[PATCH v8 4/4] Documentation: tee subsystem and op-tee driver

[Jens Wiklander <jens.wiklander-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

Signed-off-by: Jens Wiklander <jens.wiklander-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
---
 Documentation/00-INDEX |   2 +
 Documentation/tee.txt  | 117 +++++++++++++++++++++++++++++++++++++++++++++++++
 MAINTAINERS            |   1 +
 3 files changed, 120 insertions(+)
 create mode 100644 Documentation/tee.txt

diff --git a/Documentation/00-INDEX b/Documentation/00-INDEX
index cd077ca..bd3f803 100644
--- a/Documentation/00-INDEX
+++ b/Documentation/00-INDEX
@@ -435,6 +435,8 @@ sysrq.txt
 	- info on the magic SysRq key.
 target/
 	- directory with info on generating TCM v4 fabric .ko modules
+tee.txt
+	- info on the TEE subsystem and drivers
 this_cpu_ops.txt
 	- List rationale behind and the way to use this_cpu operations.
 thermal/
diff --git a/Documentation/tee.txt b/Documentation/tee.txt
new file mode 100644
index 0000000..4ac91d2
--- /dev/null
+++ b/Documentation/tee.txt
@@ -0,0 +1,117 @@
+TEE subsystem
+This document describes the TEE subsystem in Linux
+
+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 subsystem deals with:
+
+- Registration of TEE drivers
+
+- Managing shared memory between Linux and the TEE
+
+- Providing a generic API to the TEE
+
+The TEE interface
+=================
+
+include/uapi/linux/tee.h defines the generic interface to a TEE.
+
+User space (the client) connects to the driver by opening /dev/tee[0-9]* or
+/dev/teepriv[0-9]*.
+
+- TEE_IOC_SHM_ALLOC allocates shared memory and returns a file descriptor which
+  user space can mmap. When user space doesn't need the file descriptor anymore
+  it should be closed. When shared memory isn't needed any longer it should be
+  unmapped with munmap() to allow the reuse of memory.
+
+- TEE_IOC_VERSION lets user space know which TEE this driver handles and
+  the its capabilities.
+
+- TEE_IOC_OPEN_SESSION opens a new session to a Trusted Application
+
+- TEE_IOC_INVOKE invokes a function in a Trusted Application
+
+- TEE_IOC_CANCEL may cancel an ongoing TEE_IOC_OPEN_SESSION or TEE_IOC_INVOKE
+
+- TEE_IOC_CLOSE_SESSION closes a session to a Trusted Application
+
+There are two classes of clients, normal clients and supplicants. The latter is
+a helper process for the TEE to access resources in Linux, for example file
+system access. A normal client opens /dev/tee[0-9]* and a supplicant opens
+/dev/teepriv[0-9].
+
+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. In the case of
+supplicants the communication goes in the other direction, the TEE sends
+requests to the supplicant which then sends back the result.
+
+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.
+
+    User space                  Kernel                   Secure world
+    ~~~~~~~~~~                  ~~~~~~                   ~~~~~~~~~~~~
+ +--------+                                             +-------------+
+ | Client |                                             | Trusted     |
+ +--------+                                             | Application |
+    /\                                                  +-------------+
+    || +----------+                                           /\
+    || |tee-      |                                           ||
+    || |supplicant|                                           \/
+    || +----------+                                     +-------------+
+    \/      /\                                          | TEE Internal|
+ +-------+  ||                                          | API         |
+ + TEE   |  ||            +--------+--------+           +-------------+
+ | Client|  ||            | TEE    | OP-TEE |           | OP-TEE      |
+ | API   |  \/            | subsys | driver |           | Trusted OS  |
+ +-------+----------------+----+-------+----+-----------+-------------+
+ |      Generic TEE API        |       |     OP-TEE MSG               |
+ |      IOCTL (TEE_IOC_*)      |       |     SMCCC (OPTEE_SMC_CALL_*) |
+ +-----------------------------+       +------------------------------+
+
+RPC (Remote Procedure Call) are requests from secure world to kernel driver
+or tee-supplicant. An RPC is identified by a special range of SMCCC return
+values from OPTEE_SMC_CALL_WITH_ARG. RPC messages which are intended for the
+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.
+
+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.
diff --git a/MAINTAINERS b/MAINTAINERS
index f13414a..29af59a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9572,6 +9572,7 @@ S:	Maintained
 F:	include/linux/tee_drv.h
 F:	include/uapi/linux/tee.h
 F:	drivers/tee/
+F:	Documentation/tee.txt
 
 THUNDERBOLT DRIVER
 M:	Andreas Noever <andreas.noever-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
-- 
1.9.1

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