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ccf988b6 | 1 | ============================== |
764c1691 JD |
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 | ||
aeca0fe6 WS |
12 | Method 1a: Declare the I2C devices by bus number |
13 | ------------------------------------------------ | |
764c1691 JD |
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 | |
17 | which is known in advance. It is thus possible to pre-declare the I2C | |
18 | devices which live on this bus. This is done with an array of struct | |
19 | i2c_board_info which is registered by calling i2c_register_board_info(). | |
20 | ||
ccf988b6 | 21 | Example (from omap2 h4):: |
764c1691 | 22 | |
ccf988b6 | 23 | static struct i2c_board_info h4_i2c_board_info[] __initdata = { |
764c1691 JD |
24 | { |
25 | I2C_BOARD_INFO("isp1301_omap", 0x2d), | |
26 | .irq = OMAP_GPIO_IRQ(125), | |
27 | }, | |
28 | { /* EEPROM on mainboard */ | |
29 | I2C_BOARD_INFO("24c01", 0x52), | |
30 | .platform_data = &m24c01, | |
31 | }, | |
32 | { /* EEPROM on cpu card */ | |
33 | I2C_BOARD_INFO("24c01", 0x57), | |
34 | .platform_data = &m24c01, | |
35 | }, | |
ccf988b6 | 36 | }; |
764c1691 | 37 | |
ccf988b6 MCC |
38 | static void __init omap_h4_init(void) |
39 | { | |
764c1691 JD |
40 | (...) |
41 | i2c_register_board_info(1, h4_i2c_board_info, | |
42 | ARRAY_SIZE(h4_i2c_board_info)); | |
43 | (...) | |
ccf988b6 | 44 | } |
764c1691 JD |
45 | |
46 | The above code declares 3 devices on I2C bus 1, including their respective | |
47 | addresses and custom data needed by their drivers. When the I2C bus in | |
48 | question is registered, the I2C devices will be instantiated automatically | |
49 | by i2c-core. | |
50 | ||
51 | The devices will be automatically unbound and destroyed when the I2C bus | |
52 | they sit on goes away (if ever.) | |
53 | ||
54 | ||
aeca0fe6 WS |
55 | Method 1b: Declare the I2C devices via devicetree |
56 | ------------------------------------------------- | |
57 | ||
58 | This method has the same implications as method 1a. The declaration of I2C | |
59 | devices is here done via devicetree as subnodes of the master controller. | |
60 | ||
ccf988b6 | 61 | Example:: |
aeca0fe6 WS |
62 | |
63 | i2c1: i2c@400a0000 { | |
64 | /* ... master properties skipped ... */ | |
65 | clock-frequency = <100000>; | |
66 | ||
67 | flash@50 { | |
68 | compatible = "atmel,24c256"; | |
69 | reg = <0x50>; | |
70 | }; | |
71 | ||
72 | pca9532: gpio@60 { | |
73 | compatible = "nxp,pca9532"; | |
74 | gpio-controller; | |
75 | #gpio-cells = <2>; | |
76 | reg = <0x60>; | |
77 | }; | |
78 | }; | |
79 | ||
80 | Here, two devices are attached to the bus using a speed of 100kHz. For | |
81 | additional properties which might be needed to set up the device, please refer | |
82 | to its devicetree documentation in Documentation/devicetree/bindings/. | |
83 | ||
84 | ||
fde1e418 WS |
85 | Method 1c: Declare the I2C devices via ACPI |
86 | ------------------------------------------- | |
87 | ||
88 | ACPI can also describe I2C devices. There is special documentation for this | |
cb1aaebe | 89 | which is currently located at Documentation/firmware-guide/acpi/enumeration.rst. |
fde1e418 WS |
90 | |
91 | ||
764c1691 JD |
92 | Method 2: Instantiate the devices explicitly |
93 | -------------------------------------------- | |
94 | ||
95 | This method is appropriate when a larger device uses an I2C bus for | |
96 | internal communication. A typical case is TV adapters. These can have a | |
97 | tuner, a video decoder, an audio decoder, etc. usually connected to the | |
98 | main chip by the means of an I2C bus. You won't know the number of the I2C | |
99 | bus in advance, so the method 1 described above can't be used. Instead, | |
100 | you can instantiate your I2C devices explicitly. This is done by filling | |
101 | a struct i2c_board_info and calling i2c_new_device(). | |
102 | ||
ccf988b6 | 103 | Example (from the sfe4001 network driver):: |
764c1691 | 104 | |
ccf988b6 | 105 | static struct i2c_board_info sfe4001_hwmon_info = { |
764c1691 | 106 | I2C_BOARD_INFO("max6647", 0x4e), |
ccf988b6 | 107 | }; |
764c1691 | 108 | |
ccf988b6 MCC |
109 | int sfe4001_init(struct efx_nic *efx) |
110 | { | |
764c1691 JD |
111 | (...) |
112 | efx->board_info.hwmon_client = | |
113 | i2c_new_device(&efx->i2c_adap, &sfe4001_hwmon_info); | |
114 | ||
115 | (...) | |
ccf988b6 | 116 | } |
764c1691 JD |
117 | |
118 | The above code instantiates 1 I2C device on the I2C bus which is on the | |
119 | network adapter in question. | |
120 | ||
121 | A variant of this is when you don't know for sure if an I2C device is | |
122 | present or not (for example for an optional feature which is not present | |
123 | on cheap variants of a board but you have no way to tell them apart), or | |
124 | it may have different addresses from one board to the next (manufacturer | |
125 | changing its design without notice). In this case, you can call | |
126 | i2c_new_probed_device() instead of i2c_new_device(). | |
127 | ||
ccf988b6 | 128 | Example (from the nxp OHCI driver):: |
764c1691 | 129 | |
ccf988b6 | 130 | static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END }; |
764c1691 | 131 | |
ccf988b6 MCC |
132 | static int usb_hcd_nxp_probe(struct platform_device *pdev) |
133 | { | |
764c1691 JD |
134 | (...) |
135 | struct i2c_adapter *i2c_adap; | |
136 | struct i2c_board_info i2c_info; | |
137 | ||
138 | (...) | |
139 | i2c_adap = i2c_get_adapter(2); | |
140 | memset(&i2c_info, 0, sizeof(struct i2c_board_info)); | |
220ee02a | 141 | strscpy(i2c_info.type, "isp1301_nxp", sizeof(i2c_info.type)); |
764c1691 | 142 | isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info, |
9a94241a | 143 | normal_i2c, NULL); |
764c1691 JD |
144 | i2c_put_adapter(i2c_adap); |
145 | (...) | |
ccf988b6 | 146 | } |
764c1691 JD |
147 | |
148 | The above code instantiates up to 1 I2C device on the I2C bus which is on | |
149 | the OHCI adapter in question. It first tries at address 0x2c, if nothing | |
150 | is found there it tries address 0x2d, and if still nothing is found, it | |
151 | simply gives up. | |
152 | ||
153 | The driver which instantiated the I2C device is responsible for destroying | |
154 | it on cleanup. This is done by calling i2c_unregister_device() on the | |
155 | pointer that was earlier returned by i2c_new_device() or | |
156 | i2c_new_probed_device(). | |
157 | ||
158 | ||
159 | Method 3: Probe an I2C bus for certain devices | |
160 | ---------------------------------------------- | |
161 | ||
162 | Sometimes you do not have enough information about an I2C device, not even | |
163 | to call i2c_new_probed_device(). The typical case is hardware monitoring | |
164 | chips on PC mainboards. There are several dozen models, which can live | |
165 | at 25 different addresses. Given the huge number of mainboards out there, | |
166 | it is next to impossible to build an exhaustive list of the hardware | |
167 | monitoring chips being used. Fortunately, most of these chips have | |
168 | manufacturer and device ID registers, so they can be identified by | |
169 | probing. | |
170 | ||
171 | In that case, I2C devices are neither declared nor instantiated | |
172 | explicitly. Instead, i2c-core will probe for such devices as soon as their | |
173 | drivers are loaded, and if any is found, an I2C device will be | |
174 | instantiated automatically. In order to prevent any misbehavior of this | |
175 | mechanism, the following restrictions apply: | |
ccf988b6 | 176 | |
764c1691 JD |
177 | * The I2C device driver must implement the detect() method, which |
178 | identifies a supported device by reading from arbitrary registers. | |
179 | * Only buses which are likely to have a supported device and agree to be | |
180 | probed, will be probed. For example this avoids probing for hardware | |
181 | monitoring chips on a TV adapter. | |
182 | ||
183 | Example: | |
184 | See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c | |
185 | ||
186 | I2C devices instantiated as a result of such a successful probe will be | |
187 | destroyed automatically when the driver which detected them is removed, | |
188 | or when the underlying I2C bus is itself destroyed, whichever happens | |
189 | first. | |
190 | ||
191 | Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6 | |
192 | kernels will find out that this method 3 is essentially similar to what | |
193 | was done there. Two significant differences are: | |
ccf988b6 | 194 | |
764c1691 JD |
195 | * Probing is only one way to instantiate I2C devices now, while it was the |
196 | only way back then. Where possible, methods 1 and 2 should be preferred. | |
197 | Method 3 should only be used when there is no other way, as it can have | |
198 | undesirable side effects. | |
199 | * I2C buses must now explicitly say which I2C driver classes can probe | |
200 | them (by the means of the class bitfield), while all I2C buses were | |
201 | probed by default back then. The default is an empty class which means | |
202 | that no probing happens. The purpose of the class bitfield is to limit | |
203 | the aforementioned undesirable side effects. | |
204 | ||
205 | Once again, method 3 should be avoided wherever possible. Explicit device | |
206 | instantiation (methods 1 and 2) is much preferred for it is safer and | |
207 | faster. | |
99cd8e25 JD |
208 | |
209 | ||
210 | Method 4: Instantiate from user-space | |
211 | ------------------------------------- | |
212 | ||
213 | In general, the kernel should know which I2C devices are connected and | |
214 | what addresses they live at. However, in certain cases, it does not, so a | |
215 | sysfs interface was added to let the user provide the information. This | |
216 | interface is made of 2 attribute files which are created in every I2C bus | |
217 | directory: new_device and delete_device. Both files are write only and you | |
218 | must write the right parameters to them in order to properly instantiate, | |
219 | respectively delete, an I2C device. | |
220 | ||
221 | File new_device takes 2 parameters: the name of the I2C device (a string) | |
222 | and the address of the I2C device (a number, typically expressed in | |
223 | hexadecimal starting with 0x, but can also be expressed in decimal.) | |
224 | ||
225 | File delete_device takes a single parameter: the address of the I2C | |
226 | device. As no two devices can live at the same address on a given I2C | |
227 | segment, the address is sufficient to uniquely identify the device to be | |
228 | deleted. | |
229 | ||
ccf988b6 MCC |
230 | Example:: |
231 | ||
232 | # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device | |
99cd8e25 JD |
233 | |
234 | While this interface should only be used when in-kernel device declaration | |
235 | can't be done, there is a variety of cases where it can be helpful: | |
ccf988b6 | 236 | |
99cd8e25 JD |
237 | * The I2C driver usually detects devices (method 3 above) but the bus |
238 | segment your device lives on doesn't have the proper class bit set and | |
239 | thus detection doesn't trigger. | |
240 | * The I2C driver usually detects devices, but your device lives at an | |
241 | unexpected address. | |
242 | * The I2C driver usually detects devices, but your device is not detected, | |
243 | either because the detection routine is too strict, or because your | |
244 | device is not officially supported yet but you know it is compatible. | |
245 | * You are developing a driver on a test board, where you soldered the I2C | |
246 | device yourself. | |
247 | ||
248 | This interface is a replacement for the force_* module parameters some I2C | |
249 | drivers implement. Being implemented in i2c-core rather than in each | |
250 | device driver individually, it is much more efficient, and also has the | |
251 | advantage that you do not have to reload the driver to change a setting. | |
252 | You can also instantiate the device before the driver is loaded or even | |
253 | available, and you don't need to know what driver the device needs. |