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764c1691 JD |
1 | How to instantiate I2C devices |
2 | ============================== | |
3 | ||
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. | |
9 | ||
10 | ||
aeca0fe6 WS |
11 | Method 1a: Declare the I2C devices by bus number |
12 | ------------------------------------------------ | |
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13 | |
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(). | |
19 | ||
20 | Example (from omap2 h4): | |
21 | ||
42fa278d | 22 | static struct i2c_board_info h4_i2c_board_info[] __initdata = { |
764c1691 JD |
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 | ||
37 | static 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 | ||
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 | |
48 | by i2c-core. | |
49 | ||
50 | The devices will be automatically unbound and destroyed when the I2C bus | |
51 | they sit on goes away (if ever.) | |
52 | ||
53 | ||
aeca0fe6 WS |
54 | Method 1b: Declare the I2C devices via devicetree |
55 | ------------------------------------------------- | |
56 | ||
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. | |
59 | ||
60 | Example: | |
61 | ||
62 | i2c1: i2c@400a0000 { | |
63 | /* ... master properties skipped ... */ | |
64 | clock-frequency = <100000>; | |
65 | ||
66 | flash@50 { | |
67 | compatible = "atmel,24c256"; | |
68 | reg = <0x50>; | |
69 | }; | |
70 | ||
71 | pca9532: gpio@60 { | |
72 | compatible = "nxp,pca9532"; | |
73 | gpio-controller; | |
74 | #gpio-cells = <2>; | |
75 | reg = <0x60>; | |
76 | }; | |
77 | }; | |
78 | ||
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/. | |
82 | ||
83 | ||
fde1e418 WS |
84 | Method 1c: Declare the I2C devices via ACPI |
85 | ------------------------------------------- | |
86 | ||
87 | ACPI can also describe I2C devices. There is special documentation for this | |
88 | which is currently located at Documentation/acpi/enumeration.txt. | |
89 | ||
90 | ||
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91 | Method 2: Instantiate the devices explicitly |
92 | -------------------------------------------- | |
93 | ||
94 | This method is appropriate when a larger device uses an I2C bus for | |
95 | internal communication. A typical case is TV adapters. These can have a | |
96 | tuner, a video decoder, an audio decoder, etc. usually connected to the | |
97 | main chip by the means of an I2C bus. You won't know the number of the I2C | |
98 | bus in advance, so the method 1 described above can't be used. Instead, | |
99 | you can instantiate your I2C devices explicitly. This is done by filling | |
100 | a struct i2c_board_info and calling i2c_new_device(). | |
101 | ||
102 | Example (from the sfe4001 network driver): | |
103 | ||
104 | static struct i2c_board_info sfe4001_hwmon_info = { | |
105 | I2C_BOARD_INFO("max6647", 0x4e), | |
106 | }; | |
107 | ||
108 | int sfe4001_init(struct efx_nic *efx) | |
109 | { | |
110 | (...) | |
111 | efx->board_info.hwmon_client = | |
112 | i2c_new_device(&efx->i2c_adap, &sfe4001_hwmon_info); | |
113 | ||
114 | (...) | |
115 | } | |
116 | ||
117 | The above code instantiates 1 I2C device on the I2C bus which is on the | |
118 | network adapter in question. | |
119 | ||
120 | A variant of this is when you don't know for sure if an I2C device is | |
121 | present or not (for example for an optional feature which is not present | |
122 | on cheap variants of a board but you have no way to tell them apart), or | |
123 | it may have different addresses from one board to the next (manufacturer | |
124 | changing its design without notice). In this case, you can call | |
125 | i2c_new_probed_device() instead of i2c_new_device(). | |
126 | ||
28643104 | 127 | Example (from the nxp OHCI driver): |
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128 | |
129 | static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END }; | |
130 | ||
63a29f74 | 131 | static int usb_hcd_nxp_probe(struct platform_device *pdev) |
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132 | { |
133 | (...) | |
134 | struct i2c_adapter *i2c_adap; | |
135 | struct i2c_board_info i2c_info; | |
136 | ||
137 | (...) | |
138 | i2c_adap = i2c_get_adapter(2); | |
139 | memset(&i2c_info, 0, sizeof(struct i2c_board_info)); | |
28643104 | 140 | strlcpy(i2c_info.type, "isp1301_nxp", I2C_NAME_SIZE); |
764c1691 | 141 | isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info, |
9a94241a | 142 | normal_i2c, NULL); |
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143 | i2c_put_adapter(i2c_adap); |
144 | (...) | |
145 | } | |
146 | ||
147 | The above code instantiates up to 1 I2C device on the I2C bus which is on | |
148 | the OHCI adapter in question. It first tries at address 0x2c, if nothing | |
149 | is found there it tries address 0x2d, and if still nothing is found, it | |
150 | simply gives up. | |
151 | ||
152 | The driver which instantiated the I2C device is responsible for destroying | |
153 | it on cleanup. This is done by calling i2c_unregister_device() on the | |
154 | pointer that was earlier returned by i2c_new_device() or | |
155 | i2c_new_probed_device(). | |
156 | ||
157 | ||
158 | Method 3: Probe an I2C bus for certain devices | |
159 | ---------------------------------------------- | |
160 | ||
161 | Sometimes you do not have enough information about an I2C device, not even | |
162 | to call i2c_new_probed_device(). The typical case is hardware monitoring | |
163 | chips on PC mainboards. There are several dozen models, which can live | |
164 | at 25 different addresses. Given the huge number of mainboards out there, | |
165 | it is next to impossible to build an exhaustive list of the hardware | |
166 | monitoring chips being used. Fortunately, most of these chips have | |
167 | manufacturer and device ID registers, so they can be identified by | |
168 | probing. | |
169 | ||
170 | In that case, I2C devices are neither declared nor instantiated | |
171 | explicitly. Instead, i2c-core will probe for such devices as soon as their | |
172 | drivers are loaded, and if any is found, an I2C device will be | |
173 | instantiated automatically. In order to prevent any misbehavior of this | |
174 | mechanism, the following restrictions apply: | |
175 | * The I2C device driver must implement the detect() method, which | |
176 | identifies a supported device by reading from arbitrary registers. | |
177 | * Only buses which are likely to have a supported device and agree to be | |
178 | probed, will be probed. For example this avoids probing for hardware | |
179 | monitoring chips on a TV adapter. | |
180 | ||
181 | Example: | |
182 | See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c | |
183 | ||
184 | I2C devices instantiated as a result of such a successful probe will be | |
185 | destroyed automatically when the driver which detected them is removed, | |
186 | or when the underlying I2C bus is itself destroyed, whichever happens | |
187 | first. | |
188 | ||
189 | Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6 | |
190 | kernels will find out that this method 3 is essentially similar to what | |
191 | was done there. Two significant differences are: | |
192 | * Probing is only one way to instantiate I2C devices now, while it was the | |
193 | only way back then. Where possible, methods 1 and 2 should be preferred. | |
194 | Method 3 should only be used when there is no other way, as it can have | |
195 | undesirable side effects. | |
196 | * I2C buses must now explicitly say which I2C driver classes can probe | |
197 | them (by the means of the class bitfield), while all I2C buses were | |
198 | probed by default back then. The default is an empty class which means | |
199 | that no probing happens. The purpose of the class bitfield is to limit | |
200 | the aforementioned undesirable side effects. | |
201 | ||
202 | Once again, method 3 should be avoided wherever possible. Explicit device | |
203 | instantiation (methods 1 and 2) is much preferred for it is safer and | |
204 | faster. | |
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205 | |
206 | ||
207 | Method 4: Instantiate from user-space | |
208 | ------------------------------------- | |
209 | ||
210 | In general, the kernel should know which I2C devices are connected and | |
211 | what addresses they live at. However, in certain cases, it does not, so a | |
212 | sysfs interface was added to let the user provide the information. This | |
213 | interface is made of 2 attribute files which are created in every I2C bus | |
214 | directory: new_device and delete_device. Both files are write only and you | |
215 | must write the right parameters to them in order to properly instantiate, | |
216 | respectively delete, an I2C device. | |
217 | ||
218 | File new_device takes 2 parameters: the name of the I2C device (a string) | |
219 | and the address of the I2C device (a number, typically expressed in | |
220 | hexadecimal starting with 0x, but can also be expressed in decimal.) | |
221 | ||
222 | File delete_device takes a single parameter: the address of the I2C | |
223 | device. As no two devices can live at the same address on a given I2C | |
224 | segment, the address is sufficient to uniquely identify the device to be | |
225 | deleted. | |
226 | ||
227 | Example: | |
03f1805a | 228 | # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device |
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229 | |
230 | While this interface should only be used when in-kernel device declaration | |
231 | can't be done, there is a variety of cases where it can be helpful: | |
232 | * The I2C driver usually detects devices (method 3 above) but the bus | |
233 | segment your device lives on doesn't have the proper class bit set and | |
234 | thus detection doesn't trigger. | |
235 | * The I2C driver usually detects devices, but your device lives at an | |
236 | unexpected address. | |
237 | * The I2C driver usually detects devices, but your device is not detected, | |
238 | either because the detection routine is too strict, or because your | |
239 | device is not officially supported yet but you know it is compatible. | |
240 | * You are developing a driver on a test board, where you soldered the I2C | |
241 | device yourself. | |
242 | ||
243 | This interface is a replacement for the force_* module parameters some I2C | |
244 | drivers implement. Being implemented in i2c-core rather than in each | |
245 | device driver individually, it is much more efficient, and also has the | |
246 | advantage that you do not have to reload the driver to change a setting. | |
247 | You can also instantiate the device before the driver is loaded or even | |
248 | available, and you don't need to know what driver the device needs. |