feat: linux: Add information on REE FS alongside RPMB

Presently, TI's OP-TEE docs speak only of RPMB Secure Storage mechanism,
and not of REE FS. REE FS is supported by-default, so mention it too.

Signed-off-by: Suhaas Joshi <[email protected]>

Author: jsuhaas22

source/linux/Foundational_Components_OPTEE.rst

@@ -75,17 +75,34 @@ of entropy can work around these issues.
 
    $ make CROSS_COMPILE="$CROSS_COMPILE_32" CROSS_COMPILE64="$CROSS_COMPILE_64" PLATFORM=k3-|__OPTEE_PLATFORM_FLAVOR__| CFG_ARM64_core=y CFG_WITH_SOFTWARE_PRNG=y
 
+.. _optee-rpmb-flags:
 
-Secure Storage with RPMB (For HS)
-*********************************
+OP-TEE Secure Storage
+*********************
 
-OP-TEE provides secure storage functionality. TI SoCs with HS configuration have a
-KEK embedded in them that is programmed across OP-TEE instances that are distributed
-in a derived manner. Each HS device has its own HUK signing key (DKEK) which is different
-from other HS devices.
+OP-TEE provides secure storage functionality through two mechanisms:
+**REE FS** (Rich Execution Environment Filesystem) and **RPMB**
+(Replay Protected Memory Block).
 
-For enabling RPMB support along with secure storage, additional flags need to be passed to
-the build instructions. The information for the flags can be found here.
+TI SDK enables REE FS by-default, and configures OP-TEE to store
+encrypted binary blobs created by REE FS in
+:file:`/var/lib/tee/`. The SDK also keeps RPMB disabled.
+
+RPMB works in TI SoCs with HS configuration. These have a KEK embedded
+in them that is programmed across OP-TEE instances that are
+distributed in a derived manner. Each HS device has its own HUK
+signing key (DKEK) which is different from other HS devices.
+
+.. note::
+
+   Presently, AM62L does not support RPMB. This support will be added
+   in subsequent releases. It does support REE FS.
+
+   The remaining devices support both: REE FS by-default and RPMB if
+   OP-TEE binaries are re-compiled with required flags.
+
+For learning more about secure storage in OP-TEE, and instructions to
+enable RPMB, refer:
 https://optee.readthedocs.io/en/latest/architecture/secure_storage.html
 
 There is a hybrid mode in which both the flags i.e `CFG_REE_FS=y` and `CFG_RPMB_FS=y` are enabled.
@@ -100,22 +117,33 @@ E.g. For enabling hybrid mode of RPMB along with REE_FS
 
         $ export CFG_CONSOLE_UART=0x8
 
-.. parsed-literal::
+.. ifconfig:: CONFIG_part_variant not in ('AM62LX')
+
+    .. parsed-literal::
 
-    $ make CROSS_COMPILE64="$CROSS_COMPILE_64" PLATFORM=|__OPTEE_PLATFORM_FLAVOR__| CFG_ARM64_core=y CFG_REE_FS=y CFG_RPMB_FS=y
+        $ make CROSS_COMPILE64="$CROSS_COMPILE_64" PLATFORM=|__OPTEE_PLATFORM_FLAVOR__| CFG_ARM64_core=y CFG_REE_FS=y CFG_RPMB_FS=y
 
-OPTEE-client also needs to be updated to enable the use of real
-emmc instead of the virtual emmc that is enabled by default
+    OPTEE-client also needs to be updated to enable the use of real
+    emmc instead of the virtual emmc that is enabled by default
 
 Getting OP-TEE Client source code
 ---------------------------------
 
+To get optee_client source code, do:
+
+.. rubric:: Getting OP-TEE Client source code
+
 .. code-block:: console
 
   $ git clone https://github.com/OP-TEE/optee_client
 
 .. rubric:: Building OP-TEE Client with RPMB support
 
+To use emulated RPMB, set RPMB_EMU=1. Otherwise, set RPMB_EMU=0.
+
+For example, the following command builds optee_client to use the real RPMB,
+instead of the emulated one.
+
 .. code-block:: console
 
   $ make CROSS_COMPILE="$CROSS_COMPILE_64" PLATFORM=k3 CFG_TEE_SUPP_LOG_LEVEL=2 RPMB_EMU=0 CFG_ARM64_core=y