esp8266: Let machine.WDT trigger the software WDT if obj is not fed.

This patch allows scripts to have more control over the software WDT.  If
an instance of machine.WDT is created then the underlying OS is prevented
from feeding the software WDT, and it is up to the user script to feed it
instead via WDT.feed().  The timeout for this WDT is currently fixed and
will be between 1.6 and 3.2 seconds.

ports/esp8266/ets_alt_task.c

@@ -109,6 +109,7 @@ bool ets_post(uint8 prio, os_signal_t sig, os_param_t param) {
 }
 
 int ets_loop_iter_disable = 0;
+int ets_loop_dont_feed_sw_wdt = 0;
 
 // to implement a 64-bit wide microsecond counter
 static uint32_t system_time_prev = 0;
@@ -128,10 +129,18 @@ bool ets_loop_iter(void) {
     system_time_prev = system_time_cur;
     ets_intr_unlock();
 
+    // 6 words before pend_flag_noise_check is a variable that is used by
+    // the software WDT.  A 1.6 second period timer will increment this
+    // variable and if it gets to 2 then the SW WDT will trigger a reset.
+    extern uint32_t pend_flag_noise_check;
+    uint32_t *sw_wdt = &pend_flag_noise_check - 6;
+
     //static unsigned cnt;
     bool progress = false;
     for (volatile struct task_entry *t = emu_tasks; t < &emu_tasks[MP_ARRAY_SIZE(emu_tasks)]; t++) {
-        system_soft_wdt_feed();
+        if (!ets_loop_dont_feed_sw_wdt) {
+            system_soft_wdt_feed();
+        }
         ets_intr_lock();
         //printf("etc_loop_iter: "); dump_task(t - emu_tasks + FIRST_PRIO, t);
         if (t->i_get != t->i_put) {
@@ -146,7 +155,12 @@ bool ets_loop_iter(void) {
                 t->i_get = 0;
             }
             //ets_intr_unlock();
+            uint32_t old_sw_wdt = *sw_wdt;
             t->task(&t->queue[idx]);
+            if (ets_loop_dont_feed_sw_wdt) {
+                // Restore previous SW WDT counter, in case task fed/cleared it
+                *sw_wdt = old_sw_wdt;
+            }
             //ets_intr_lock();
             //printf("Done calling task %d\n", t - emu_tasks + FIRST_PRIO);
         }

ports/esp8266/ets_alt_task.h

@@ -2,6 +2,7 @@
 #define MICROPY_INCLUDED_ESP8266_ETS_ALT_TASK_H
 
 extern int ets_loop_iter_disable;
+extern int ets_loop_dont_feed_sw_wdt;
 extern uint32_t system_time_high_word;
 
 bool ets_loop_iter(void);

ports/esp8266/machine_wdt.c

@@ -30,6 +30,7 @@
 #include "py/runtime.h"
 #include "user_interface.h"
 #include "etshal.h"
+#include "ets_alt_task.h"
 
 const mp_obj_type_t esp_wdt_type;
 
@@ -49,6 +50,8 @@ STATIC mp_obj_t machine_wdt_make_new(const mp_obj_type_t *type_in, size_t n_args
 
     switch (id) {
     case 0:
+        ets_loop_dont_feed_sw_wdt = 1;
+        system_soft_wdt_feed();
         return &wdt_default;
     default:
         mp_raise_ValueError(NULL);