Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
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 11 Method 1a: Declare the I2C devices by bus number 12 ------------------------------------------------ 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 22 static struct i2c_board_info h4_i2c_board_info[] __initdata = { 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 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 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 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(). 126 127 Example (from the nxp OHCI driver): 128 129 static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END }; 130 131 static int usb_hcd_nxp_probe(struct platform_device *pdev) 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)); 140 strlcpy(i2c_info.type, "isp1301_nxp", I2C_NAME_SIZE); 141 isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info, 142 normal_i2c, NULL); 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. 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: 228 # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device 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.