1 How to instantiate I2C devices
2 ==============================
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.
11 Method 1a: Declare the I2C devices by bus number
12 ------------------------------------------------
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().
20 Example (from omap2 h4):
22 static struct i2c_board_info h4_i2c_board_info[] __initdata = {
24 I2C_BOARD_INFO("isp1301_omap", 0x2d),
25 .irq = OMAP_GPIO_IRQ(125),
27 { /* EEPROM on mainboard */
28 I2C_BOARD_INFO("24c01", 0x52),
29 .platform_data = &m24c01,
31 { /* EEPROM on cpu card */
32 I2C_BOARD_INFO("24c01", 0x57),
33 .platform_data = &m24c01,
37 static void __init omap_h4_init(void)
40 i2c_register_board_info(1, h4_i2c_board_info,
41 ARRAY_SIZE(h4_i2c_board_info));
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
50 The devices will be automatically unbound and destroyed when the I2C bus
51 they sit on goes away (if ever.)
54 Method 1b: Declare the I2C devices via devicetree
55 -------------------------------------------------
57 This method has the same implications as method 1a. The declaration of I2C
58 devices is here done via devicetree as subnodes of the master controller.
63 /* ... master properties skipped ... */
64 clock-frequency = <100000>;
67 compatible = "atmel,24c256";
72 compatible = "nxp,pca9532";
79 Here, two devices are attached to the bus using a speed of 100kHz. For
80 additional properties which might be needed to set up the device, please refer
81 to its devicetree documentation in Documentation/devicetree/bindings/.
84 Method 2: Instantiate the devices explicitly
85 --------------------------------------------
87 This method is appropriate when a larger device uses an I2C bus for
88 internal communication. A typical case is TV adapters. These can have a
89 tuner, a video decoder, an audio decoder, etc. usually connected to the
90 main chip by the means of an I2C bus. You won't know the number of the I2C
91 bus in advance, so the method 1 described above can't be used. Instead,
92 you can instantiate your I2C devices explicitly. This is done by filling
93 a struct i2c_board_info and calling i2c_new_device().
95 Example (from the sfe4001 network driver):
97 static struct i2c_board_info sfe4001_hwmon_info = {
98 I2C_BOARD_INFO("max6647", 0x4e),
101 int sfe4001_init(struct efx_nic *efx)
104 efx->board_info.hwmon_client =
105 i2c_new_device(&efx->i2c_adap, &sfe4001_hwmon_info);
110 The above code instantiates 1 I2C device on the I2C bus which is on the
111 network adapter in question.
113 A variant of this is when you don't know for sure if an I2C device is
114 present or not (for example for an optional feature which is not present
115 on cheap variants of a board but you have no way to tell them apart), or
116 it may have different addresses from one board to the next (manufacturer
117 changing its design without notice). In this case, you can call
118 i2c_new_probed_device() instead of i2c_new_device().
120 Example (from the nxp OHCI driver):
122 static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
124 static int usb_hcd_nxp_probe(struct platform_device *pdev)
127 struct i2c_adapter *i2c_adap;
128 struct i2c_board_info i2c_info;
131 i2c_adap = i2c_get_adapter(2);
132 memset(&i2c_info, 0, sizeof(struct i2c_board_info));
133 strlcpy(i2c_info.type, "isp1301_nxp", I2C_NAME_SIZE);
134 isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info,
136 i2c_put_adapter(i2c_adap);
140 The above code instantiates up to 1 I2C device on the I2C bus which is on
141 the OHCI adapter in question. It first tries at address 0x2c, if nothing
142 is found there it tries address 0x2d, and if still nothing is found, it
145 The driver which instantiated the I2C device is responsible for destroying
146 it on cleanup. This is done by calling i2c_unregister_device() on the
147 pointer that was earlier returned by i2c_new_device() or
148 i2c_new_probed_device().
151 Method 3: Probe an I2C bus for certain devices
152 ----------------------------------------------
154 Sometimes you do not have enough information about an I2C device, not even
155 to call i2c_new_probed_device(). The typical case is hardware monitoring
156 chips on PC mainboards. There are several dozen models, which can live
157 at 25 different addresses. Given the huge number of mainboards out there,
158 it is next to impossible to build an exhaustive list of the hardware
159 monitoring chips being used. Fortunately, most of these chips have
160 manufacturer and device ID registers, so they can be identified by
163 In that case, I2C devices are neither declared nor instantiated
164 explicitly. Instead, i2c-core will probe for such devices as soon as their
165 drivers are loaded, and if any is found, an I2C device will be
166 instantiated automatically. In order to prevent any misbehavior of this
167 mechanism, the following restrictions apply:
168 * The I2C device driver must implement the detect() method, which
169 identifies a supported device by reading from arbitrary registers.
170 * Only buses which are likely to have a supported device and agree to be
171 probed, will be probed. For example this avoids probing for hardware
172 monitoring chips on a TV adapter.
175 See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
177 I2C devices instantiated as a result of such a successful probe will be
178 destroyed automatically when the driver which detected them is removed,
179 or when the underlying I2C bus is itself destroyed, whichever happens
182 Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6
183 kernels will find out that this method 3 is essentially similar to what
184 was done there. Two significant differences are:
185 * Probing is only one way to instantiate I2C devices now, while it was the
186 only way back then. Where possible, methods 1 and 2 should be preferred.
187 Method 3 should only be used when there is no other way, as it can have
188 undesirable side effects.
189 * I2C buses must now explicitly say which I2C driver classes can probe
190 them (by the means of the class bitfield), while all I2C buses were
191 probed by default back then. The default is an empty class which means
192 that no probing happens. The purpose of the class bitfield is to limit
193 the aforementioned undesirable side effects.
195 Once again, method 3 should be avoided wherever possible. Explicit device
196 instantiation (methods 1 and 2) is much preferred for it is safer and
200 Method 4: Instantiate from user-space
201 -------------------------------------
203 In general, the kernel should know which I2C devices are connected and
204 what addresses they live at. However, in certain cases, it does not, so a
205 sysfs interface was added to let the user provide the information. This
206 interface is made of 2 attribute files which are created in every I2C bus
207 directory: new_device and delete_device. Both files are write only and you
208 must write the right parameters to them in order to properly instantiate,
209 respectively delete, an I2C device.
211 File new_device takes 2 parameters: the name of the I2C device (a string)
212 and the address of the I2C device (a number, typically expressed in
213 hexadecimal starting with 0x, but can also be expressed in decimal.)
215 File delete_device takes a single parameter: the address of the I2C
216 device. As no two devices can live at the same address on a given I2C
217 segment, the address is sufficient to uniquely identify the device to be
221 # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
223 While this interface should only be used when in-kernel device declaration
224 can't be done, there is a variety of cases where it can be helpful:
225 * The I2C driver usually detects devices (method 3 above) but the bus
226 segment your device lives on doesn't have the proper class bit set and
227 thus detection doesn't trigger.
228 * The I2C driver usually detects devices, but your device lives at an
230 * The I2C driver usually detects devices, but your device is not detected,
231 either because the detection routine is too strict, or because your
232 device is not officially supported yet but you know it is compatible.
233 * You are developing a driver on a test board, where you soldered the I2C
236 This interface is a replacement for the force_* module parameters some I2C
237 drivers implement. Being implemented in i2c-core rather than in each
238 device driver individually, it is much more efficient, and also has the
239 advantage that you do not have to reload the driver to change a setting.
240 You can also instantiate the device before the driver is loaded or even
241 available, and you don't need to know what driver the device needs.