Improve interrupt handling response time

Change the way ISRs are stored. Store now in a list on a ‘first come
first served basis’, the first interrupt that gets attached will be
served first. This improves the speed of the ISR calling from about 6us
to 2us for a single interrupt on extint14

See #200

cores/arduino/WInterrupts.c

@@ -21,12 +21,17 @@
 
 #include <string.h>
 
-static voidFuncPtr callbacksInt[EXTERNAL_NUM_INTERRUPTS];
+static voidFuncPtr ISRcallback[EXTERNAL_NUM_INTERRUPTS];
+static EExt_Interrupts ISRlist[EXTERNAL_NUM_INTERRUPTS];
+static uint32_t nints; // Stores total number of attached interrupts
+
 
 /* Configure I/O interrupt sources */
 static void __initialize()
 {
-  memset(callbacksInt, 0, sizeof(callbacksInt));
+  memset(ISRlist, 0, sizeof(ISRlist));
+  memset(ISRcallback, 0, sizeof(ISRcallback));
+  nints = 0;
 
   NVIC_DisableIRQ(EIC_IRQn);
   NVIC_ClearPendingIRQ(EIC_IRQn);
@@ -76,42 +81,65 @@ void attachInterrupt(uint32_t pin, voidFuncPtr callback, uint32_t mode)
   // Assign pin to EIC
   pinPeripheral(pin, PIO_EXTINT);
 
-  // Assign callback to interrupt
-  callbacksInt[in] = callback;
+  // Only store when there is really an ISR to call.
+  // This allow for calling attachInterrupt(pin, NULL, mode), we set up all needed register
+  // but won't service the interrupt, this way we also don't need to check it inside the ISR.
+  if (callback)
+  {
+    // Store interrupts to service in order of when they were attached
+    // to allow for first come first serve handler
+    uint32_t current=0;
+
+    // Check if we already have this interrupt
+    int id = -1;
+    for (uint32_t i=0; i<nints; i++) {
+      if (ISRlist[i] == in) id = in;
+    }
 
-  // Look for right CONFIG register to be addressed
-  if (in > EXTERNAL_INT_7) {
-    config = 1;
-  } else {
-    config = 0;
-  }
+    if (id == -1) {
+      // Need to make a new entry
+      current = nints;
+      nints++;
+    } else {
+      // We already have an entry for this pin
+      current = id;
+    }
+    ISRlist[current] = in; // List with nr of interrupt in order of when they were attached
+    ISRcallback[current] = callback; // List of callback adresses
+
+    // Look for right CONFIG register to be addressed
+    if (in > EXTERNAL_INT_7) {
+      config = 1;
+    } else {
+      config = 0;
+    }
 
-  // Configure the interrupt mode
-  pos = (in - (8 * config)) << 2;
-  EIC->CONFIG[config].reg &=~ (EIC_CONFIG_SENSE0_Msk << pos);//reset sense mode, important when changing trigger mode during runtime
-  switch (mode)
-  {
-    case LOW:
-      EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_LOW_Val << pos;
-      break;
+    // Configure the interrupt mode
+    pos = (in - (8 * config)) << 2;
+    EIC->CONFIG[config].reg &=~ (EIC_CONFIG_SENSE0_Msk << pos); // Reset sense mode, important when changing trigger mode during runtime
+    switch (mode)
+    {
+      case LOW:
+        EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_LOW_Val << pos;
+        break;
 
-    case HIGH:
-      EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_HIGH_Val << pos;
-      break;
+      case HIGH:
+        EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_HIGH_Val << pos;
+        break;
 
-    case CHANGE:
-      EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_BOTH_Val << pos;
-      break;
+      case CHANGE:
+        EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_BOTH_Val << pos;
+        break;
 
-    case FALLING:
-      EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_FALL_Val << pos;
-      break;
+      case FALLING:
+        EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_FALL_Val << pos;
+        break;
 
-    case RISING:
-      EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_RISE_Val << pos;
-      break;
+      case RISING:
+        EIC->CONFIG[config].reg |= EIC_CONFIG_SENSE0_RISE_Val << pos;
+        break;
+    }
   }
-
   // Enable the interrupt
   EIC->INTENSET.reg = EIC_INTENSET_EXTINT(1 << in);
 }
@@ -133,6 +161,23 @@ void detachInterrupt(uint32_t pin)
   
   // Disable wakeup capability on pin during sleep
   EIC->WAKEUP.reg &= ~(1 << in);
+
+  // Remove callback from the ISR list
+  int id = -1;
+  for (uint32_t i=0; i<nints; i++) {
+      if (ISRlist[i] == in) id = in;
+  }
+  if (id == -1) return; // We didn't have it
+
+  // Shift the reminder down
+  for (uint32_t i=id; i<nints-1; i++) {
+      ISRlist[i] = ISRlist[i+1];
+      ISRcallback[i] = ISRcallback[i+1];
+  }
+  // And remove the top item
+  ISRlist[nints]=0;
+  ISRcallback[nints]=NULL;
+  nints--;
 }
 
 /*
@@ -140,18 +185,18 @@ void detachInterrupt(uint32_t pin)
  */
 void EIC_Handler(void)
 {
-  // Test the 16 normal interrupts
-  for (uint32_t i=EXTERNAL_INT_0; i<=EXTERNAL_INT_15; i++)
+  // Calling the routine directly from -here- takes about 1us
+  // Depending on where you are in the list it will take longer
+
+  // Loop over all enabled interrupts in the list
+  for (uint32_t i=0; i<nints; i++)
   {
-    if ((EIC->INTFLAG.reg & (1 << i)) != 0)
+    if ((EIC->INTFLAG.reg & 1<<ISRlist[i]) != 0)
     {
-      // Call the callback function if assigned
-      if (callbacksInt[i]) {
-        callbacksInt[i]();
-      }
-
+      // Call the callback function
+      ISRcallback[i]();
       // Clear the interrupt
-      EIC->INTFLAG.reg = 1 << i;
+      EIC->INTFLAG.reg = 1<<ISRlist[i];
     }
   }
 }