Documentation: tee: add AMD-TEE driver details

Update tee.txt with AMD-TEE driver details. The driver is written
to communicate with AMD's TEE.

Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Co-developed-by: Devaraj Rangasamy <Devaraj.Rangasamy@amd.com>
Signed-off-by: Devaraj Rangasamy <Devaraj.Rangasamy@amd.com>
Signed-off-by: Rijo Thomas <Rijo-john.Thomas@amd.com>
Reviewed-by: Gary R Hook <gary.hook@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/tee.txt

@@ -112,6 +112,83 @@ kernel are handled by the kernel driver. Other RPC messages will be forwarded to
 tee-supplicant without further involvement of the driver, except switching
 shared memory buffer representation.
 
+AMD-TEE driver
+==============
+
+The AMD-TEE driver handles the communication with AMD's TEE environment. The
+TEE environment is provided by AMD Secure Processor.
+
+The AMD Secure Processor (formerly called Platform Security Processor or PSP)
+is a dedicated processor that features ARM TrustZone technology, along with a
+software-based Trusted Execution Environment (TEE) designed to enable
+third-party Trusted Applications. This feature is currently enabled only for
+APUs.
+
+The following picture shows a high level overview of AMD-TEE::
+
+                                             |
+    x86                                      |
+                                             |
+ User space            (Kernel space)        |    AMD Secure Processor (PSP)
+ ~~~~~~~~~~            ~~~~~~~~~~~~~~        |    ~~~~~~~~~~~~~~~~~~~~~~~~~~
+                                             |
+ +--------+                                  |       +-------------+
+ | Client |                                  |       | Trusted     |
+ +--------+                                  |       | Application |
+     /\                                      |       +-------------+
+     ||                                      |             /\
+     ||                                      |             ||
+     ||                                      |             \/
+     ||                                      |         +----------+
+     ||                                      |         |   TEE    |
+     ||                                      |         | Internal |
+     \/                                      |         |   API    |
+ +---------+           +-----------+---------+         +----------+
+ | TEE     |           | TEE       | AMD-TEE |         | AMD-TEE  |
+ | Client  |           | subsystem | driver  |         | Trusted  |
+ | API     |           |           |         |         |   OS     |
+ +---------+-----------+----+------+---------+---------+----------+
+ |   Generic TEE API        |      | ASP     |      Mailbox       |
+ |   IOCTL (TEE_IOC_*)      |      | driver  | Register Protocol  |
+ +--------------------------+      +---------+--------------------+
+
+At the lowest level (in x86), the AMD Secure Processor (ASP) driver uses the
+CPU to PSP mailbox regsister to submit commands to the PSP. The format of the
+command buffer is opaque to the ASP driver. It's role is to submit commands to
+the secure processor and return results to AMD-TEE driver. The interface
+between AMD-TEE driver and AMD Secure Processor driver can be found in [6].
+
+The AMD-TEE driver packages the command buffer payload for processing in TEE.
+The command buffer format for the different TEE commands can be found in [7].
+
+The TEE commands supported by AMD-TEE Trusted OS are:
+* TEE_CMD_ID_LOAD_TA          - loads a Trusted Application (TA) binary into
+                                TEE environment.
+* TEE_CMD_ID_UNLOAD_TA        - unloads TA binary from TEE environment.
+* TEE_CMD_ID_OPEN_SESSION     - opens a session with a loaded TA.
+* TEE_CMD_ID_CLOSE_SESSION    - closes session with loaded TA
+* TEE_CMD_ID_INVOKE_CMD       - invokes a command with loaded TA
+* TEE_CMD_ID_MAP_SHARED_MEM   - maps shared memory
+* TEE_CMD_ID_UNMAP_SHARED_MEM - unmaps shared memory
+
+AMD-TEE Trusted OS is the firmware running on AMD Secure Processor.
+
+The AMD-TEE driver registers itself with TEE subsystem and implements the
+following driver function callbacks:
+
+* get_version - returns the driver implementation id and capability.
+* open - sets up the driver context data structure.
+* release - frees up driver resources.
+* open_session - loads the TA binary and opens session with loaded TA.
+* close_session -  closes session with loaded TA and unloads it.
+* invoke_func - invokes a command with loaded TA.
+
+cancel_req driver callback is not supported by AMD-TEE.
+
+The GlobalPlatform TEE Client API [5] can be used by the user space (client) to
+talk to AMD's TEE. AMD's TEE provides a secure environment for loading, opening
+a session, invoking commands and clossing session with TA.
+
 References
 ==========
 
@@ -125,3 +202,7 @@ References
 
 [5] http://www.globalplatform.org/specificationsdevice.asp look for
     "TEE Client API Specification v1.0" and click download.
+
+[6] include/linux/psp-tee.h
+
+[7] drivers/tee/amdtee/amdtee_if.h