Root/Documentation/i2c/instantiating-devices

1How to instantiate I2C devices
2==============================
3
4Unlike PCI or USB devices, I2C devices are not enumerated at the hardware
5level. Instead, the software must know which devices are connected on each
6I2C bus segment, and what address these devices are using. For this
7reason, the kernel code must instantiate I2C devices explicitly. There are
8several ways to achieve this, depending on the context and requirements.
9
10
11Method 1: Declare the I2C devices by bus number
12-----------------------------------------------
13
14This method is appropriate when the I2C bus is a system bus as is the case
15for many embedded systems. On such systems, each I2C bus has a number
16which is known in advance. It is thus possible to pre-declare the I2C
17devices which live on this bus. This is done with an array of struct
18i2c_board_info which is registered by calling i2c_register_board_info().
19
20Example (from omap2 h4):
21
22static struct i2c_board_info __initdata h4_i2c_board_info[] = {
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
37static 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
45The above code declares 3 devices on I2C bus 1, including their respective
46addresses and custom data needed by their drivers. When the I2C bus in
47question is registered, the I2C devices will be instantiated automatically
48by i2c-core.
49
50The devices will be automatically unbound and destroyed when the I2C bus
51they sit on goes away (if ever.)
52
53
54Method 2: Instantiate the devices explicitly
55--------------------------------------------
56
57This method is appropriate when a larger device uses an I2C bus for
58internal communication. A typical case is TV adapters. These can have a
59tuner, a video decoder, an audio decoder, etc. usually connected to the
60main chip by the means of an I2C bus. You won't know the number of the I2C
61bus in advance, so the method 1 described above can't be used. Instead,
62you can instantiate your I2C devices explicitly. This is done by filling
63a struct i2c_board_info and calling i2c_new_device().
64
65Example (from the sfe4001 network driver):
66
67static struct i2c_board_info sfe4001_hwmon_info = {
68    I2C_BOARD_INFO("max6647", 0x4e),
69};
70
71int 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
80The above code instantiates 1 I2C device on the I2C bus which is on the
81network adapter in question.
82
83A variant of this is when you don't know for sure if an I2C device is
84present or not (for example for an optional feature which is not present
85on cheap variants of a board but you have no way to tell them apart), or
86it may have different addresses from one board to the next (manufacturer
87changing its design without notice). In this case, you can call
88i2c_new_probed_device() instead of i2c_new_device().
89
90Example (from the pnx4008 OHCI driver):
91
92static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
93
94static int __devinit usb_hcd_pnx4008_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.name, "isp1301_pnx", 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
110The above code instantiates up to 1 I2C device on the I2C bus which is on
111the OHCI adapter in question. It first tries at address 0x2c, if nothing
112is found there it tries address 0x2d, and if still nothing is found, it
113simply gives up.
114
115The driver which instantiated the I2C device is responsible for destroying
116it on cleanup. This is done by calling i2c_unregister_device() on the
117pointer that was earlier returned by i2c_new_device() or
118i2c_new_probed_device().
119
120
121Method 3: Probe an I2C bus for certain devices
122----------------------------------------------
123
124Sometimes you do not have enough information about an I2C device, not even
125to call i2c_new_probed_device(). The typical case is hardware monitoring
126chips on PC mainboards. There are several dozen models, which can live
127at 25 different addresses. Given the huge number of mainboards out there,
128it is next to impossible to build an exhaustive list of the hardware
129monitoring chips being used. Fortunately, most of these chips have
130manufacturer and device ID registers, so they can be identified by
131probing.
132
133In that case, I2C devices are neither declared nor instantiated
134explicitly. Instead, i2c-core will probe for such devices as soon as their
135drivers are loaded, and if any is found, an I2C device will be
136instantiated automatically. In order to prevent any misbehavior of this
137mechanism, 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
144Example:
145See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
146
147I2C devices instantiated as a result of such a successful probe will be
148destroyed automatically when the driver which detected them is removed,
149or when the underlying I2C bus is itself destroyed, whichever happens
150first.
151
152Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6
153kernels will find out that this method 3 is essentially similar to what
154was 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
165Once again, method 3 should be avoided wherever possible. Explicit device
166instantiation (methods 1 and 2) is much preferred for it is safer and
167faster.
168
169
170Method 4: Instantiate from user-space
171-------------------------------------
172
173In general, the kernel should know which I2C devices are connected and
174what addresses they live at. However, in certain cases, it does not, so a
175sysfs interface was added to let the user provide the information. This
176interface is made of 2 attribute files which are created in every I2C bus
177directory: new_device and delete_device. Both files are write only and you
178must write the right parameters to them in order to properly instantiate,
179respectively delete, an I2C device.
180
181File new_device takes 2 parameters: the name of the I2C device (a string)
182and the address of the I2C device (a number, typically expressed in
183hexadecimal starting with 0x, but can also be expressed in decimal.)
184
185File delete_device takes a single parameter: the address of the I2C
186device. As no two devices can live at the same address on a given I2C
187segment, the address is sufficient to uniquely identify the device to be
188deleted.
189
190Example:
191# echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
192
193While this interface should only be used when in-kernel device declaration
194can'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
206This interface is a replacement for the force_* module parameters some I2C
207drivers implement. Being implemented in i2c-core rather than in each
208device driver individually, it is much more efficient, and also has the
209advantage that you do not have to reload the driver to change a setting.
210You can also instantiate the device before the driver is loaded or even
211available, and you don't need to know what driver the device needs.
212

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