i.MX: Add provisions to boot from IRAM

This commit add a very basic code to allow Barebox to be booted from
IRAM. Given that the amount of IRAM on most i.MX variants is
insufficient to contain a copy of Barebox with any reasonable degree
of functionality this code uses IRAM only as a temporary location and
eventually bootstraps from DRAM. But the presense of the intermediate
IRAM-only stage allows to add provisions to test the area of DRAM that
Barebox would be using to facilitate various testing scenarious.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 27 insertions, 0 deletions

diff --git a/Documentation/boards/imx.rst b/Documentation/boards/imx.rst
index 466f69c5ba..ef1a6a507e 100644
--- a/Documentation/boards/imx.rst
+++ b/Documentation/boards/imx.rst
@@ -46,6 +46,33 @@ The images can also always be started second stage::
 
   bootm /mnt/tftp/barebox-freescale-imx51-babbage.img
 
+Internal Boot Mode Through Internal RAM(IRAM)
+---------------------------------------
+
+The Internal Boot Mode Through Internal RAM is supported on:
+
+* i.MX51
+
+As can be easily deduced from its name, the Internal Boot Mode Through
+Internal RAM is just a variant of Internal Boot Mode so all of the
+stated above still applies in this case. What it differs in is the following:
+
+* Boot process is done in two stages(First stage binary can be
+  produced with ``imx_v7-xload_defconfig``)
+* DCD of the first stage image is set such that the image is fetched
+  into an unoccupied area or IRAM
+* First stage image once uncompressed and set up will look for a
+  second stage bootloader on the same media it booted from and start
+  it(see mach-imx/xload.c for more details)
+* Second stage images are just regular i.MX boot images
+
+Since on a typical i.MX SoC unused IRAM area is not enough to run
+anything but a PBL this mode, due to its very limited usability,
+serves only one purpose -- allow for a portion of a bootloader to be
+executed without depending on DRAM to be functional. This peculiarity
+of the mode can be used to implement various memory testing
+scenarious.
+
 USB Boot
 ^^^^^^^^