opt-in ownership of PWM peripheral

Author: henrygab

cores/nRF5/HardwarePWM.cpp

@@ -53,6 +53,25 @@ HardwarePWM* HwPWMx[] =
 #endif
 };
 
+// returns true ONLY when (1) no PWM channel has a pin, and (2) the owner token is nullptr
+bool HardwarePWM::takeOwnership(uintptr_t token)
+{
+  if (this->_owner_token       != 0) return false; // doesn't matter if it's actually a match ... it's not legal to take ownership twice
+  if (this->usedChannelCount() != 0) return false; // at least one channel is already in use
+  // use gcc built-in intrinsic to ensure atomicity
+  // See https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html
+  return __sync_bool_compare_and_swap(&(this->_owner_token), 0, token);
+}
+// returns true ONLY when (1) no PWM channel has a pin attached, and (2) the owner token matches
+bool HardwarePWM::releaseOwnership(uintptr_t token)
+{
+  if (!this->isOwner(token)         ) return false; // don't even look at peripheral
+  if ( this->usedChannelCount() != 0) return false; // fail if any channels still have pins
+  // use gcc built-in intrinsic to ensure atomicity
+  // See https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html
+  return __sync_bool_compare_and_swap(&(this->_owner_token), token, 0);
+}
+
 HardwarePWM::HardwarePWM(NRF_PWM_Type* pwm)
 {
   _pwm = pwm;

cores/nRF5/HardwarePWM.h

@@ -50,6 +50,7 @@ class HardwarePWM
   private:
     enum { MAX_CHANNELS = 4 }; // Max channel per group
     NRF_PWM_Type* _pwm;
+    uintptr_t _owner_token = 0;
 
     uint16_t _seq0[MAX_CHANNELS];
 
@@ -67,6 +68,18 @@ class HardwarePWM
 
     void setClockDiv(uint8_t div);      // value is PWM_PRESCALER_PRESCALER_DIV_x, DIV1 is 16Mhz
 
+    // Cooperative ownership sharing
+
+    // returns true ONLY when (1) no PWM channel has a pin, and (2) the owner token is nullptr
+    bool takeOwnership(uintptr_t token);
+    // returns true ONLY when (1) no PWM channel has a pin attached, and (2) the owner token matches
+    bool releaseOwnership(uintptr_t token);
+    // allows caller to verify that they own the peripheral
+    __INLINE bool isOwner(uintptr_t token) __attribute__((__always_inline__))
+    {
+      return this->_owner_token == token;
+    }
+
     bool addPin     (uint8_t pin);
     bool removePin  (uint8_t pin);