[PATCH 3/3] doc: environment: Further expand on Image locations and provide example

[Tom Rini]

Start by elaborating on what some of our constraints tend to be with
image location values, and document where these external constraints
come from.  Provide a new subsection, an example based on the TI ARMv7
OMAP2PLUS families of chips, that gives sample values and explains why
we use these particular values.  This is based on what is in
include/configs/ti_armv7_common.h as of fb3ad9bd923d ("TI: Add, use a
DEFAULT_LINUX_BOOT_ENV environment string") as this contains just the
values referenced in this document now and not some of the further
additions that are less generic.

Signed-off-by: Tom Rini <trini at konsulko.com>
---
 doc/usage/environment.rst | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/doc/usage/environment.rst b/doc/usage/environment.rst
index a9a4702632d2..f70ccd6a58ee 100644
--- a/doc/usage/environment.rst
+++ b/doc/usage/environment.rst
@@ -404,6 +404,42 @@ device tree blob  fdtfile        fdt_addr_r       fdt_addr
 ramdisk           ramdiskfile    ramdisk_addr_r   ramdisk_addr
 ================= ============== ================ ==============
 
+When setting the RAM addresses for `kernel_addr_r`, `fdt_addr_r` and
+`ramdisk_addr_r` there are several constraints to keep in mind. When booting
+Linux, the `Booting ARM Linux`_ and `Booting AArch64 Linux`_ documents lay out
+the requirements for booting all ARM platforms, including both alignment and
+where within memory various things must be.  These guidelines tend to also be
+correct for other OSes and unless specifically contradicted by documentation
+specific to another architecture, are good rules to follow for other
+architectures as well.
+
+Example Image locations
+^^^^^^^^^^^^^^^^^^^^^^^
+
+If we take the Texas Instruments OMAP2PLUS family of ARMv7 processors as an
+example for the above listed variables, we would do::
+
+    loadaddr=0x82000000
+    kernel_addr_r=${loadaddr}
+    fdt_addr_r=0x88000000
+    ramdisk_addr_r=0x88080000
+    bootm_size=0x10000000
+
+To explain this, we start by noting that DRAM starts at 0x80000000.  A 32MiB
+buffer from the start of memory as our default load address, and so where the
+kernel would also be loaded to.  This will hopefully allow for us to have the
+whole of the compressed kernel image exist in memory above where the whole of
+the decompressed kernel image will be, and allow for a quicker boot.  Next, we
+say that the device tree will be placed at 128MiB offset from the start of
+memory.  This is suggested by the kernel documment as it is exceedingly
+unlikely to be overwritten by the kernel itself given other architectural
+constraints.  We then allow for the device tree to be up to 512KiB in size
+before placing the ramdisk in memory.  We then say that everything should be
+within the first 256MiB of memory so that U-Boot can relocate things as needed
+to ensure proper alignment.  We pick 256MiB as our value here because we know
+there are very few platforms on in this family with less memory.  It could be
+as high as 768MiB and still ensure that everything would be visible to the
+kernel, but again we go with what we assume is the safest assumption.
 
 Automatically updated variables
 -------------------------------
@@ -472,3 +508,6 @@ Implementation
 --------------
 
 See :doc:`../develop/environment` for internal development details.
+
+.. _`Booting ARM Linux`: https://www.kernel.org/doc/html/latest/arm/booting.html
+.. _`Booting AArch64 Linux`: https://www.kernel.org/doc/html/latest/arm64/booting.html
-- 
2.25.1