Documentation: tee: Document TEE kernel interface

Update documentation with TEE bus infrastructure which provides an
interface for kernel client drivers to communicate with corresponding
Trusted Application.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Link: https://lore.kernel.org/r/1591253979-29067-1-git-send-email-sumit.garg@linaro.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/tee.txt

@@ -53,6 +53,66 @@ 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.
 
+The TEE kernel interface
+========================
+
+Kernel provides a TEE bus infrastructure where a Trusted Application is
+represented as a device identified via Universally Unique Identifier (UUID) and
+client drivers register a table of supported device UUIDs.
+
+TEE bus infrastructure registers following APIs:
+-  match(): iterates over the client driver UUID table to find a corresponding
+   match for device UUID. If a match is found, then this particular device is
+   probed via corresponding probe API registered by the client driver. This
+   process happens whenever a device or a client driver is registered with TEE
+   bus.
+-  uevent(): notifies user-space (udev) whenever a new device is registered on
+   TEE bus for auto-loading of modularized client drivers.
+
+TEE bus device enumeration is specific to underlying TEE implementation, so it
+is left open for TEE drivers to provide corresponding implementation.
+
+Then TEE client driver can talk to a matched Trusted Application using APIs
+listed in include/linux/tee_drv.h.
+
+TEE client driver example
+-------------------------
+
+Suppose a TEE client driver needs to communicate with a Trusted Application
+having UUID: ``ac6a4085-0e82-4c33-bf98-8eb8e118b6c2``, so driver registration
+snippet would look like::
+
+	static const struct tee_client_device_id client_id_table[] = {
+		{UUID_INIT(0xac6a4085, 0x0e82, 0x4c33,
+			   0xbf, 0x98, 0x8e, 0xb8, 0xe1, 0x18, 0xb6, 0xc2)},
+		{}
+	};
+
+	MODULE_DEVICE_TABLE(tee, client_id_table);
+
+	static struct tee_client_driver client_driver = {
+		.id_table	= client_id_table,
+		.driver		= {
+			.name		= DRIVER_NAME,
+			.bus		= &tee_bus_type,
+			.probe		= client_probe,
+			.remove		= client_remove,
+		},
+	};
+
+	static int __init client_init(void)
+	{
+		return driver_register(&client_driver.driver);
+	}
+
+	static void __exit client_exit(void)
+	{
+		driver_unregister(&client_driver.driver);
+	}
+
+	module_init(client_init);
+	module_exit(client_exit);
+
 OP-TEE driver
 =============
 
@@ -112,6 +172,14 @@ 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.
 
+OP-TEE device enumeration
+-------------------------
+
+OP-TEE provides a pseudo Trusted Application: drivers/tee/optee/device.c in
+order to support device enumeration. In other words, OP-TEE driver invokes this
+application to retrieve a list of Trusted Applications which can be registered
+as devices on the TEE bus.
+
 AMD-TEE driver
 ==============