Doc: Migrate Using Kernel Threads

Migrate page Using Kernel Threads from Confluence wiki to official wiki

Signed-off-by: Ludovic Vanasse <ludovicvanasse@gmail.com>

Documentation/guides/index.rst

@@ -37,3 +37,4 @@ Guides
   devicetree.rst
   debuggingflash_nuttxonarm.rst
   changing_systemclockconfig.rst
+  usingkernelthreads.rst

Documentation/guides/usingkernelthreads.rst

@@ -0,0 +1,60 @@
+====================
+Using Kernel Threads
+====================
+
+.. warning:: 
+    Migrated from: 
+    https://cwiki.apache.org/confluence/display/NUTTX/Using+Kernel+Threads
+
+
+Build Configurations
+====================
+
+NuttX can be built in three different configurations: (1) as a FLAT build where 
+all of the code resides in a common address space, (2) as a PROTECTED build 
+where a memory protection unit (MPU) is used to separate the memory into 
+privileged memory for the OS and unprivileged memory for all applications, 
+or (3) as a KERNEL build where a memory management unit (MMU) is used place 
+the OS in a privileged address space and to place to task (or process) in its 
+own virtual address space.
+
+In the last two configurations, applications reside outside of the OS address 
+space and in all configurations applications do not have have access to any 
+internal resources of the OS.
+
+More information about these build configurations can be found on `the Memory 
+Configuration Wiki page <https://cwiki.apache.org/confluence/display/NUTTX/Memory+Configurations>`_.
+
+Thread Types
+============
+
+NuttX supports three classes of threads: tasks, pthreads, and kernel threads. 
+tasks and pthreads are both application threads and are distinguished by some 
+usage semantics and by their hierarchical relationship. tasks are created via 
+several different mechanisms: ``task_create()``, ``task_spawn()``, ``execv()``, 
+``posix_spawn()``, and others. Tasks may then create pthreads using 
+``pthread_create()``.
+
+More information about tasks and pthreads can be found on the 
+`NuttX Tasking <https://cwiki.apache.org/confluence/display/NUTTX/NuttX+Tasking>`_ 
+Wiki page.
+
+Kernel Threads
+==============
+
+Kernel threads are really like tasks except that they run inside the operating 
+system and are started with ``kernel_thread()`` which is prototyped in 
+``include/nuttx/kthread.h``. The differ from tasks in that (1) in PROTECTED and 
+KERNEL builds, they have full supervisor privileges, and (2) they have full 
+access to all internal OS resources.
+
+In order to build the task into the OS as a kernel thread, you simply have to: 
+(1) place the kernel thread code in your board source code directory, and (2) 
+start it with ``kernel_thread()`` in your board bring-up logic. There a few 
+examples of this in the NuttX source tree. Here is one: 
+`https://github.com/apache/nuttx/blob/master/boards/arm/stm32/viewtool-stm32f107/src/stm32_highpri.c <https://github.com/apache/nuttx/blob/master/boards/arm/stm32/viewtool-stm32f107/src/stm32_highpri.c>`_
+
+So that is another trick that you can use to architecture optimal solutions: 
+Create parts of your applications as kernel threads: They need to reside in 
+your board/src directory and the need to be started with ``kernel_thread()`` in 
+your board bring-up logic. And that is it.