Simplify interrupt and RTOS timer tick handlers

RTOS Timer tick handler is now the same as any other ISR.

This causes a few subtle behaviour changes that seem OK but are worth noting:

* RTOS tick handler sdk__xt_timer_int() is now called from one stack
  frame deeper (inside _xt_isr_handler()), whereas before it was called
  from the level above in UserHandleInterrupt. I can't see any way that
  the extra ~40 bytes of stack use here hurt, though.

* sdk__xt_timer_int() was previous called after all other interrupts
  flagged in the handler, now it's called before the TIMER FRC1 & FRC2
  handlers. The tick handler doesn't appear to do anything particularly
  timing intensive, though.

* GPIO interrupt (value 3) is now lower priority than the SPI
  interrupt (value 2), whereas before it would have been called before
  SPI if both interrupts triggered at once.

FreeRTOS/Source/portable/esp8266/port.c

@@ -198,6 +198,8 @@ portBASE_TYPE xPortStartScheduler( void )
     }
 
     /* Initialize system tick timer interrupt and schedule the first tick. */
+    _xt_isr_attach(INUM_TICK, sdk__xt_timer_int);
+    _xt_isr_unmask(BIT(INUM_TICK));
     sdk__xt_tick_timer_init();
 
     vTaskSwitchContext();

core/esp_interrupts.c

@@ -14,47 +14,26 @@ void IRAM _xt_isr_attach(uint8_t i, _xt_isr func)
     isr[i] = func;
 }
 
-/* This ISR handler is taken directly from the FreeRTOS port and
-   probably could use a cleanup.
+/* Generic ISR handler.
+
+   Handles all flags set for interrupts in 'intset'.
 */
-uint16_t IRAM _xt_isr_handler(uint16_t i)
+uint16_t IRAM _xt_isr_handler(uint16_t intset)
 {
-    uint8_t index;
-
-    /* I think this is implementing some kind of interrupt priority or
-       short-circuiting an expensive ffs for most common interrupts - ie
-       WDT And GPIO are common or high priority, then remaining flags.
-    */
-    if (i & (1 << INUM_WDT)) {
-	index = INUM_WDT;
-    }
-    else if (i & (1 << INUM_GPIO)) {
-	index = INUM_GPIO;
-    }else {
-	index = __builtin_ffs(i) - 1;
-
-	if (index == INUM_MAX) {
-	    /* I don't understand what happens here. INUM_MAX is not
-	       the highest interrupt number listed (and the isr array
-	       has 16 entries).
-
-	       Clearing that flag and then setting index to
-	       __builtin_ffs(i)-1 may result in index == 255 if no
-	       higher flags are set, unless this is guarded against
-	       somehow by the caller?
-
-	       I also don't understand why the code is written like
-	       this in esp_iot_rtos_sdk instead of just putting the i
-	       &= line near the top... Probably no good reason?
-	    */
-	    i &= ~(1 << INUM_MAX);
-	    index = __builtin_ffs(i) - 1;
-	}
+    /* WDT has highest priority (occasional WDT resets otherwise) */
+    if(intset & BIT(INUM_WDT)) {
+        _xt_clear_ints(BIT(INUM_WDT));
+        isr[INUM_WDT]();
+        intset -= BIT(INUM_WDT);
     }
 
-    _xt_clear_ints(1<<index);
-
+    while(intset) {
+        uint8_t index = __builtin_ffs(intset) - 1;
+        uint16_t mask = BIT(index);
+        _xt_clear_ints(mask);
         isr[index]();
+        intset -= mask;
+    }
 
-    return i & ~(1 << index);
+    return 0;
 }

core/exception_vectors.S

@@ -469,7 +469,6 @@ CallNMIExceptionHandler:
  * LoadStoreCause. */
 
         .literal_position
-
         .balign  4
 UserExceptionHandler:
         .type   UserExceptionHandler, @function
@@ -490,46 +489,28 @@ UserExceptionHandler:
         wsr     a0, ps
         rsync
         rsr     a2, exccause
-        beqi    a2, CAUSE_LVL1INT, UserHandleInterrupt
-        /* Any UserException cause other than level 1 interrupt should panic */
-UserFailOtherExceptionCause:
-        break   1, 1
-        call0   sdk_user_fatal_exception_handler
-UserHandleInterrupt:
+        /* Any UserException cause other than a level 1 interrupt is fatal */
+        bnei    a2, CAUSE_LVL1INT, .UserFailOtherExceptionCause
+.UserHandleInterrupt:
         rsil    a0, 1
         rsr     a2, intenable
         rsr     a3, interrupt
         movi    a4, 0x3fff
         and     a2, a2, a3
         and     a2, a2, a4       # a2 = 0x3FFF & INTENABLE & INTERRUPT
-UserHandleTimer:
-        movi    a3, 0xffbf
-        and     a3, a2, a3      # a3 = a2 with bit 6 cleared
-        bnez    a3, UserTimerDone   # If any non-timer interrupt bits set
-        movi    a3, 0x40
-        sub     a12, a2, a3     # a12 = a2 - 0x40 -- Will be zero if bit 6 set
-        call0   sdk__xt_timer_int  # tick timer interrupt
-        mov     a2, a12         # restore a2 from a12, ie zero
-        beqz    a2, UserIntDone
-UserTimerDone:
         call0   _xt_isr_handler
-        bnez    a2, UserHandleTimer
-UserIntDone:
-        beqz    a2, UserIntExit
-        /* FIXME: this code will never be reached */
+        j       sdk__xt_int_exit # once finished, jumps to _xt_user_exit via stack
+
+        .literal_position
+.UserFailOtherExceptionCause:
         break   1, 1
         call0   sdk_user_fatal_exception_handler
-UserIntExit:
-        call0   sdk__xt_int_exit  # jumps to _xt_user_exit. Never returns here
 
-        .section .text
-
-/* _xt_user_exit is used to exit interrupt context. */
-/* TODO: Find a better place for this to live. */
+/* _xt_user_exit is pushed onto the stack as part of the user exception handler,
+   restores same set registers which were saved there and returns from exception */
 _xt_user_exit:
         .global _xt_user_exit
         .type _xt_user_exit, @function
-
         l32i    a0, sp, 0x8
         wsr     a0, ps
         l32i    a0, sp, 0x4

core/include/esp/interrupts.h

@@ -20,7 +20,7 @@ typedef enum {
     INUM_SPI = 2,
     INUM_GPIO = 4,
     INUM_UART = 5,
-    INUM_MAX = 6, /* in some places this is documented as timer0 CCOMPARE0 interrupt */
+    INUM_TICK = 6, /* RTOS timer tick, possibly xtensa CPU CCOMPARE0(?) */
     INUM_SOFT = 7,
     INUM_WDT = 8,
     INUM_TIMER_FRC1 = 9,
@@ -35,6 +35,7 @@ typedef enum {
 void sdk__xt_int_exit (void);
 void _xt_user_exit (void);
 void sdk__xt_tick_timer_init (void);
+void sdk__xt_timer_int(void);
 void sdk__xt_timer_int1(void);
 
 INLINED uint32_t _xt_get_intlevel(void)