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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.