add watchdog_enable_caused_reboot (minor compatibility issue) (#594)

add watchdog_enable_caused_reboot. Note there is a small backwards incompatibility in so far as watchdog_enable sets scratch[4] to a magic value not 0 now, however this behavior was not documented before.

src/rp2_common/hardware_watchdog/include/hardware/watchdog.h

@@ -8,6 +8,7 @@
 #define _HARDWARE_WATCHDOG_H
 
 #include "pico.h"
+#include "hardware/structs/watchdog.h"
 
 /** \file hardware/watchdog.h
  *  \defgroup hardware_watchdog hardware_watchdog
@@ -66,6 +67,11 @@ void watchdog_update(void);
  *
  * By default the SDK assumes a 12MHz XOSC and sets the \ref watchdog_start_tick appropriately.
  *
+ * This method sets a marker in the watchdog scratch register 4 that is checked by \ref watchdog_enable_caused_reboot.
+ * If the device is subsequently reset via a call to watchdog_reboot (including for example by dragging a UF2
+ * onto the RPI-RP2), then this value will be cleared, and so \ref watchdog_enable_caused_reboot will
+ * return false.
+ *
  * \param delay_ms Number of milliseconds before watchdog will reboot without watchdog_update being called. Maximum of 0x7fffff, which is approximately 8.3 seconds
  * \param pause_on_debug If the watchdog should be paused when the debugger is stepping through code
  */
@@ -80,6 +86,23 @@ void watchdog_enable(uint32_t delay_ms, bool pause_on_debug);
  */
 bool watchdog_caused_reboot(void);
 
+/**
+ * \brief Did watchdog_enable cause the last reboot?
+ * \ingroup hardware_watchdog
+ *
+ * Perform additional checking along with \ref watchdog_caused_reboot to determine if a watchdog timeout initiated by
+ * \ref watchdog_enable caused the last reboot.
+ *
+ * This method checks for a special value in watchdog scratch register 4 placed there by \ref watchdog_enable.
+ * This would not be present if a watchdog reset is initiated by \ref watchdog_reboot or by the RP2040 bootrom
+ * (e.g. dragging a UF2 onto the RPI-RP2 drive).
+ *
+ * @return true if the watchdog timer or a watchdog force caused (see \reg watchdog_caused_reboot) the last reboot
+ *              and the watchdog reboot happened after \ref watchdog_enable was called
+ * @return false there has been no watchdog reboot since run has been
+ */
+bool watchdog_enable_caused_reboot(void);
+
 /**
  * @brief Returns the number of microseconds before the watchdog will reboot the chip.
  * \ingroup hardware_watchdog

src/rp2_common/hardware_watchdog/watchdog.c

@@ -65,10 +65,12 @@ void _watchdog_enable(uint32_t delay_ms, bool pause_on_debug) {
 }
 // end::watchdog_enable[]
 
+#define WATCHDOG_NON_REBOOT_MAGIC 0x6ab73121
+
 void watchdog_enable(uint32_t delay_ms, bool pause_on_debug) {
-    // This watchdog enable doesn't reboot so clear scratch register
+    // update scratch[4] to distinguish from magic used for reboot to specific address, or 0 used to reboot
-    // with magic word to jump into code
+    // into regular flash path
-    watchdog_hw->scratch[4] = 0;
+    watchdog_hw->scratch[4] = WATCHDOG_NON_REBOOT_MAGIC;
     _watchdog_enable(delay_ms, pause_on_debug);
 }
 
@@ -98,3 +100,7 @@ bool watchdog_caused_reboot(void) {
     // If any reason bits are set this is true
     return watchdog_hw->reason;
 }
+
+bool watchdog_enable_caused_reboot(void) {
+    return watchdog_hw->reason && watchdog_hw->scratch[4] == WATCHDOG_NON_REBOOT_MAGIC;
+}