Documentation: tee uclass and op-tee driver

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

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+=============
+TEE uclass
+=============
+
+This document describes the TEE uclass in U-Boot
+
+A TEE (Trusted Execution Environment) is a trusted OS running in some
+secure environment, for example, TrustZone on ARM CPUs, or a separate
+secure co-processor etc. A TEE driver handles the details needed to
+communicate with the TEE.
+
+This uclass deals with:
+
+- Registration of TEE drivers
+
+- Managing shared memory between U-Boot and the TEE
+
+- Providing a generic API to the TEE
+
+The TEE interface
+=================
+
+include/tee.h defines the generic interface to a TEE.
+
+A client finds the TEE device via tee_find_device(). Other important functions
+when interfacing with a TEE are:
+
+- tee_shm_alloc(), tee_shm_register() and tee_shm_free() to manage shared
+  memory objects often needed when communicating with the TEE.
+
+- tee_get_version() lets the client know which the capabilities of the TEE
+  device.
+
+- tee_open_session() opens a session to a Trusted Application
+
+- tee_invoke_func() invokes a function in a Trusted Application
+
+- tee_close_session() closes a session to a Trusted Application
+
+Much of the communication between clients and the TEE is opaque to the
+driver. The main job for the driver is to receive requests from the
+clients, forward them to the TEE and send back the results.
+
+OP-TEE driver
+=============
+
+The OP-TEE driver handles OP-TEE [1] based TEEs. Currently it is only the ARM
+TrustZone based OP-TEE solution that is supported.
+
+Lowest level of communication with OP-TEE builds on ARM SMC Calling
+Convention (SMCCC) [2], which is the foundation for OP-TEE's SMC interface
+[3] used internally by the driver. Stacked on top of that is OP-TEE Message
+Protocol [4].
+
+OP-TEE SMC interface provides the basic functions required by SMCCC and some
+additional functions specific for OP-TEE. The most interesting functions are:
+
+- OPTEE_SMC_FUNCID_CALLS_UID (part of SMCCC) returns the version information
+  which is then returned by TEE_IOC_VERSION
+
+- OPTEE_SMC_CALL_GET_OS_UUID returns the particular OP-TEE implementation, used
+  to tell, for instance, a TrustZone OP-TEE apart from an OP-TEE running on a
+  separate secure co-processor.
+
+- OPTEE_SMC_CALL_WITH_ARG drives the OP-TEE message protocol
+
+- OPTEE_SMC_GET_SHM_CONFIG lets the driver and OP-TEE agree on which memory
+  range to used for shared memory between Linux and OP-TEE.
+
+The GlobalPlatform TEE Client API [5] is implemented on top of the generic
+TEE API.
+
+Picture of the relationship between the different components in the
+OP-TEE architecture:
+
+                   U-Boot                  Secure world
+                   ~~~~~~                  ~~~~~~~~~~~~
+                 +------------+           +-------------+
+                 | Client     |           | Trusted     |
+                 |            |           | Application |
+                 +------------+           +-------------+
+                       /\                       /\
+                       ||                       ||
+                       \/                       \/
+                 +------------+           +-------------+
+                 | TEE        |           | TEE Internal|
+                 | uclass     |           | API         |
+                 +------------+           +-------------+
+                 | OP-TEE     |           | OP-TEE      |
+                 | driver     |           | Trusted OS  |
+                 +------------+-----------+-------------+
+                 |             OP-TEE MSG               |
+                 |      SMCCC (OPTEE_SMC_CALL_*)        |
+                 +--------------------------------------+
+
+RPC (Remote Procedure Call) are requests from secure world to the driver.
+An RPC is identified by a special range of SMCCC return values from
+OPTEE_SMC_CALL_WITH_ARG.
+
+References
+==========
+
+[1] https://github.com/OP-TEE/optee_os
+
+[2] http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html
+
+[3] drivers/tee/optee/optee_smc.h
+
+[4] drivers/tee/optee/optee_msg.h
+
+[5] http://www.globalplatform.org/specificationsdevice.asp look for
+    "TEE Client API Specification v1.0" and click download.