stmhal: Add pyb.fault_debug() function, to control hard-fault behaviour.

This new function controls what happens on a hard-fault:
- debugging disabled: board will do a reset
- debugging enabled: board will print registers and stack and flash LEDs

The default is disabled, ie to do a reset.  This is different to previous
behaviour which flashed the LEDs and waited indefinitely.

docs/library/pyb.rst

@@ -80,6 +80,19 @@ Reset related functions
 
    Activate the bootloader without BOOT\* pins.
 
+.. function:: fault_debug(value)
+
+   Enable or disable hard-fault debugging.  A hard-fault is when there is a fatal
+   error in the underlying system, like an invalid memory access.
+
+   If the `value` argument is `False` then the board will automatically reset if
+   there is a hard fault.
+
+   If `value` is `True` then, when the board has a hard fault, it will print the
+   registers and the stack trace, and then cycle the LEDs indefinitely.
+
+   The default value is disabled, i.e. to automatically reset.
+
 Interrupt related functions
 ---------------------------
 

stmhal/modpyb.c

@@ -38,6 +38,7 @@
 #include "lib/oofatfs/ff.h"
 #include "lib/oofatfs/diskio.h"
 #include "gccollect.h"
+#include "stm32_it.h"
 #include "irq.h"
 #include "systick.h"
 #include "led.h"
@@ -64,6 +65,12 @@
 #include "extmod/vfs.h"
 #include "extmod/utime_mphal.h"
 
+STATIC mp_obj_t pyb_fault_debug(mp_obj_t value) {
+    pyb_hard_fault_debug = mp_obj_is_true(value);
+    return mp_const_none;
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(pyb_fault_debug_obj, pyb_fault_debug);
+
 /// \function millis()
 /// Returns the number of milliseconds since the board was last reset.
 ///
@@ -131,6 +138,8 @@ MP_DECLARE_CONST_FUN_OBJ_KW(pyb_main_obj); // defined in main.c
 STATIC const mp_map_elem_t pyb_module_globals_table[] = {
     { MP_OBJ_NEW_QSTR(MP_QSTR___name__), MP_OBJ_NEW_QSTR(MP_QSTR_pyb) },
 
+    { MP_OBJ_NEW_QSTR(MP_QSTR_fault_debug), (mp_obj_t)&pyb_fault_debug_obj },
+
     { MP_OBJ_NEW_QSTR(MP_QSTR_bootloader), (mp_obj_t)&machine_bootloader_obj },
     { MP_OBJ_NEW_QSTR(MP_QSTR_hard_reset), (mp_obj_t)&machine_reset_obj },
     { MP_OBJ_NEW_QSTR(MP_QSTR_info), (mp_obj_t)&machine_info_obj },

stmhal/stm32_it.c

@@ -71,6 +71,7 @@
 #include STM32_HAL_H
 
 #include "py/obj.h"
+#include "py/mphal.h"
 #include "pendsv.h"
 #include "irq.h"
 #include "pybthread.h"
@@ -95,11 +96,6 @@ extern PCD_HandleTypeDef pcd_hs_handle;
 // Set the following to 1 to get some more information on the Hard Fault
 // More information about decoding the fault registers can be found here:
 // http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0646a/Cihdjcfc.html
-#define REPORT_HARD_FAULT_REGS  0
-
-#if REPORT_HARD_FAULT_REGS
-
-#include "py/mphal.h"
 
 STATIC char *fmt_hex(uint32_t val, char *buf) {
     const char *hexDig = "0123456789abcdef";
@@ -142,7 +138,13 @@ typedef struct {
     uint32_t    r0, r1, r2, r3, r12, lr, pc, xpsr;
 } ExceptionRegisters_t;
 
+int pyb_hard_fault_debug = 0;
+
 void HardFault_C_Handler(ExceptionRegisters_t *regs) {
+    if (!pyb_hard_fault_debug) {
+        NVIC_SystemReset();
+    }
+
     // We need to disable the USB so it doesn't try to write data out on
     // the VCP and then block indefinitely waiting for the buffer to drain.
     pyb_usb_flags = 0;
@@ -209,14 +211,6 @@ void HardFault_Handler(void) {
     " b HardFault_C_Handler \n" // Off to C land
     );
 }
-#else
-void HardFault_Handler(void) {
-    /* Go to infinite loop when Hard Fault exception occurs */
-    while (1) {
-        __fatal_error("HardFault");
-    }
-}
-#endif // REPORT_HARD_FAULT_REGS
 
 /**
   * @brief   This function handles NMI exception.

stmhal/stm32_it.h

@@ -63,6 +63,8 @@
   ******************************************************************************
   */
 
+extern int pyb_hard_fault_debug;
+
 void NMI_Handler(void);
 void HardFault_Handler(void);
 void MemManage_Handler(void);

tests/pyb/pyb1.py

@@ -40,3 +40,6 @@ pyb.sync()
 print(len(pyb.unique_id()))
 
 pyb.wfi()
+
+pyb.fault_debug(True)
+pyb.fault_debug(False)