user: Rework overview

Move "Overview", "Real-time Applicaiton Systems" and "Real-time
Executive" chapters as sections into new "Introduction" chapter.  Add
"Feature" section.

1 files changed, 173 insertions, 30 deletions

diff --git a/user/overview/index.rst b/user/overview/index.rst
index 76bac36..84a6033 100644
--- a/user/overview/index.rst
+++ b/user/overview/index.rst
@@ -1,60 +1,189 @@
 .. comment SPDX-License-Identifier: CC-BY-SA-4.0
 
+Introduction
+============
+
 Overview
-========
+--------
+
+You are someone looking for a real-time operating system.  This document
+
+- presents the basic features of RTEMS, so that you can decide if it is worth to
+  look at,
+
+- gives you a :ref:`quick start <QuickStart>` to install all the tools
+  necessary to work with RTEMS, and
+
+- helps you to build an example application on top of RTEMS.
+
+Features
+--------
+
+The Real-Time Executive for Multiprocessor Systems (:ref:term:`RTEMS`) is a
+multi-threaded, single address-space, real-time operating system with no
+kernel-space/user-space separation.  It is capable to operate in an
+:ref:term:`SMP` configuration providing a state of the art feature set.
+
+RTEMS is licensed under a
+`modified GPL 2.0 or later license with an exception for static linking <https://git.rtems.org/rtems/tree/LICENSE>`_
+[#]_.  It exposes no license requirements on application code.  The third-party
+software used and distributed by RTEMS which may be linked to the application
+is licensed under permissive open source licenses.  Everything necessary to
+build RTEMS applications is available as open source software.  This makes you
+completely vendor independent.
+
+RTEMS provides the following basic feature set:
+
+- :ref:term:`APIs <API>`
+
+    - :ref:term:`POSIX` with
+      `pthreads <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/pthread.h.html>`_
+      (enables a broad range of standard software to run on RTEMS)
+
+    - `Classic <https://docs.rtems.org/branches/master/c-user.pdf>`_
+
+    - :ref:term:`C11` (including
+      `thread <https://en.cppreference.com/w/c/thread>`_ support)
+
+    - :ref:term:`C++11` (including
+      `thread <https://en.cppreference.com/w/cpp/thread>`_ support)
+
+    - Newlib and :ref:term:`GCC` internal
+
+- Programming languages
+
+    - C/C++/OpenMP (RTEMS Source Builder, RSB)
+
+    - Ada (RSB, ``--with-ada``)
+
+    - Erlang
+
+    - Fortran (RSB, ``--with-fortran``)
+
+    - Python and MicroPython
+
+- Parallel languages
+
+    - :ref:term:`EMB²`
+
+    - Google Go [#]_
+
+    - :ref:term:`OpenMP` 4.5
+
+- Thread synchronization and communication
+
+    - Mutexes with and without locking protocols
+
+    - Counting semaphores
+
+    - Binary semaphores
+
+    - Condition variables
 
-Welcome to the :ref:term:`RTEMS` User Manual.
+    - Events
 
-This document covers the topics a user of RTEMS needs to be able to install,
-configure, build and create applications for the RTEMS operating system.
+    - Message queues
 
-RTEMS, Real-Time Executive for Multiprocessor Systems, is a real-time executive
-(kernel) which provides a high performance environment for embedded
-applications with the following features:
+    - Barriers
 
-.. sidebar:: Developers
+    - :ref:term:`Futex` (used by :ref:term:`OpenMP` barriers)
 
-  Developers should look at the :r:url:`devel` for technical information. The
-  design and development of RTEMS is located there.
+    - Epoch Based Reclamation (libbsd)
 
-- standards based user interfaces
+- Locking protocols
 
-- multitasking capabilities
+    - Transitive Priority Inheritance
 
-- homogeneous and heterogeneous multiprocessor systems
+    - :ref:term:`OMIP` (SMP feature)
 
-- event-driven, priority-based, preemptive scheduling
+    - Priority Ceiling
 
-- optional rate monotonic scheduling
+    - :ref:term:`MrsP` (SMP feature)
 
-- intertask communication and synchronization
+- Scalable timer and timeout support
 
-- priority inheritance
+- Lock-free timestamps (FreeBSD timecounters)
 
-- responsive interrupt management
+- Responsive interrupt management
 
-- dynamic memory allocation
+- C11/C++11 :ref:term:`TLS` [#]_
 
-- high level of user configurability
+- Link-time configurable schedulers
 
-- open source with a friendly user license
+    - Fixed-priority
 
-RTEMS provides features found in modern operating systems:
+    - Job-level fixed-priority (:ref:term:`EDF`)
 
-- file systems
+    - Constant Bandwidth Server (experimental)
 
-- networking
+- Clustered scheduling (SMP feature)
 
-- USB
+    - Flexible link-time configuration
 
-- permanent media such as flash disks, cards and USB devices
+    - Job-level fixed-priority scheduler (:ref:term:`EDF`) with support for
+      one-to-one and one-to-all thread to processor affinities (default SMP
+      scheduler)
 
-- support for various languages
+    - Fixed-priority scheduler
 
-- parallel programming language support
+    - Proof-of-concept strong :ref:term:`APA` scheduler
+
+- Focus on link-time application-specific configuration
+
+- Linker-set based initialization (similar to global C++ constructors)
+
+- Operating system uses fine-grained locking (SMP feature)
+
+- Dynamic memory allocators
+
+    - First-fit (default)
+
+    - Universal Memory Allocator
+      (`UMA <https://www.freebsd.org/cgi/man.cgi?query=uma&sektion=9>`_ ,
+      libbsd)
+
+- File systems
+
+    - :ref:term:`IMFS`
+
+    - :ref:term:`FAT`
+
+    - :ref:term:`RFS`
+
+    - :ref:term:`NFSv2`
+
+    - :ref:term:`JFFS2` (NOR flashes)
+
+    - :ref:term:`YAFFS2` (NAND flashes, GPL or commercial license required)
+
+- Device drivers
+
+    - Termios (serial interfaces)
+
+    - I2C (Linux user-space API compatible)
+
+    - SPI (Linux user-space API compatible)
+
+    - Network stacks (legacy, libbsd, lwIP)
+
+    - USB stack (libbsd)
+
+    - SD/MMC card stack (libbsd)
+
+    - Framebuffer (Linux user-space API compatible, Qt)
+
+    - Application runs in kernel-space and can access hardware directly
+
+- libbsd
+
+    - Port of FreeBSD user-space and kernel-space components to RTEMS
+
+    - Easy access to FreeBSD software for RTEMS
+
+    - Support to stay in synchronization with FreeBSD
 
 Real-time Application Systems
-=============================
+-----------------------------
 
 Real-time application systems are a special class of computer applications.
 They have a complex set of characteristics that distinguish them from other
@@ -89,7 +218,7 @@ competing processors are introduced.  The ramifications of multiple processors
 complicate each and every characteristic of a real-time system.
 
 Real-time Executive
-===================
+-------------------
 
 Fortunately, real-time operating systems, or real-time executives, serve as a
 cornerstone on which to build the application system.  A real-time multitasking
@@ -127,3 +256,17 @@ routines to manage memory, pass messages, or provide mutual exclusion.  The
 developer is then able to concentrate solely on the application.  By using
 standard software components, the time and cost required to develop
 sophisticated real-time applications are significantly reduced.
+
+.. [#] The goal is to use the
+       `BSD 2-Clause license
+       <https://git.rtems.org/rtems/tree/LICENSE.BSD-2-Clause>`_ for new code
+       or code those copyright holder agreed to a license change, see `#3053
+       <https://devel.rtems.org/ticket/3053>`_ for the details.
+
+.. [#] See `#2832 <https://devel.rtems.org/ticket/2832>`_.
+
+.. [#] Thread-local storage requires some support by the tool chain and the
+       RTEMS architecture support, e.g. context-switch code.  It is supported
+       at least on ARM, PowerPC, RISC-V, SPARC and m68k.  Check the
+       `RTEMS CPU Architecture Supplement <https://docs.rtems.org/branches/master/cpu-supplement.pdf>`_
+       if it is supported.