🐛 Prevent AVR watchdogpile (#23075)

Marlin/Configuration_adv.h

  */
 //#define SOFT_RESET_VIA_SERIAL         // 'KILL' and '^X' commands will soft-reset the controller
 //#define SOFT_RESET_ON_KILL            // Use a digital button to soft-reset the controller after KILL
+
+// Report uncleaned reset reason from register r2 instead of MCUSR. Supported by Optiboot on AVR.
+//#define OPTIBOOT_RESET_REASON

Marlin/src/HAL/AVR/HAL.cpp

 // Public Variables
 // ------------------------
 
-//uint8_t MCUSR;
+// Don't initialize/override variable (which would happen in .init4)
+uint8_t reset_reason __attribute__((section(".noinit")));
 
 // ------------------------
 // Public functions
 // ------------------------
 
+__attribute__((naked))             // Don't output function pro- and epilogue
+__attribute__((used))              // Output the function, even if "not used"
+__attribute__((section(".init3"))) // Put in an early user definable section
+void HAL_save_reset_reason() {
+  #if ENABLED(OPTIBOOT_RESET_REASON)
+    __asm__ __volatile__(
+      A("STS %0, r2")
+      : "=m"(reset_reason)
+    );
+  #else
+    reset_reason = MCUSR;
+  #endif
+
+  // Clear within 16ms since WDRF bit enables a 16ms watchdog timer -> Boot loop
+  MCUSR = 0;
+  wdt_disable();
+}
+
 void HAL_init() {
   // Init Servo Pins
   #define INIT_SERVO(N) OUT_WRITE(SERVO##N##_PIN, LOW)

Marlin/src/HAL/AVR/HAL.h

 // Public Variables
 // ------------------------
 
-//extern uint8_t MCUSR;
+extern uint8_t reset_reason;
 
 // Serial ports
 #ifdef USBCON
 
 //void _delay_ms(const int delay);
 
-inline void HAL_clear_reset_source() { MCUSR = 0; }
-inline uint8_t HAL_get_reset_source() { return MCUSR; }
+inline void HAL_clear_reset_source() { }
+inline uint8_t HAL_get_reset_source() { return reset_reason; }
 
 void HAL_reboot();
 

Marlin/src/inc/SanityCheck.h

   #error "An encoder button is required or SOFT_RESET_ON_KILL will reset the printer without notice!"
 #endif
 
+// Reset reason for AVR
+#if ENABLED(OPTIBOOT_RESET_REASON) && !defined(__AVR__)
+  #error "OPTIBOOT_RESET_REASON only applies to AVR."
+#endif
+
 /**
  * I2C bus
  */