doc: Migrate DesignPrinciples wiki page to Sphinx

Move the current DesignPrinciples wiki page to
doc/develop/designprinciples.rst.  The changes here are for formatting
or slight rewording so that it reads well when linking to other Sphinx
documents.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>

doc/develop/designprinciples.rst

@@ -0,0 +1,205 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+U-Boot Design Principles
+========================
+
+The 10 Golden Rules of U-Boot design
+------------------------------------
+
+Keep it Small
+^^^^^^^^^^^^^
+
+U-Boot is a Boot Loader, i.e. its primary purpose in the shipping
+system is to load some operating system.
+That means that U-Boot is
+necessary to perform a certain task, but it's nothing you want to
+throw any significant resources at. Typically U-Boot is stored in
+relatively small NOR flash memory, which is expensive
+compared to the much larger NAND devices often used to store the
+operating system and the application.
+
+At the moment, U-Boot supports boards with just 128 KiB ROM or with
+256 KiB NOR flash. We should not easily ignore such configurations -
+they may be the exception in among all the other supported boards,
+but if a design uses such a resource-constrained hardware setup it is
+usually because costs are critical, i. e. because the number of
+manufactured boards might be tens or hundreds of thousands or even
+millions...
+
+A usable and useful configuration of U-Boot, including a basic
+interactive command interpreter, support for download over Ethernet
+and the capability to program the flash shall fit in no more than 128 !KiB.
+
+Keep it Fast
+^^^^^^^^^^^^
+
+The end user is not interested in running U-Boot. In most embedded
+systems he is not even aware that U-Boot exists. The user wants to
+run some application code, and that as soon as possible after switching
+on his device.
+
+It is therefore essential that U-Boot is as fast as possible,
+especially that it loads and boots the operating system as fast as possible.
+
+To achieve this, the following design principles shall be followed:
+
+* Enable caches as soon and whenever possible
+
+* Initialize devices only when they are needed within U-Boot, i.e. don't
+  initialize the Ethernet interface(s) unless U-Boot performs a download over
+  Ethernet; don't  initialize any IDE or USB devices unless U-Boot actually
+  tries to load files from these, etc.  (and don't forget to shut down these
+  devices after using them  - otherwise nasty things may happen when you try to
+  boot your OS).
+
+Also, building of U-Boot shall be as fast as possible.
+This makes it easier to run a build for all supported configurations
+or at least for all configurations of a specific architecture,
+which is essential for quality assurance.
+If building is cumbersome and slow, most people will omit
+this important step.
+
+Keep it Simple
+^^^^^^^^^^^^^^
+
+U-Boot is a boot loader, but it is also a tool used for board
+bring-up, for production testing, and for other activities
+
+Keep it Portable
+^^^^^^^^^^^^^^^^
+
+U-Boot is a boot loader, but it is also a tool used for board
+bring-up, for production testing, and for other activities that are
+very closely related to hardware development. So far, it has been
+ported to several hundreds of different boards on about 30 different
+processor families - please make sure that any code you add can be
+used on as many different platforms as possible.
+
+Avoid assembly language whenever possible - only the reset code with
+basic CPU initialization, maybe a static DRAM initialization and the C
+stack setup should be in assembly.
+All further initializations should be done in C using assembly/C
+subroutines or inline macros. These functions represent some
+kind of HAL functionality and should be defined consistently on all
+architectures, e.g. basic MMU and cache control, stack pointer manipulation.
+Non-existing functions should expand into empty macros or error codes.
+
+Don't make assumptions about the environment where U-Boot is running.
+It may be communicating with a human operator on directly attached
+serial console, but it may be through a GSM modem as well, or driven
+by some automatic test or control system. So don't output any fancy
+control character sequences or similar.
+
+Keep it Configurable
+^^^^^^^^^^^^^^^^^^^^
+
+Section "Keep it Small" already explains about the size restrictions
+for U-Boot on one side. On the other side, U-Boot is a powerful tool
+with many, many extremely useful features. The maintainer or user of
+each board will have to decide which features are important to him and
+what shall be included with his specific board configuration to meet
+his current requirements and restrictions.
+
+Please make sure that it is easy to add or remove features from a
+board configuration, so everybody can make the best use of U-Boot on
+his system.
+
+If a feature is not included, it should not have any residual code
+bloating the build.
+
+Keep it Debuggable
+^^^^^^^^^^^^^^^^^^
+
+Of course debuggable code is a big benefit for all of us contributing
+in one way or another to the development of the U-Boot project. But
+as already mentioned in section "Keep it Portable" above, U-Boot is
+not only a tool in itself, it is often also used for hardware
+bring-up, so debugging U-Boot often means that we don't know if we are
+tracking down a problem in the U-Boot software or in the hardware we
+are running on. Code that is clean and easy to understand and to
+debug is all the more important to many of us.
+
+* One important feature of U-Boot is to enable output to the (usually serial)
+  console as soon as possible in the boot process, even if this causes
+  tradeoffs in other areas like memory footprint.
+
+* All initialization steps shall print some "begin doing this" message before
+  they actually start, and some "done" message when they complete. For example,
+  RAM initialization and size detection may print a "RAM: " before they start,
+  and "256 MB\n" when done.  The purpose of this is that you can always see
+  which initialization step was running if there should be any problem.  This
+  is important not only during software development, but also for the service
+  people dealing with broken hardware in the field.
+
+* U-Boot should be debuggable with simple JTAG or BDM equipment.  It shall use
+  a simple, single-threaded execution model.  Avoid any magic, which could
+  prevent easy debugging even when only 1 or 2 hardware breakpoints are
+  available.
+
+Keep it Usable
+^^^^^^^^^^^^^^
+
+Please always keep in mind that there are at least three different
+groups of users for U-Boot, with completely different expectations
+and requirements:
+
+* The end user of an embedded device just wants to run some application; he
+  does not even want to know that U-Boot exists and only rarely interacts with
+  it (for example to perform a reset to factory default settings etc.)
+
+* System designers and engineers working on the development of the application
+  and/or the operating system want a powerful tool that can boot from any boot
+  device they can imagine, they want it fast and scriptable and whatever - in
+  short, they want as many features supported as possible. And some more.
+
+* The engineer who ports U-Boot to a new board and the board maintainer want
+  U-Boot to be as simple as possible so porting it to and maintaining it on
+  their hardware is easy for them.
+
+* Make it easy to test. Add debug code (but don't re-invent the wheel - use
+  existing macros like log_debug() or debug() depending on context).
+
+Please always keep in mind that U-Boot tries to meet all these
+different requirements.
+
+Keep it Maintainable
+^^^^^^^^^^^^^^^^^^^^
+
+* Avoid ``#ifdefs`` where possible
+
+* Use "weak" functions
+
+* Always follow the :doc:`codingstyle` requirements.
+
+Keep it Beautiful
+^^^^^^^^^^^^^^^^^
+
+* Keep the source code clean: strictly follow the :doc:`codingstyle`,
+  keep lists (target names in the Makefiles, board names, etc.)
+  alphabetically sorted, etc.
+
+* Keep U-Boot console output clean: output only really necessary information,
+  be terse but precise, keep output vertically aligned, do not use control
+  character sequences (e.g. backspaces or \\r to do "spinning wheel" activity
+  indicators), etc.
+
+Keep it Open
+^^^^^^^^^^^^
+
+Contribute your work back to the whole community. Submit your changes
+and extensions as patches to the U-Boot mailing list.
+
+Lemmas from the golden rules
+----------------------------
+
+Generic Code is Good Code
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+New code shall be as generic as possible and added to the U-Boot
+abstraction hierarchy as high as possible. As few code as possible shall be
+added in board directories as people usually do not expect re-usable code
+there.  Thus peripheral drivers should be put below
+"drivers" even if they start out supporting only one specific
+configuration.  Note that it is not a requirement for such a first
+instance to be generic as genericity generally cannot be extrapolated
+from a single data point.

doc/develop/index.rst

@@ -10,6 +10,7 @@ General
    :maxdepth: 1
 
    codingstyle
+   designprinciples
 
 Implementation
 --------------