[PR] xtensa: set PS.EXCM initial value to 1 while new thread created [nuttx]

[via GitHub]

sdc-g opened a new pull request, #15985:
URL: https://github.com/apache/nuttx/pull/15985

   ## Summary
   
   To avoid level-1 interrupt break retrieve PC/A0/SP/A2 register, PS.EXCM set 
to 1 by CPU HW while handling exception/interrupt.
   
   But if context switching happens and new thread created, the thread initial 
value of PS.EXCM is used.
   
   Same behevior as ESP-IDF code:
   https://github.com/espressif/esp-idf/blob/master/
   components/freertos/FreeRTOS-Kernel/portable/xtensa/port.c#L366
   
   ## Impact
   
   PS.EXCM initial value while new thread created.
   
   ## Testing
   
   tested by internal test case as below code
   
   ```
   static void *BusyThread(void *arg) {
     (void)arg;  // Avoid compiler warning
     while (1) {
     }
     return NULL;
   }
   
   
//-----------------------------------------------------------------------------
   static void *DebugTestThread(void *arg) {
     printf("Test Thread\n");
     return NULL;
   }
   
   
//-----------------------------------------------------------------------------
   static void *TimeSliceThread(void *arg) {
     printf("Start TimeSliceThread\n");
     while (1) {
       usleep(500 * 1000);
     }
     return NULL;
   }
   
   
//-----------------------------------------------------------------------------
   int main(int argc, char *argv[]) {
   
     // Start priority=120 thread to induce Time Slice
     pthread_t thread;
     pthread_attr_t attr;
     struct sched_param param;
   
     pthread_attr_init(&attr);
     pthread_attr_getschedparam(&attr, &param);
     param.sched_priority = 120;
     pthread_attr_setschedparam(&attr, &param);
     pthread_create(&thread, &attr, TimeSliceThread, NULL);
   
     printf("\n\n******** Test Start ********\n");
   
     pthread_t busy_thread;
     pthread_create(&busy_thread, NULL, BusyThread, NULL);
   
     pthread_t test_thread;
     while(1) {
       pthread_create(&test_thread, NULL, DebugTestThread, NULL);
       pthread_join(test_thread, NULL);
     }
   }
   ```
   
   


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