Add support for static mutexes

Add PlatformMutexStatic which is lazily initialized in static context
which allows it to be garbage collected.  The lazy initialization also
allows it to be used earlier than normal mutexes.

Author: c1728p9

hal/api/AnalogIn.h

@@ -118,7 +118,7 @@ class AnalogIn {
     }
 
     analogin_t _adc;
-    static PlatformMutex _mutex;
+    static PlatformMutexStatic _mutex;
 };
 
 } // namespace mbed

hal/api/FileBase.h

@@ -65,7 +65,7 @@ class FileBase {
     /* disallow copy constructor and assignment operators */
 private:
     static FileBase *_head;
-    static PlatformMutex _mutex;
+    static PlatformMutexStatic _mutex;
 
     FileBase   *_next;
     const char * const _name;

hal/api/I2C.h

@@ -181,7 +181,7 @@ class I2C {
     i2c_t _i2c;
     static I2C  *_owner;
     int         _hz;
-    static PlatformMutex _mutex;
+    static PlatformMutexStatic _mutex;
 };
 
 } // namespace mbed

hal/api/platform.h

@@ -21,6 +21,7 @@
 #include "device.h"
 #include "PinNames.h"
 #include "PeripheralNames.h"
+#include "critical.h"
 
 #include <cstddef>
 #include <cstdlib>
@@ -54,4 +55,55 @@ class PlatformMutex {
 
 #endif
 
+/** The static version of a PlatformMutex
+ *
+ * This class must only be used in a static context -
+ * this class must never be allocated or created on the
+ * stack.
+ *
+ * This class is lazily initialized on first use.
+ * This class is a POD type so if it is not used it will
+ * be garbage collected.
+ */
+struct PlatformMutexStatic {
+    PlatformMutex* _mutex;
+
+    void _init() {
+        PlatformMutex* current = _mutex;
+        PlatformMutex* new_mutex;
+
+        if (NULL == current) {
+            bool done = false;
+            new_mutex = new PlatformMutex;
+            while (!done) {
+                done = core_util_atomic_cas_ptr((void**)&_mutex, (void**)&current, (void*)new_mutex);
+                if (current != NULL) {
+                    // Mutex was created on another thread first
+                    // so delete ours
+                    delete new_mutex;
+                    break;
+                }
+            }
+        }
+    }
+
+    /** Wait until this Mutex becomes available.
+     */
+    void lock() {
+        if (NULL == _mutex) {
+            _init();
+        }
+        _mutex->lock();
+    }
+
+    /** Unlock the mutex that has previously been locked by the same thread
+     */
+    void unlock() {
+        if (NULL == _mutex) {
+            _init();
+        }
+        _mutex->unlock();
+    }
+};
+
 #endif

hal/common/AnalogIn.cpp

@@ -22,7 +22,7 @@
 
 namespace mbed {
 
-PlatformMutex AnalogIn::_mutex;
+PlatformMutexStatic AnalogIn::_mutex;
 
 };
 

hal/common/FileBase.cpp

@@ -18,7 +18,7 @@
 namespace mbed {
 
 FileBase *FileBase::_head = NULL;
-PlatformMutex FileBase::_mutex;
+PlatformMutexStatic FileBase::_mutex;
 
 FileBase::FileBase(const char *name, PathType t) : _next(NULL),
                                                    _name(name),

hal/common/I2C.cpp

@@ -20,7 +20,7 @@
 namespace mbed {
 
 I2C *I2C::_owner = NULL;
-PlatformMutex I2C::_mutex;
+PlatformMutexStatic I2C::_mutex;
 
 I2C::I2C(PinName sda, PinName scl) :
 #if DEVICE_I2C_ASYNCH

hal/common/retarget.cpp

@@ -72,7 +72,7 @@ extern const char __stderr_name[] = "/stderr";
  * (or rather index+3, as filehandles 0-2 are stdin/out/err).
  */
 static FileHandle *filehandles[OPEN_MAX];
-static PlatformMutex filehandle_mutex;
+static PlatformMutexStatic filehandle_mutex;
 
 FileHandle::~FileHandle() {
     filehandle_mutex.lock();