1 ==============================
2 How to instantiate I2C devices
3 ==============================
4
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.
10
11
12 Method 1: Declare the I2C devices statically
13 --------------------------------------------
14
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.
19
20 This information is provided to the kernel in a different way on different
21 architectures: device tree, ACPI or board files.
22
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).
26
27
28 Declare the I2C devices via devicetree
29 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
30
31 On platforms using devicetree, the declaration of I2C devices is done in
32 subnodes of the master controller.
33
34 Example:
35
36 .. code-block:: dts
37
38 i2c1: i2c@400a0000 {
39 /* ... master properties skipped ... */
40 clock-frequency = <100000>;
41
42 flash@50 {
43 compatible = "atmel,24c256";
44 reg = <0x50>;
45 };
46
47 pca9532: gpio@60 {
48 compatible = "nxp,pca9532";
49 gpio-controller;
50 #gpio-cells = <2>;
51 reg = <0x60>;
52 };
53 };
54
55 Here, two devices are attached to the bus using a speed of 100kHz. For
56 additional properties which might be needed to set up the device, please refer
57 to its devicetree documentation in Documentation/devicetree/bindings/.
58
59
60 Declare the I2C devices via ACPI
61 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
62
63 ACPI can also describe I2C devices. There is special documentation for this
64 which is currently located at Documentation/firmware-guide/acpi/enumeration.rst.
65
66
67 Declare the I2C devices in board files
68 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
69
70 In many embedded architectures, devicetree has replaced the old hardware
71 description based on board files, but the latter are still used in old
72 code. Instantiating I2C devices via board files is done with an array of
73 struct i2c_board_info which is registered by calling
74 i2c_register_board_info().
75
76 Example (from omap2 h4):
77
78 .. code-block:: c
79
80 static struct i2c_board_info h4_i2c_board_info[] __initdata = {
81 {
82 I2C_BOARD_INFO("isp1301_omap", 0x2d),
83 .irq = OMAP_GPIO_IRQ(125),
84 },
85 { /* EEPROM on mainboard */
86 I2C_BOARD_INFO("24c01", 0x52),
87 .platform_data = &m24c01,
88 },
89 { /* EEPROM on cpu card */
90 I2C_BOARD_INFO("24c01", 0x57),
91 .platform_data = &m24c01,
92 },
93 };
94
95 static void __init omap_h4_init(void)
96 {
97 (...)
98 i2c_register_board_info(1, h4_i2c_board_info,
99 ARRAY_SIZE(h4_i2c_board_info));
100 (...)
101 }
102
103 The above code declares 3 devices on I2C bus 1, including their respective
104 addresses and custom data needed by their drivers.
105
106
107 Method 2: Instantiate the devices explicitly
108 --------------------------------------------
109
110 This method is appropriate when a larger device uses an I2C bus for
111 internal communication. A typical case is TV adapters. These can have a
112 tuner, a video decoder, an audio decoder, etc. usually connected to the
113 main chip by the means of an I2C bus. You won't know the number of the I2C
114 bus in advance, so the method 1 described above can't be used. Instead,
115 you can instantiate your I2C devices explicitly. This is done by filling
116 a struct i2c_board_info and calling i2c_new_client_device().
117
118 Example (from the sfe4001 network driver):
119
120 .. code-block:: c
121
122 static struct i2c_board_info sfe4001_hwmon_info = {
123 I2C_BOARD_INFO("max6647", 0x4e),
124 };
125
126 int sfe4001_init(struct efx_nic *efx)
127 {
128 (...)
129 efx->board_info.hwmon_client =
130 i2c_new_client_device(&efx->i2c_adap, &sfe4001_hwmon_info);
131
132 (...)
133 }
134
135 The above code instantiates 1 I2C device on the I2C bus which is on the
136 network adapter in question.
137
138 A variant of this is when you don't know for sure if an I2C device is
139 present or not (for example for an optional feature which is not present
140 on cheap variants of a board but you have no way to tell them apart), or
141 it may have different addresses from one board to the next (manufacturer
142 changing its design without notice). In this case, you can call
143 i2c_new_scanned_device() instead of i2c_new_client_device().
144
145 Example (from the nxp OHCI driver):
146
147 .. code-block:: c
148
149 static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
150
151 static int usb_hcd_nxp_probe(struct platform_device *pdev)
152 {
153 (...)
154 struct i2c_adapter *i2c_adap;
155 struct i2c_board_info i2c_info;
156
157 (...)
158 i2c_adap = i2c_get_adapter(2);
159 memset(&i2c_info, 0, sizeof(struct i2c_board_info));
160 strscpy(i2c_info.type, "isp1301_nxp", sizeof(i2c_info.type));
161 isp1301_i2c_client = i2c_new_scanned_device(i2c_adap, &i2c_info,
162 normal_i2c, NULL);
163 i2c_put_adapter(i2c_adap);
164 (...)
165 }
166
167 The above code instantiates up to 1 I2C device on the I2C bus which is on
168 the OHCI adapter in question. It first tries at address 0x2c, if nothing
169 is found there it tries address 0x2d, and if still nothing is found, it
170 simply gives up.
171
172 The driver which instantiated the I2C device is responsible for destroying
173 it on cleanup. This is done by calling i2c_unregister_device() on the
174 pointer that was earlier returned by i2c_new_client_device() or
175 i2c_new_scanned_device().
176
177
178 Method 3: Probe an I2C bus for certain devices
179 ----------------------------------------------
180
181 Sometimes you do not have enough information about an I2C device, not even
182 to call i2c_new_scanned_device(). The typical case is hardware monitoring
183 chips on PC mainboards. There are several dozen models, which can live
184 at 25 different addresses. Given the huge number of mainboards out there,
185 it is next to impossible to build an exhaustive list of the hardware
186 monitoring chips being used. Fortunately, most of these chips have
187 manufacturer and device ID registers, so they can be identified by
188 probing.
189
190 In that case, I2C devices are neither declared nor instantiated
191 explicitly. Instead, i2c-core will probe for such devices as soon as their
192 drivers are loaded, and if any is found, an I2C device will be
193 instantiated automatically. In order to prevent any misbehavior of this
194 mechanism, the following restrictions apply:
195
196 * The I2C device driver must implement the detect() method, which
197 identifies a supported device by reading from arbitrary registers.
198 * Only buses which are likely to have a supported device and agree to be
199 probed, will be probed. For example this avoids probing for hardware
200 monitoring chips on a TV adapter.
201
202 Example:
203 See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c
204
205 I2C devices instantiated as a result of such a successful probe will be
206 destroyed automatically when the driver which detected them is removed,
207 or when the underlying I2C bus is itself destroyed, whichever happens
208 first.
209
210 Those of you familiar with the I2C subsystem of 2.4 kernels and early 2.6
211 kernels will find out that this method 3 is essentially similar to what
212 was done there. Two significant differences are:
213
214 * Probing is only one way to instantiate I2C devices now, while it was the
215 only way back then. Where possible, methods 1 and 2 should be preferred.
216 Method 3 should only be used when there is no other way, as it can have
217 undesirable side effects.
218 * I2C buses must now explicitly say which I2C driver classes can probe
219 them (by the means of the class bitfield), while all I2C buses were
220 probed by default back then. The default is an empty class which means
221 that no probing happens. The purpose of the class bitfield is to limit
222 the aforementioned undesirable side effects.
223
224 Once again, method 3 should be avoided wherever possible. Explicit device
225 instantiation (methods 1 and 2) is much preferred for it is safer and
226 faster.
227
228
229 Method 4: Instantiate from user-space
230 -------------------------------------
231
232 In general, the kernel should know which I2C devices are connected and
233 what addresses they live at. However, in certain cases, it does not, so a
234 sysfs interface was added to let the user provide the information. This
235 interface is made of 2 attribute files which are created in every I2C bus
236 directory: ``new_device`` and ``delete_device``. Both files are write
237 only and you must write the right parameters to them in order to properly
238 instantiate, respectively delete, an I2C device.
239
240 File ``new_device`` takes 2 parameters: the name of the I2C device (a
241 string) and the address of the I2C device (a number, typically expressed
242 in hexadecimal starting with 0x, but can also be expressed in decimal.)
243
244 File ``delete_device`` takes a single parameter: the address of the I2C
245 device. As no two devices can live at the same address on a given I2C
246 segment, the address is sufficient to uniquely identify the device to be
247 deleted.
248
249 Example::
250
251 # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
252
253 While this interface should only be used when in-kernel device declaration
254 can't be done, there is a variety of cases where it can be helpful:
255
256 * The I2C driver usually detects devices (method 3 above) but the bus
257 segment your device lives on doesn't have the proper class bit set and
258 thus detection doesn't trigger.
259 * The I2C driver usually detects devices, but your device lives at an
260 unexpected address.
261 * The I2C driver usually detects devices, but your device is not detected,
262 either because the detection routine is too strict, or because your
263 device is not officially supported yet but you know it is compatible.
264 * You are developing a driver on a test board, where you soldered the I2C
265 device yourself.
266
267 This interface is a replacement for the force_* module parameters some I2C
268 drivers implement. Being implemented in i2c-core rather than in each
269 device driver individually, it is much more efficient, and also has the
270 advantage that you do not have to reload the driver to change a setting.
271 You can also instantiate the device before the driver is loaded or even
272 available, and you don't need to know what driver the device needs.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.