[RFCv1] doc: develop: Describe using CONFIG vs CFG for values

[Tom Rini]

Document how and when to use CONFIG or CFG namespace for options.  There
are times where Kconfig is not a great fit for our needs, so we want to
use the CFG namespace instead.

Signed-off-by: Tom Rini <trini at konsulko.com>
---
RFCv1:
- This is essentially my idea on how to better handle the problem of
  CONFIG values that just don't fit in Kconfig because it makes much
  more sense to define them statically for a given SoC or calculate them
  from other values, and so on.  One example here would be to revert
  c7fad78ec0ee ("Convert CONFIG_SYS_BR0_PRELIM et al to Kconfig") and
  re-name these to CFG_SYS_.. instead. Another big example here would be
  a global search-and-replace of 's/CONFIG_HPS_/CFG_HPS_/g' as that's
  all tool-generated. Not all CONFIG_SYS_ options would get this as
  boolean choices are well handled in Kconfig, and that may not be clear
  enough in what I wrote here?
---
 doc/develop/build_configuration.rst | 36 +++++++++++++++++++++++++++++
 doc/develop/index.rst               |  1 +
 2 files changed, 37 insertions(+)
 create mode 100644 doc/develop/build_configuration.rst

diff --git a/doc/develop/build_configuration.rst b/doc/develop/build_configuration.rst
new file mode 100644
index 000000000000..541ce528b36a
--- /dev/null
+++ b/doc/develop/build_configuration.rst
@@ -0,0 +1,36 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Using CONFIG or CFG to configure the build
+==========================================
+
+There are two namespaces used to control the build-time configuration of
+U-Boot, ``CONFIG`` and ``CFG``. These are used when it is either not possible
+or not practical to make a run-time determination about some functionality of
+the hardware or a required software feature or similar. Each of these has their
+own places where they are better suited than the other for use.
+
+The first of these, ``CONFIG``` is managed in the `Kconfig language
+<https://www.kernel.org/doc/html/latest/kbuild/kconfig-language.html>`_ that is
+most commonly associated with the Linux kernel and the Kconfig files found
+throughout our sources. Adding options to this namespace is the preferred way of
+exposing new configuration options as there are a number of ways for both users
+and system integrators to manage and change these options. Some common examples
+here are to enable a specific command within U-Boot or even a whole subsystem
+such as NAND flash or network connectivity.
+
+The second of these, ``CFG`` is implemented directly as C preprocessor values or
+macros, depending on what they are in turn describing. The nature of U-Boot
+means that while we have some functionality that is very reasonable to expose to
+the end user to enable or disable we have other places where we need to describe
+things such as register locations or values, memory map ranges and so on. When
+practical, we should be getting these values from the :doc:`device tree
+<devicetree/control>`. However, there are cases where this is either not
+practical due to when we need the information and may not have a device tree yet
+or due to legacy reasons code has not been rewritten. As this information tends
+to be specific to a given SoC (or family of SoCs) or if board specific something
+that is mandated by the physical design rather than truly user configurable, it
+is acceptable to ``#define`` these values in the ``CFG`` namespace. It is
+strongly encouraged to place these in the architecture header files, if they are
+generic to a given SoC, or under the board directory if board specific. Placing
+them under the board.h file in the *include/configs/* directory should be seen
+as a last resort.
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 73741ceb6a2f..c8ec7c210d35 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -9,6 +9,7 @@ General
 .. toctree::
    :maxdepth: 1
 
+   build_configuration
    codingstyle
    designprinciples
    process
-- 
2.25.1