SPL: Add README.omap3

This document describes the SPL process for OMAP3 (and related) boards
as well as a partial memory map and how to verify certain aspects
outside of running on the target.

Signed-off-by: Tom Rini <trini@ti.com>

doc/SPL/README.omap3

@@ -0,0 +1,74 @@
+Overview of SPL on OMAP3 devices
+================================
+
+Introduction
+------------
+
+This document provides an overview of how SPL functions on OMAP3 (and related
+such as am35x and am37x) processors.
+
+Methodology
+-----------
+
+On these platforms the ROM supports trying a sequence of boot devices.  Once
+one has been used successfully to load SPL this information is stored in memory
+and the location stored in a register.  We will read this to determine where to
+read U-Boot from in turn.
+
+Memory Map
+----------
+
+This is an example of a typical setup.  See top-level README for documentation
+of which CONFIG variables control these values.  For a given board and the
+amount of DRAM available to it different values may need to be used.
+Note that the size of the SPL text rodata and data is enforced with a CONFIG
+option and growing over that size results in a link error.  The SPL stack
+starts at the top of SRAM (which is configurable) and grows downward.  The
+space between the top of SRAM and the enforced upper bound on the size of the
+SPL text, data and rodata is considered the safe stack area.  Details on
+confirming this behavior are shown below.
+
+A portion of the system memory map looks as follows:
+SRAM: 0x40200000 - 0x4020FFFF
+DDR1: 0x80000000 - 0xBFFFFFFF
+
+Option 1 (SPL only):
+0x40200800 - 0x4020BBFF: Area for SPL text, data and rodata
+0x4020BC00 - 0x4020FFFC: Area for the SPL stack.
+0x80000000 - 0x8007FFFF: Area for the SPL BSS.
+0x80100000: CONFIG_SYS_TEXT_BASE of U-Boot
+0x80208000 - 0x80307FFF: malloc() pool available to SPL.
+
+Option 2 (SPL or X-Loader):
+0x40200800 - 0x4020BBFF: Area for SPL text, data and rodata
+0x4020BC00 - 0x4020FFFC: Area for the SPL stack.
+0x80008000: CONFIG_SYS_TEXT_BASE of U-Boot
+0x87000000 - 0x8707FFFF: Area for the SPL BSS.
+0x87080000 - 0x870FFFFF: malloc() pool available to SPL.
+
+For the areas that reside within DDR1 they must not be used prior to s_init()
+completing.  Note that CONFIG_SYS_TEXT_BASE must be clear of the areas that SPL
+uses while running.  This is why we have two versions of the memory map that
+only vary in where the BSS and malloc pool reside.
+
+Estimating stack usage
+----------------------
+
+With gcc 4.6 (and later) and the use of GNU cflow it is possible to estimate
+stack usage at various points in run sequence of SPL.  The -fstack-usage option
+to gcc will produce '.su' files (such as arch/arm/cpu/armv7/syslib.su) that
+will give stack usage information and cflow can construct program flow.
+
+Must have gcc 4.6 or later, which supports -fstack-usage
+
+1) Build normally
+2) Perform the following shell command to generate a list of C files used in
+SPL:
+$ find spl -name '*.su' | sed -e 's:^spl/::' -e 's:[.]su$:.c:' > used-spl.list
+3) Execute cflow:
+$ cflow --main=board_init_r `cat used-spl.list` 2>&1 | $PAGER
+
+cflow will spit out a number of warnings as it does not parse
+the config files and picks functions based on #ifdef.  Parsing the '.i'
+files instead introduces another set of headaches.  These warnings are
+not usually important to understanding the flow, however.