1.. SPDX-License-Identifier: GPL-2.0
2
3==============================================
4Intel(R) Management Engine (ME) Client bus API
5==============================================
6
7
8Rationale
9=========
10
11The MEI character device is useful for dedicated applications to send and receive
12data to the many FW appliance found in Intel's ME from the user space.
13However, for some of the ME functionalities it makes sense to leverage existing software
14stack and expose them through existing kernel subsystems.
15
16In order to plug seamlessly into the kernel device driver model we add kernel virtual
17bus abstraction on top of the MEI driver. This allows implementing Linux kernel drivers
18for the various MEI features as a stand alone entities found in their respective subsystem.
19Existing device drivers can even potentially be re-used by adding an MEI CL bus layer to
20the existing code.
21
22
23MEI CL bus API
24==============
25
26A driver implementation for an MEI Client is very similar to any other existing bus
27based device drivers. The driver registers itself as an MEI CL bus driver through
28the ``struct mei_cl_driver`` structure defined in :file:`include/linux/mei_cl_bus.c`
29
30.. code-block:: C
31
32        struct mei_cl_driver {
33                struct device_driver driver;
34                const char *name;
35
36                const struct mei_cl_device_id *id_table;
37
38                int (*probe)(struct mei_cl_device *dev, const struct mei_cl_id *id);
39                int (*remove)(struct mei_cl_device *dev);
40        };
41
42
43
44The mei_cl_device_id structure defined in :file:`include/linux/mod_devicetable.h` allows a
45driver to bind itself against a device name.
46
47.. code-block:: C
48
49        struct mei_cl_device_id {
50                char name[MEI_CL_NAME_SIZE];
51                uuid_le uuid;
52                __u8    version;
53                kernel_ulong_t driver_info;
54        };
55
56To actually register a driver on the ME Client bus one must call the :c:func:`mei_cl_add_driver`
57API. This is typically called at module initialization time.
58
59Once the driver is registered and bound to the device, a driver will typically
60try to do some I/O on this bus and this should be done through the :c:func:`mei_cl_send`
61and :c:func:`mei_cl_recv` functions. More detailed information is in :ref:`api` section.
62
63In order for a driver to be notified about pending traffic or event, the driver
64should register a callback via :c:func:`mei_cl_devev_register_rx_cb` and
65:c:func:`mei_cldev_register_notify_cb` function respectively.
66
67.. _api:
68
69API:
70----
71.. kernel-doc:: drivers/misc/mei/bus.c
72    :export: drivers/misc/mei/bus.c
73
74
75
76Example
77=======
78
79As a theoretical example let's pretend the ME comes with a "contact" NFC IP.
80The driver init and exit routines for this device would look like:
81
82.. code-block:: C
83
84        #define CONTACT_DRIVER_NAME "contact"
85
86        static struct mei_cl_device_id contact_mei_cl_tbl[] = {
87                { CONTACT_DRIVER_NAME, },
88
89                /* required last entry */
90                { }
91        };
92        MODULE_DEVICE_TABLE(mei_cl, contact_mei_cl_tbl);
93
94        static struct mei_cl_driver contact_driver = {
95                .id_table = contact_mei_tbl,
96                .name = CONTACT_DRIVER_NAME,
97
98                .probe = contact_probe,
99                .remove = contact_remove,
100        };
101
102        static int contact_init(void)
103        {
104                int r;
105
106                r = mei_cl_driver_register(&contact_driver);
107                if (r) {
108                        pr_err(CONTACT_DRIVER_NAME ": driver registration failed\n");
109                        return r;
110                }
111
112                return 0;
113        }
114
115        static void __exit contact_exit(void)
116        {
117                mei_cl_driver_unregister(&contact_driver);
118        }
119
120        module_init(contact_init);
121        module_exit(contact_exit);
122
123And the driver's simplified probe routine would look like that:
124
125.. code-block:: C
126
127        int contact_probe(struct mei_cl_device *dev, struct mei_cl_device_id *id)
128        {
129                [...]
130                mei_cldev_enable(dev);
131
132                mei_cldev_register_rx_cb(dev, contact_rx_cb);
133
134                return 0;
135        }
136
137In the probe routine the driver first enable the MEI device and then registers
138an rx handler which is as close as it can get to registering a threaded IRQ handler.
139The handler implementation will typically call :c:func:`mei_cldev_recv` and then
140process received data.
141
142.. code-block:: C
143
144        #define MAX_PAYLOAD 128
145        #define HDR_SIZE 4
146        static void conntact_rx_cb(struct mei_cl_device *cldev)
147        {
148                struct contact *c = mei_cldev_get_drvdata(cldev);
149                unsigned char payload[MAX_PAYLOAD];
150                ssize_t payload_sz;
151
152                payload_sz = mei_cldev_recv(cldev, payload,  MAX_PAYLOAD)
153                if (reply_size < HDR_SIZE) {
154                        return;
155                }
156
157                c->process_rx(payload);
158
159        }
160
161MEI Client Bus Drivers
162======================
163
164.. toctree::
165   :maxdepth: 2
166
167   hdcp
168   nfc
169