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Documentation / misc-devices / mei / mei-client-bus.txt




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Based on kernel version 4.0. Page generated on 2015-04-14 21:25 EST.

1	Intel(R) Management Engine (ME) Client bus API
2	==============================================
3	
4	
5	Rationale
6	=========
7	
8	MEI misc character device is useful for dedicated applications to send and receive
9	data to the many FW appliance found in Intel's ME from the user space.
10	However for some of the ME functionalities it make sense to leverage existing software
11	stack and expose them through existing kernel subsystems.
12	
13	In order to plug seamlessly into the kernel device driver model we add kernel virtual
14	bus abstraction on top of the MEI driver. This allows implementing linux kernel drivers
15	for the various MEI features as a stand alone entities found in their respective subsystem.
16	Existing device drivers can even potentially be re-used by adding an MEI CL bus layer to
17	the existing code.
18	
19	
20	MEI CL bus API
21	==============
22	
23	A driver implementation for an MEI Client is very similar to existing bus
24	based device drivers. The driver registers itself as an MEI CL bus driver through
25	the mei_cl_driver structure:
26	
27	struct mei_cl_driver {
28		struct device_driver driver;
29		const char *name;
30	
31		const struct mei_cl_device_id *id_table;
32	
33		int (*probe)(struct mei_cl_device *dev, const struct mei_cl_id *id);
34		int (*remove)(struct mei_cl_device *dev);
35	};
36	
37	struct mei_cl_id {
38		char name[MEI_NAME_SIZE];
39		kernel_ulong_t driver_info;
40	};
41	
42	The mei_cl_id structure allows the driver to bind itself against a device name.
43	
44	To actually register a driver on the ME Client bus one must call the mei_cl_add_driver()
45	API. This is typically called at module init time.
46	
47	Once registered on the ME Client bus, a driver will typically try to do some I/O on
48	this bus and this should be done through the mei_cl_send() and mei_cl_recv()
49	routines. The latter is synchronous (blocks and sleeps until data shows up).
50	In order for drivers to be notified of pending events waiting for them (e.g.
51	an Rx event) they can register an event handler through the
52	mei_cl_register_event_cb() routine. Currently only the MEI_EVENT_RX event
53	will trigger an event handler call and the driver implementation is supposed
54	to call mei_recv() from the event handler in order to fetch the pending
55	received buffers.
56	
57	
58	Example
59	=======
60	
61	As a theoretical example let's pretend the ME comes with a "contact" NFC IP.
62	The driver init and exit routines for this device would look like:
63	
64	#define CONTACT_DRIVER_NAME "contact"
65	
66	static struct mei_cl_device_id contact_mei_cl_tbl[] = {
67		{ CONTACT_DRIVER_NAME, },
68	
69		/* required last entry */
70		{ }
71	};
72	MODULE_DEVICE_TABLE(mei_cl, contact_mei_cl_tbl);
73	
74	static struct mei_cl_driver contact_driver = {
75		.id_table = contact_mei_tbl,
76		.name = CONTACT_DRIVER_NAME,
77	
78		.probe = contact_probe,
79		.remove = contact_remove,
80	};
81	
82	static int contact_init(void)
83	{
84		int r;
85	
86		r = mei_cl_driver_register(&contact_driver);
87		if (r) {
88			pr_err(CONTACT_DRIVER_NAME ": driver registration failed\n");
89			return r;
90		}
91	
92		return 0;
93	}
94	
95	static void __exit contact_exit(void)
96	{
97		mei_cl_driver_unregister(&contact_driver);
98	}
99	
100	module_init(contact_init);
101	module_exit(contact_exit);
102	
103	And the driver's simplified probe routine would look like that:
104	
105	int contact_probe(struct mei_cl_device *dev, struct mei_cl_device_id *id)
106	{
107		struct contact_driver *contact;
108	
109		[...]
110		mei_cl_enable_device(dev);
111	
112		mei_cl_register_event_cb(dev, contact_event_cb, contact);
113	
114		return 0;
115	}
116	
117	In the probe routine the driver first enable the MEI device and then registers
118	an ME bus event handler which is as close as it can get to registering a
119	threaded IRQ handler.
120	The handler implementation will typically call some I/O routine depending on
121	the pending events:
122	
123	#define MAX_NFC_PAYLOAD 128
124	
125	static void contact_event_cb(struct mei_cl_device *dev, u32 events,
126				     void *context)
127	{
128		struct contact_driver *contact = context;
129	
130		if (events & BIT(MEI_EVENT_RX)) {
131			u8 payload[MAX_NFC_PAYLOAD];
132			int payload_size;
133	
134			payload_size = mei_recv(dev, payload, MAX_NFC_PAYLOAD);
135			if (payload_size <= 0)
136				return;
137	
138			/* Hook to the NFC subsystem */
139			nfc_hci_recv_frame(contact->hdev, payload, payload_size);
140		}
141	}
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