device state and save to flash for a crash report.

  • A combination of the above, using logic that fits the needs of your project.

Module Usage

Basic usage of this module entails applications supplying a definition for pw_cpu_exception_DefaultHandler(). pw_cpu_exception_DefaultHandler() should contain any logic to determine if a exception can be recovered from, as well as necessary actions to properly recover. If the device cannot recover from the exception, the function should not return.

pw_cpu_exception_DefaultHandler() is called indirectly, and may be overridden at runtime via pw_cpu_exception_SetHandler(). The handler can also be reset to point to pw_cpu_exception_DefaultHandler() by calling pw_cpu_exception_RestoreDefaultHandler().

When writing an exception handler, prefer to use the functions provided by this interface rather than relying on the backend implementation of pw_cpu_exception_State. This allows better code portability as it helps prevent an application fault handler from being tied to a single backend.

For example; when logging or dumping CPU state, prefer ToString() or RawFaultingCpuState() over directly accessing members of a pw_cpu_exception_State object.

Some exception handling behavior may require architecture-specific CPU state to attempt to correct a fault. In this situation, the application’s exception handler will be tied to the backend implementation of the CPU exception module.

Backend Expectations

CPU exception backends do not provide an exception handler, but instead provide mechanisms to capture CPU state for use by an application’s exception handler, and allow recovery from CPU exceptions when possible.

  • A backend should provide a definition for the pw_cpu_exception_State struct that provides suitable means to access and modify any captured CPU state.

  • If an application’s exception handler modifies the captured CPU state, the state should be treated as though it were the original state of the CPU when the exception occurred. The backend may need to manually restore some of the modified state to ensure this on exception handler return.

  • A backend should implement the pw_cpu_exception_Entry() function that will call pw_cpu_exception_HandleException() after performing any necessary actions prior to handing control to the application’s exception handler (e.g. capturing necessary CPU state).

Compatibility

Most of the pw_cpu_exception module is C-compatible. The exception to this is the “support” facade and library, which requires C++.

Dependencies

  • pw_span

  • pw_preprocessor