doc: spl: Add info regarding memory reservation

Add details regarding scheme which need to be followed in SPL and
further stages for those regions which need to be preserved across
bootstages.

Signed-off-by: Devarsh Thakkar <devarsht@ti.com>
Reviewed-by: Simon Glass <sjg@chromium>

1 files changed, 28 insertions, 0 deletions

diff --git a/doc/develop/spl.rst b/doc/develop/spl.rst
index 814530d3486..0a3e572310a 100644
--- a/doc/develop/spl.rst
+++ b/doc/develop/spl.rst
@@ -173,3 +173,31 @@ 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.
+
+
+Reserving memory in SPL
+-----------------------
+
+If memory needs to be reserved in RAM during SPL stage with the requirement that
+the SPL reserved memory remains preserved across further boot stages too
+then it needs to be reserved mandatorily starting from end of RAM. This is to
+ensure that further stages can simply skip this region before carrying out
+further reservations or updating the relocation address.
+
+Also out of these regions which are to be preserved across further stages of
+boot, video framebuffer memory region must be reserved first starting from
+end of RAM for which helper function spl_reserve_video_from_ram_top is provided
+which makes sure that video memory is placed at top of reservation area with
+further reservations below it.
+
+The corresponding information of reservation for those regions can be passed to
+further boot stages using a bloblist. For e.g. the information for
+framebuffer area reserved by SPL can be passed onto U-boot using
+BLOBLISTT_U_BOOT_VIDEO.
+
+The further boot stages need to parse each of the bloblist passed from SPL stage
+starting from video bloblist and skip this whole SPL reserved memory area from
+end of RAM as per the bloblists received, before carrying out further
+reservations or updating the relocation address. For e.g, U-boot proper uses
+function "setup_relocaddr_from_bloblist" to parse the bloblists passed from
+previous stage and skip the memory reserved from previous stage accordingly.