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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

1	How to instantiate I2C devices
2	==============================
3	
4	Unlike PCI or USB devices, I2C devices are not enumerated at the hardware
5	level. Instead, the software must know which devices are connected on each
6	I2C bus segment, and what address these devices are using. For this
7	reason, the kernel code must instantiate I2C devices explicitly. There are
8	several ways to achieve this, depending on the context and requirements.
9	
10	
11	Method 1: Declare the I2C devices by bus number
12	-----------------------------------------------
13	
14	This method is appropriate when the I2C bus is a system bus as is the case
15	for many embedded systems. On such systems, each I2C bus has a number
16	which is known in advance. It is thus possible to pre-declare the I2C
17	devices which live on this bus. This is done with an array of struct
18	i2c_board_info which is registered by calling i2c_register_board_info().
19	
20	Example (from omap2 h4):
21	
22	static struct i2c_board_info h4_i2c_board_info[] __initdata = {
23		{
24			I2C_BOARD_INFO("isp1301_omap", 0x2d),
25			.irq		= OMAP_GPIO_IRQ(125),
26		},
27		{	/* EEPROM on mainboard */
28			I2C_BOARD_INFO("24c01", 0x52),
29			.platform_data	= &m24c01,
30		},
31		{	/* EEPROM on cpu card */
32			I2C_BOARD_INFO("24c01", 0x57),
33			.platform_data	= &m24c01,
34		},
35	};
36	
37	static void __init omap_h4_init(void)
38	{
39		(...)
40		i2c_register_board_info(1, h4_i2c_board_info,
41				ARRAY_SIZE(h4_i2c_board_info));
42		(...)
43	}
44	
45	The above code declares 3 devices on I2C bus 1, including their respective
46	addresses and custom data needed by their drivers. When the I2C bus in
47	question is registered, the I2C devices will be instantiated automatically
48	by i2c-core.
49	
50	The devices will be automatically unbound and destroyed when the I2C bus
51	they sit on goes away (if ever.)
52	
53	
54	Method 2: Instantiate the devices explicitly
55	--------------------------------------------
56	
57	This method is appropriate when a larger device uses an I2C bus for
58	internal communication. A typical case is TV adapters. These can have a
59	tuner, a video decoder, an audio decoder, etc. usually connected to the
60	main chip by the means of an I2C bus. You won't know the number of the I2C
61	bus in advance, so the method 1 described above can't be used. Instead,
62	you can instantiate your I2C devices explicitly. This is done by filling
63	a struct i2c_board_info and calling i2c_new_device().
64	
65	Example (from the sfe4001 network driver):
66	
67	static struct i2c_board_info sfe4001_hwmon_info = {
68		I2C_BOARD_INFO("max6647", 0x4e),
69	};
70	
71	int sfe4001_init(struct efx_nic *efx)
72	{
73		(...)
74		efx->board_info.hwmon_client =
75			i2c_new_device(&efx->i2c_adap, &sfe4001_hwmon_info);
76	
77		(...)
78	}
79	
80	The above code instantiates 1 I2C device on the I2C bus which is on the
81	network adapter in question.
82	
83	A variant of this is when you don't know for sure if an I2C device is
84	present or not (for example for an optional feature which is not present
85	on cheap variants of a board but you have no way to tell them apart), or
86	it may have different addresses from one board to the next (manufacturer
87	changing its design without notice). In this case, you can call
88	i2c_new_probed_device() instead of i2c_new_device().
89	
90	Example (from the nxp OHCI driver):
91	
92	static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
93	
94	static int usb_hcd_nxp_probe(struct platform_device *pdev)
95	{
96		(...)
97		struct i2c_adapter *i2c_adap;
98		struct i2c_board_info i2c_info;
99	
100		(...)
101		i2c_adap = i2c_get_adapter(2);
102		memset(&i2c_info, 0, sizeof(struct i2c_board_info));
103		strlcpy(i2c_info.type, "isp1301_nxp", I2C_NAME_SIZE);
104		isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info,
105							   normal_i2c, NULL);
106		i2c_put_adapter(i2c_adap);
107		(...)
108	}
109	
110	The above code instantiates up to 1 I2C device on the I2C bus which is on
111	the OHCI adapter in question. It first tries at address 0x2c, if nothing
112	is found there it tries address 0x2d, and if still nothing is found, it
113	simply gives up.
114	
115	The driver which instantiated the I2C device is responsible for destroying
116	it on cleanup. This is done by calling i2c_unregister_device() on the
117	pointer that was earlier returned by i2c_new_device() or
118	i2c_new_probed_device().
119	
120	
121	Method 3: Probe an I2C bus for certain devices
122	----------------------------------------------
123	
124	Sometimes you do not have enough information about an I2C device, not even
125	to call i2c_new_probed_device(). The typical case is hardware monitoring
126	chips on PC mainboards. There are several dozen models, which can live
127	at 25 different addresses. Given the huge number of mainboards out there,
128	it is next to impossible to build an exhaustive list of the hardware
129	monitoring chips being used. Fortunately, most of these chips have
130	manufacturer and device ID registers, so they can be identified by
131	probing.
132	
133	In that case, I2C devices are neither declared nor instantiated
134	explicitly. Instead, i2c-core will probe for such devices as soon as their
135	drivers are loaded, and if any is found, an I2C device will be
136	instantiated automatically. In order to prevent any misbehavior of this
137	mechanism, the following restrictions apply:
138	* The I2C device driver must implement the detect() method, which
139	  identifies a supported device by reading from arbitrary registers.
140	* Only buses which are likely to have a supported device and agree to be
141	  probed, will be probed. For example this avoids probing for hardware
142	  monitoring chips on a TV adapter.
143	
144	Example:
145	See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
146	
147	I2C devices instantiated as a result of such a successful probe will be
148	destroyed automatically when the driver which detected them is removed,
149	or when the underlying I2C bus is itself destroyed, whichever happens
150	first.
151	
152	Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6
153	kernels will find out that this method 3 is essentially similar to what
154	was done there. Two significant differences are:
155	* Probing is only one way to instantiate I2C devices now, while it was the
156	  only way back then. Where possible, methods 1 and 2 should be preferred.
157	  Method 3 should only be used when there is no other way, as it can have
158	  undesirable side effects.
159	* I2C buses must now explicitly say which I2C driver classes can probe
160	  them (by the means of the class bitfield), while all I2C buses were
161	  probed by default back then. The default is an empty class which means
162	  that no probing happens. The purpose of the class bitfield is to limit
163	  the aforementioned undesirable side effects.
164	
165	Once again, method 3 should be avoided wherever possible. Explicit device
166	instantiation (methods 1 and 2) is much preferred for it is safer and
167	faster.
168	
169	
170	Method 4: Instantiate from user-space
171	-------------------------------------
172	
173	In general, the kernel should know which I2C devices are connected and
174	what addresses they live at. However, in certain cases, it does not, so a
175	sysfs interface was added to let the user provide the information. This
176	interface is made of 2 attribute files which are created in every I2C bus
177	directory: new_device and delete_device. Both files are write only and you
178	must write the right parameters to them in order to properly instantiate,
179	respectively delete, an I2C device.
180	
181	File new_device takes 2 parameters: the name of the I2C device (a string)
182	and the address of the I2C device (a number, typically expressed in
183	hexadecimal starting with 0x, but can also be expressed in decimal.)
184	
185	File delete_device takes a single parameter: the address of the I2C
186	device. As no two devices can live at the same address on a given I2C
187	segment, the address is sufficient to uniquely identify the device to be
188	deleted.
189	
190	Example:
191	# echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
192	
193	While this interface should only be used when in-kernel device declaration
194	can't be done, there is a variety of cases where it can be helpful:
195	* The I2C driver usually detects devices (method 3 above) but the bus
196	  segment your device lives on doesn't have the proper class bit set and
197	  thus detection doesn't trigger.
198	* The I2C driver usually detects devices, but your device lives at an
199	  unexpected address.
200	* The I2C driver usually detects devices, but your device is not detected,
201	  either because the detection routine is too strict, or because your
202	  device is not officially supported yet but you know it is compatible.
203	* You are developing a driver on a test board, where you soldered the I2C
204	  device yourself.
205	
206	This interface is a replacement for the force_* module parameters some I2C
207	drivers implement. Being implemented in i2c-core rather than in each
208	device driver individually, it is much more efficient, and also has the
209	advantage that you do not have to reload the driver to change a setting.
210	You can also instantiate the device before the driver is loaded or even
211	available, and you don't need to know what driver the device needs.
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