[cm3/sync] Add mutex_trylock()

Allows non-blocking use in user code.

include/libopencm3/cm3/sync.h

@@ -47,6 +47,7 @@ typedef uint32_t mutex_t;
 #define MUTEX_LOCKED	 1
 
 void mutex_lock(mutex_t *m);
+uint32_t mutex_trylock(mutex_t *m);
 void mutex_unlock(mutex_t *m);
 
 #endif

lib/cm3/sync.c

@@ -61,6 +61,25 @@ void mutex_lock(mutex_t *m)
 	__dmb();
 }
 
+/* returns 1 if the lock was acquired */
+uint32_t mutex_trylock(mutex_t *m)
+{
+	uint32_t status = 0;
+
+	/* If the mutex is unlocked. */
+	if (__ldrex(m) == MUTEX_UNLOCKED) {
+		/* Try to lock it. */
+		status = __strex(MUTEX_LOCKED, m);
+	}
+
+	/* Execute the mysterious Data Memory Barrier instruction! */
+	__dmb();
+
+	/* Did we get the lock? If not then try again
+	 * by calling this function once more. */
+	return status == 0;
+}
+
 void mutex_unlock(mutex_t *m)
 {
 	/* Ensure accesses to protected resource are finished */