doc: add documentation for condition variables

Add documentation for condition variables in the synchronization section
of the kernel.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

doc/reference/kernel/index.rst

@@ -38,6 +38,7 @@ synchronization.
    other/polling.rst
    synchronization/semaphores.rst
    synchronization/mutexes.rst
+   synchronization/condvar.rst
    smp/smp.rst
 
 Data Passing

doc/reference/kernel/synchronization/condvar.rst

@@ -0,0 +1,139 @@
+.. _condvar:
+
+Condition ariables
+####################
+
+A :dfn:`condition variable` is a synchronization primitive
+that enables threads to wait until a particular condition occurs.
+
+.. contents::
+    :local:
+    :depth: 2
+
+Concepts
+********
+
+Any number of condition variables can be defined (limited only by available RAM). Each
+condition variable is referenced by its memory address.
+
+To wait for a condition to become true, a thread can make use of a condition
+variable.
+
+A condition variable is basically a queue of threads that threads can put
+themselves on when some state of execution (i.e., some condition) is not as
+desired (by waiting on the condition). The function
+:c:func:`k_condvar_wait` performs atomically the following steps;
+
+# Releases the last acquired mutex.
+# Puts the current thread in the condition variable queue.
+
+Some other thread, when it changes said state, can then wake one (or more)
+of those waiting threads and thus allow them to continue by signaling on
+the condition using :c:func:`k_condvar_signal` or
+:c:func:`k_condvar_broadcast` then it:
+
+# Re-acquires the mutex previously released.
+# Returns from :c:func:`k_condvar_wait`.
+
+A condition variable must be initialized before it can be used.
+
+
+Implementation
+**************
+
+Defining a Condition Variable
+=============================
+
+A condition variable is defined using a variable of type :c:struct:`k_condvar`.
+It must then be initialized by calling :c:func:`k_condvar_init`.
+
+The following code defines a condition variable:
+
+.. code-block:: c
+
+    struct k_condvar my_condvar;
+
+    k_condvar_init(&my_condvar);
+
+Alternatively, a condition variable can be defined and initialized at compile time
+by calling :c:macro:`K_CONDVAR_DEFINE`.
+
+The following code has the same effect as the code segment above.
+
+.. code-block:: c
+
+    K_CONDVAR_DEFINE(my_condvar);
+
+Waiting on a Condition Variable
+===============================
+
+A thread can wait on a condition by calling :c:func:`k_condvar_wait`.
+
+The following code waits on the condition variable.
+
+
+.. code-block:: c
+
+    K_MUTEX_DEFINE(mutex);
+    K_CONDVAR_DEFINE(condvar)
+
+    void main(void)
+    {
+        k_mutex_lock(&mutex, K_FOREVER);
+
+        /* block this thread until another thread signals cond. While
+         * blocked, the mutex is released, then re-acquired before this
+         * thread is woken up and the call returns.
+         */
+        k_condvar_wait(&condvar, &mutex, K_FOREVER);
+        ...
+        k_mutex_unlock(&mutex);
+    }
+
+Signaling a Condition Variable
+===============================
+
+A condition variable is signaled on by calling :c:func:`k_condvar_signal` for
+one thread or by calling :c:func:`k_condvar_broadcast` for multiple threads.
+
+The following code builds on the example above.
+
+.. code-block:: c
+
+    void worker_thread(void)
+    {
+        k_mutex_lock(&mutex, K_FOREVER);
+
+        /*
+         * Do some work and fullfill the condition
+         */
+        ...
+        ...
+        k_condvar_signal(&condvar);
+        k_mutex_unlock(&mutex);
+    }
+
+Suggested Uses
+**************
+
+Use condition variables with a mutex to signal changing states (conditions) from
+one thread to another thread.
+Condition variables are not the condition itself and they are not events.
+The condition is contained in the surrounding programming logic.
+
+Mutexes alone are not designed for use as a notification/synchronization
+mechanism. They are meant to provide mutually exclusive access to a shared
+resource only.
+
+Configuration Options
+*********************
+
+Related configuration options:
+
+* None.
+
+API Reference
+**************
+
+.. doxygengroup:: condvar_apis
+   :project: Zephyr