doc: Move init-related things out of README

Move this section to rst, changing it just enough so that it builds.

Signed-off-by: Simon Glass <sjg@chromium.org>

1 files changed, 92 insertions, 0 deletions

diff --git a/doc/develop/init.rst b/doc/develop/init.rst
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+.. SPDX-License-Identifier: GPL-2.0+
+
+Board Initialisation Flow
+-------------------------
+
+This is the intended start-up flow for boards. This should apply for both
+SPL and U-Boot proper (i.e. they both follow the same rules).
+
+Note: "SPL" stands for "Secondary Program Loader," which is explained in
+more detail later in this file.
+
+At present, SPL mostly uses a separate code path, but the function names
+and roles of each function are the same. Some boards or architectures
+may not conform to this.  At least most ARM boards which use
+CONFIG_SPL_FRAMEWORK conform to this.
+
+Execution typically starts with an architecture-specific (and possibly
+CPU-specific) start.S file, such as:
+
+- arch/arm/cpu/armv7/start.S
+- arch/powerpc/cpu/mpc83xx/start.S
+- arch/mips/cpu/start.S
+
+and so on. From there, three functions are called; the purpose and
+limitations of each of these functions are described below.
+
+lowlevel_init()
+~~~~~~~~~~~~~~~
+
+- purpose: essential init to permit execution to reach board_init_f()
+- no global_data or BSS
+- there is no stack (ARMv7 may have one but it will soon be removed)
+- must not set up SDRAM or use console
+- must only do the bare minimum to allow execution to continue to
+  board_init_f()
+- this is almost never needed
+- return normally from this function
+
+board_init_f()
+~~~~~~~~~~~~~~
+
+- purpose: set up the machine ready for running board_init_r():
+  i.e. SDRAM and serial UART
+- global_data is available
+- stack is in SRAM
+- BSS is not available, so you cannot use global/static variables,
+  only stack variables and global_data
+
+Non-xPL-specific notes:
+
+    - dram_init() is called to set up DRAM. If already done in SPL this
+      can do nothing
+
+xPL-specific notes:
+
+    - you can override the entire board_init_f() function with your own
+      version as needed.
+    - preloader_console_init() can be called here in extremis
+    - should set up SDRAM, and anything needed to make the UART work
+    - there is no need to clear BSS, it will be done by crt0.S
+    - for specific scenarios on certain architectures an early BSS *can*
+      be made available (via CONFIG_SPL_EARLY_BSS by moving the clearing
+      of BSS prior to entering board_init_f()) but doing so is discouraged.
+      Instead it is strongly recommended to architect any code changes
+      or additions such to not depend on the availability of BSS during
+      board_init_f() as indicated in other sections of this README to
+      maintain compatibility and consistency across the entire code base.
+    - must return normally from this function (don't call board_init_r()
+      directly)
+
+Here the BSS is cleared. For SPL, if CONFIG_SPL_STACK_R is defined, then at
+this point the stack and global_data are relocated to below
+CONFIG_SPL_STACK_R_ADDR. For non-SPL, U-Boot is relocated to run at the top of
+memory.
+
+board_init_r()
+~~~~~~~~~~~~~~
+
+    - purpose: main execution, common code
+    - global_data is available
+    - SDRAM is available
+    - BSS is available, all static/global variables can be used
+    - execution eventually continues to main_loop()
+
+Non-SPL-specific notes:
+
+    - U-Boot is relocated to the top of memory and is now running from
+      there.
+
+SPL-specific notes:
+
+    - stack is optionally in SDRAM, if CONFIG_SPL_STACK_R is defined