[PATCH 4/4] user: add documentation for OP-TEE loading

[Rouven Czerwinski <r.czerwinski@pengutronix.de>]

Some rudimentary documentation how to load OP-TEE.

Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
---
 Documentation/user/optee.rst       | 29 +++++++++++++++++++++++++++++
 Documentation/user/user-manual.rst |  1 +
 2 files changed, 30 insertions(+)
 create mode 100644 Documentation/user/optee.rst

diff --git a/Documentation/user/optee.rst b/Documentation/user/optee.rst
new file mode 100644
index 0000000000..950917b446
--- /dev/null
+++ b/Documentation/user/optee.rst
@@ -0,0 +1,29 @@
+
+.. _optee:
+
+OP-TEE
+======
+
+Barebox is able to start the Open Portable Trusted Execution Environment
+(OP-TEE) either before starting the linux kernel or during lowlevel board
+initialization in the Pre Bootloader ``PBL``.
+
+Before Linux start
+------------------
+Enable the `CONFIG_BOOTM_OPTEE` configuration variable and configure the
+`CONFIG_OPTEE_SIZE` variable. This will reserve a memory area at the end
+of memory for OP-TEE to run, usually Barebox would relocate itself there. To
+load OP-TEE before the kernel is started, configure the global ``bootm.tee``
+variable to point to a valid OPTEE v1 binary.
+
+During the PBL
+--------------
+To start OP-TEE during the lowlevel initialization of your board in the ``PBL``,
+enable the ``CONFIG_PBL_OPTEE`` configuration variable. your board should then
+call the function ``start_optee_early(void* tee, void* fdt)`` with a valid tee
+and FDT. Ensure that your OP-TEE is compiled with ``CFG_NS_ENTRY_ADDR`` unset,
+otherwise OP-TEE will not correctly return to barebox after startup.
+Since OP-TEE in the default configuration also modifies the device tree, don't
+pass the barebox internal device tree, instead copy it into a different memory
+location and pass it to OP-TEE afterwards.
+The modified device tree can then be passed to the main barebox start function.
diff --git a/Documentation/user/user-manual.rst b/Documentation/user/user-manual.rst
index 41fdb8805c..827683eaa0 100644
--- a/Documentation/user/user-manual.rst
+++ b/Documentation/user/user-manual.rst
@@ -33,6 +33,7 @@ Contents:
    system-reset
    state
    random
+   optee
    debugging
    watchdog
 
-- 
2.25.0


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