Amend critical section function descriptions

hal/critical_section_api.h

@@ -30,38 +30,52 @@ extern "C" {
 /**
  * Mark the start of a critical section
  *
-  * This function is called directly by core_util_critical_section_enter on
-  * first entrance to a critical section.
+ * This function will be called by core_util_critical_section_enter() each time
+ * the application requests to enter a critical section. The purpose of the
+ * critical section is to ensure mutual-exclusion synchronisation of the
+ * processor by preventing any change in processor control, the default
+ * behaviour requires storing the state of interrupts in the system before
+ * disabling them.
  *
-  * There is a default implementation of this function which will save the
-  * current state of interrupts before disabling them. This implementation can
-  * be found in mbed_critical_section_api.c. This behaviour is can be overridden
-  * on a per platform basis by providing a different implementation within the
-  * correct targets directory.
+ * The critical section code supports nesting. When a thread has entered a
+ * critical section it can make additional calls to
+ * core_util_critical_section_enter() without deadlocking itself. The critical
+ * section driver API tracks the number of nested calls to the critical section.
+ * The critical section will only be exited when
+ * core_util_critical_section_exit() has been called once for each time it
+ * entered the critical section.
  *
-  * The function is only called once per critical section by
-  * core_util_critical_section_enter. When implementing this function for a
-  * platform you must store any state that you intend to alter within this
-  * function so it can be restored when exiting the critical section.
+ * On the first call to enter a critical section this function MUST store the
+ * state of any interrupts or other application settings it will modify to
+ * facilitate the critical section.
  *
+ * Each successive call to enter the critical section MUST ignore storing or
+ * modifying any application state.
+ *
+ * The default implementation of this function which will save the current state
+ * of interrupts before disabling them. This implementation can be found in
+ * mbed_critical_section_api.c. This behaviour is can be overridden on a per
+ * platform basis by providing a different implementation within the correct
+ * targets directory.
  */
 void hal_critical_section_enter(void);
 
-/** Mark the end of a critical section
+/** Mark the end of a critical section.
  *
-  * This function is called directly by core_util_critical_section_exit on the
-  * final exit from a critical section.
+ * The purpose of this function is to restore any state that was modified upon
+ * entering the critical section, allowing other threads or interrupts to change
+ * the processor control.
  *
-  * There is a default implementation of this function, it will restore the
-  * state of the interrupts as they were prior to entering this critical
-  * section, this implementation can be found in mbed_critical_section_api.c.
-  * This behavior is overridable by providing a different function
-  * implementation within the correct targets directory.
-  *
-  * This function is only called once per critical section. When implemented
-  * for a specific platform it must restore any state that was saved upon
-  * entering the current critical section.
+ * This function will be called once by core_util_critical_section_exit() per
+ * critical section on last call to exit. When called, the application MUST
+ * restore the saved interrupt/application state that was saved when entering
+ * the critical section.
  *
+ * There is a default implementation of this function, it will restore the state
+ * of interrupts that were previously saved when hal_critical_section_enter was
+ * first called, this implementation can be found in
+ * mbed_critical_section_api.c. This behaviour is overridable by providing a
+ * different function implementation within the correct targets directory.
  */
 void hal_critical_section_exit(void);