1 ==============================
2 How to instantiate I2C devices
3 ==============================
5 Unlike PCI or USB devices, I2C devices are not enumerated at the hardware
6 level. Instead, the software must know which devices are connected on each
7 I2C bus segment, and what address these devices are using. For this
8 reason, the kernel code must instantiate I2C devices explicitly. There are
9 several ways to achieve this, depending on the context and requirements.
12 Method 1: Declare the I2C devices statically
13 --------------------------------------------
15 This method is appropriate when the I2C bus is a system bus as is the case
16 for many embedded systems. On such systems, each I2C bus has a number which
17 is known in advance. It is thus possible to pre-declare the I2C devices
18 which live on this bus.
20 This information is provided to the kernel in a different way on different
21 architectures: device tree, ACPI or board files.
23 When the I2C bus in question is registered, the I2C devices will be
24 instantiated automatically by i2c-core. The devices will be automatically
25 unbound and destroyed when the I2C bus they sit on goes away (if ever).
28 Declare the I2C devices via devicetree
29 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
31 On platforms using devicetree, the declaration of I2C devices is done in
32 subnodes of the master controller.
37 /* ... master properties skipped ... */
38 clock-frequency = <100000>;
41 compatible = "atmel,24c256";
46 compatible = "nxp,pca9532";
53 Here, two devices are attached to the bus using a speed of 100kHz. For
54 additional properties which might be needed to set up the device, please refer
55 to its devicetree documentation in Documentation/devicetree/bindings/.
58 Declare the I2C devices via ACPI
59 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
61 ACPI can also describe I2C devices. There is special documentation for this
62 which is currently located at Documentation/firmware-guide/acpi/enumeration.rst.
65 Declare the I2C devices in board files
66 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
68 In many embedded architectures, devicetree has replaced the old hardware
69 description based on board files, but the latter are still used in old
70 code. Instantiating I2C devices via board files is done with an array of
71 struct i2c_board_info which is registered by calling
72 i2c_register_board_info().
74 Example (from omap2 h4)::
76 static struct i2c_board_info h4_i2c_board_info[] __initdata = {
78 I2C_BOARD_INFO("isp1301_omap", 0x2d),
79 .irq = OMAP_GPIO_IRQ(125),
81 { /* EEPROM on mainboard */
82 I2C_BOARD_INFO("24c01", 0x52),
83 .platform_data = &m24c01,
85 { /* EEPROM on cpu card */
86 I2C_BOARD_INFO("24c01", 0x57),
87 .platform_data = &m24c01,
91 static void __init omap_h4_init(void)
94 i2c_register_board_info(1, h4_i2c_board_info,
95 ARRAY_SIZE(h4_i2c_board_info));
99 The above code declares 3 devices on I2C bus 1, including their respective
100 addresses and custom data needed by their drivers.
103 Method 2: Instantiate the devices explicitly
104 --------------------------------------------
106 This method is appropriate when a larger device uses an I2C bus for
107 internal communication. A typical case is TV adapters. These can have a
108 tuner, a video decoder, an audio decoder, etc. usually connected to the
109 main chip by the means of an I2C bus. You won't know the number of the I2C
110 bus in advance, so the method 1 described above can't be used. Instead,
111 you can instantiate your I2C devices explicitly. This is done by filling
112 a struct i2c_board_info and calling i2c_new_client_device().
114 Example (from the sfe4001 network driver)::
116 static struct i2c_board_info sfe4001_hwmon_info = {
117 I2C_BOARD_INFO("max6647", 0x4e),
120 int sfe4001_init(struct efx_nic *efx)
123 efx->board_info.hwmon_client =
124 i2c_new_client_device(&efx->i2c_adap, &sfe4001_hwmon_info);
129 The above code instantiates 1 I2C device on the I2C bus which is on the
130 network adapter in question.
132 A variant of this is when you don't know for sure if an I2C device is
133 present or not (for example for an optional feature which is not present
134 on cheap variants of a board but you have no way to tell them apart), or
135 it may have different addresses from one board to the next (manufacturer
136 changing its design without notice). In this case, you can call
137 i2c_new_scanned_device() instead of i2c_new_client_device().
139 Example (from the nxp OHCI driver)::
141 static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
143 static int usb_hcd_nxp_probe(struct platform_device *pdev)
146 struct i2c_adapter *i2c_adap;
147 struct i2c_board_info i2c_info;
150 i2c_adap = i2c_get_adapter(2);
151 memset(&i2c_info, 0, sizeof(struct i2c_board_info));
152 strscpy(i2c_info.type, "isp1301_nxp", sizeof(i2c_info.type));
153 isp1301_i2c_client = i2c_new_scanned_device(i2c_adap, &i2c_info,
155 i2c_put_adapter(i2c_adap);
159 The above code instantiates up to 1 I2C device on the I2C bus which is on
160 the OHCI adapter in question. It first tries at address 0x2c, if nothing
161 is found there it tries address 0x2d, and if still nothing is found, it
164 The driver which instantiated the I2C device is responsible for destroying
165 it on cleanup. This is done by calling i2c_unregister_device() on the
166 pointer that was earlier returned by i2c_new_client_device() or
167 i2c_new_scanned_device().
170 Method 3: Probe an I2C bus for certain devices
171 ----------------------------------------------
173 Sometimes you do not have enough information about an I2C device, not even
174 to call i2c_new_scanned_device(). The typical case is hardware monitoring
175 chips on PC mainboards. There are several dozen models, which can live
176 at 25 different addresses. Given the huge number of mainboards out there,
177 it is next to impossible to build an exhaustive list of the hardware
178 monitoring chips being used. Fortunately, most of these chips have
179 manufacturer and device ID registers, so they can be identified by
182 In that case, I2C devices are neither declared nor instantiated
183 explicitly. Instead, i2c-core will probe for such devices as soon as their
184 drivers are loaded, and if any is found, an I2C device will be
185 instantiated automatically. In order to prevent any misbehavior of this
186 mechanism, the following restrictions apply:
188 * The I2C device driver must implement the detect() method, which
189 identifies a supported device by reading from arbitrary registers.
190 * Only buses which are likely to have a supported device and agree to be
191 probed, will be probed. For example this avoids probing for hardware
192 monitoring chips on a TV adapter.
195 See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
197 I2C devices instantiated as a result of such a successful probe will be
198 destroyed automatically when the driver which detected them is removed,
199 or when the underlying I2C bus is itself destroyed, whichever happens
202 Those of you familiar with the I2C subsystem of 2.4 kernels and early 2.6
203 kernels will find out that this method 3 is essentially similar to what
204 was done there. Two significant differences are:
206 * Probing is only one way to instantiate I2C devices now, while it was the
207 only way back then. Where possible, methods 1 and 2 should be preferred.
208 Method 3 should only be used when there is no other way, as it can have
209 undesirable side effects.
210 * I2C buses must now explicitly say which I2C driver classes can probe
211 them (by the means of the class bitfield), while all I2C buses were
212 probed by default back then. The default is an empty class which means
213 that no probing happens. The purpose of the class bitfield is to limit
214 the aforementioned undesirable side effects.
216 Once again, method 3 should be avoided wherever possible. Explicit device
217 instantiation (methods 1 and 2) is much preferred for it is safer and
221 Method 4: Instantiate from user-space
222 -------------------------------------
224 In general, the kernel should know which I2C devices are connected and
225 what addresses they live at. However, in certain cases, it does not, so a
226 sysfs interface was added to let the user provide the information. This
227 interface is made of 2 attribute files which are created in every I2C bus
228 directory: ``new_device`` and ``delete_device``. Both files are write
229 only and you must write the right parameters to them in order to properly
230 instantiate, respectively delete, an I2C device.
232 File ``new_device`` takes 2 parameters: the name of the I2C device (a
233 string) and the address of the I2C device (a number, typically expressed
234 in hexadecimal starting with 0x, but can also be expressed in decimal.)
236 File ``delete_device`` takes a single parameter: the address of the I2C
237 device. As no two devices can live at the same address on a given I2C
238 segment, the address is sufficient to uniquely identify the device to be
243 # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
245 While this interface should only be used when in-kernel device declaration
246 can't be done, there is a variety of cases where it can be helpful:
248 * The I2C driver usually detects devices (method 3 above) but the bus
249 segment your device lives on doesn't have the proper class bit set and
250 thus detection doesn't trigger.
251 * The I2C driver usually detects devices, but your device lives at an
253 * The I2C driver usually detects devices, but your device is not detected,
254 either because the detection routine is too strict, or because your
255 device is not officially supported yet but you know it is compatible.
256 * You are developing a driver on a test board, where you soldered the I2C
259 This interface is a replacement for the force_* module parameters some I2C
260 drivers implement. Being implemented in i2c-core rather than in each
261 device driver individually, it is much more efficient, and also has the
262 advantage that you do not have to reload the driver to change a setting.
263 You can also instantiate the device before the driver is loaded or even
264 available, and you don't need to know what driver the device needs.