Documentation: Add Thread safety page

Move the section of the Thread support page dealing with thread safety
to a dedicated .dox file at Documentation/. This is done to support the
splitting of the Documentation into a public and internal version. With
a separate page, references can be made to thread safety without having
to include the Thread class in the doxygen run. Some sections of the new
page are still specific to internal implementations and so are hidden
with the \internal flag and an internal section which is conditionally
included. For now, hardcode it to be included in the Doxyfile.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/Documentation/thread-safety.dox b/Documentation/thread-safety.dox
new file mode 100644
index 00000000..df4c457c
--- /dev/null
+++ b/Documentation/thread-safety.dox
@@ -0,0 +1,44 @@
+/**
+ * \page thread-safety Reentrancy and Thread-Safety
+ *
+ * Through the documentation, several terms are used to define how classes and
+ * their member functions can be used from multiple threads.
+ *
+ * - A **reentrant** function may be called simultaneously from multiple
+ *   threads if and only if each invocation uses a different instance of the
+ *   class. This is the default for all member functions not explictly marked
+ *   otherwise.
+ *
+ * - \anchor thread-safe A **thread-safe** function may be called
+ *   simultaneously from multiple threads on the same instance of a class. A
+ *   thread-safe function is thus reentrant. Thread-safe functions may also be
+ *   called simultaneously with any other reentrant function of the same class
+ *   on the same instance.
+ *
+ * \internal
+ * - \anchor thread-bound A **thread-bound** function may be called only from
+ *   the thread that the class instances lives in (see section \ref
+ *   thread-objects). For instances of classes that do not derive from the
+ *   Object class, this is the thread in which the instance was created. A
+ *   thread-bound function is not thread-safe, and may or may not be reentrant.
+ * \endinternal
+ *
+ * Neither reentrancy nor thread-safety, in this context, mean that a function
+ * may be called simultaneously from the same thread, for instance from a
+ * callback invoked by the function. This may deadlock and isn't allowed unless
+ * separately documented.
+ *
+ * \if internal
+ * A class is defined as reentrant, thread-safe or thread-bound if all its
+ * member functions are reentrant, thread-safe or thread-bound respectively.
+ * \else
+ * A class is defined as reentrant or thread-safe if all its member functions
+ * are reentrant or thread-safe respectively.
+ * \endif
+ * Some member functions may additionally be documented as having additional
+ * thread-related attributes.
+ *
+ * Most classes are reentrant but not thread-safe, as making them fully
+ * thread-safe would incur locking costs considered prohibitive for the
+ * expected use cases.
+ */