1============================== 2How to instantiate I2C devices 3============================== 4 5Unlike PCI or USB devices, I2C devices are not enumerated at the hardware 6level. Instead, the software must know which devices are connected on each 7I2C bus segment, and what address these devices are using. For this 8reason, the kernel code must instantiate I2C devices explicitly. There are 9several ways to achieve this, depending on the context and requirements. 10 11 12Method 1: Declare the I2C devices statically 13-------------------------------------------- 14 15This method is appropriate when the I2C bus is a system bus as is the case 16for many embedded systems. On such systems, each I2C bus has a number which 17is known in advance. It is thus possible to pre-declare the I2C devices 18which live on this bus. 19 20This information is provided to the kernel in a different way on different 21architectures: device tree, ACPI or board files. 22 23When the I2C bus in question is registered, the I2C devices will be 24instantiated automatically by i2c-core. The devices will be automatically 25unbound and destroyed when the I2C bus they sit on goes away (if ever). 26 27 28Declare the I2C devices via devicetree 29^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 30 31On platforms using devicetree, the declaration of I2C devices is done in 32subnodes of the master controller. 33 34Example: 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 55Here, two devices are attached to the bus using a speed of 100kHz. For 56additional properties which might be needed to set up the device, please refer 57to its devicetree documentation in Documentation/devicetree/bindings/. 58 59 60Declare the I2C devices via ACPI 61^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 62 63ACPI can also describe I2C devices. There is special documentation for this 64which is currently located at Documentation/firmware-guide/acpi/enumeration.rst. 65 66 67Declare the I2C devices in board files 68^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 69 70In many embedded architectures, devicetree has replaced the old hardware 71description based on board files, but the latter are still used in old 72code. Instantiating I2C devices via board files is done with an array of 73struct i2c_board_info which is registered by calling 74i2c_register_board_info(). 75 76Example (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 103The above code declares 3 devices on I2C bus 1, including their respective 104addresses and custom data needed by their drivers. 105 106 107Method 2: Instantiate the devices explicitly 108-------------------------------------------- 109 110This method is appropriate when a larger device uses an I2C bus for 111internal communication. A typical case is TV adapters. These can have a 112tuner, a video decoder, an audio decoder, etc. usually connected to the 113main chip by the means of an I2C bus. You won't know the number of the I2C 114bus in advance, so the method 1 described above can't be used. Instead, 115you can instantiate your I2C devices explicitly. This is done by filling 116a struct i2c_board_info and calling i2c_new_client_device(). 117 118Example (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 135The above code instantiates 1 I2C device on the I2C bus which is on the 136network adapter in question. 137 138A variant of this is when you don't know for sure if an I2C device is 139present or not (for example for an optional feature which is not present 140on cheap variants of a board but you have no way to tell them apart), or 141it may have different addresses from one board to the next (manufacturer 142changing its design without notice). In this case, you can call 143i2c_new_scanned_device() instead of i2c_new_client_device(). 144 145Example (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 167The above code instantiates up to 1 I2C device on the I2C bus which is on 168the OHCI adapter in question. It first tries at address 0x2c, if nothing 169is found there it tries address 0x2d, and if still nothing is found, it 170simply gives up. 171 172The driver which instantiated the I2C device is responsible for destroying 173it on cleanup. This is done by calling i2c_unregister_device() on the 174pointer that was earlier returned by i2c_new_client_device() or 175i2c_new_scanned_device(). 176 177 178Method 3: Probe an I2C bus for certain devices 179---------------------------------------------- 180 181Sometimes you do not have enough information about an I2C device, not even 182to call i2c_new_scanned_device(). The typical case is hardware monitoring 183chips on PC mainboards. There are several dozen models, which can live 184at 25 different addresses. Given the huge number of mainboards out there, 185it is next to impossible to build an exhaustive list of the hardware 186monitoring chips being used. Fortunately, most of these chips have 187manufacturer and device ID registers, so they can be identified by 188probing. 189 190In that case, I2C devices are neither declared nor instantiated 191explicitly. Instead, i2c-core will probe for such devices as soon as their 192drivers are loaded, and if any is found, an I2C device will be 193instantiated automatically. In order to prevent any misbehavior of this 194mechanism, 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 202Example: 203See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c 204 205I2C devices instantiated as a result of such a successful probe will be 206destroyed automatically when the driver which detected them is removed, 207or when the underlying I2C bus is itself destroyed, whichever happens 208first. 209 210Those of you familiar with the I2C subsystem of 2.4 kernels and early 2.6 211kernels will find out that this method 3 is essentially similar to what 212was 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 224Once again, method 3 should be avoided wherever possible. Explicit device 225instantiation (methods 1 and 2) is much preferred for it is safer and 226faster. 227 228 229Method 4: Instantiate from user-space 230------------------------------------- 231 232In general, the kernel should know which I2C devices are connected and 233what addresses they live at. However, in certain cases, it does not, so a 234sysfs interface was added to let the user provide the information. This 235interface is made of 2 attribute files which are created in every I2C bus 236directory: ``new_device`` and ``delete_device``. Both files are write 237only and you must write the right parameters to them in order to properly 238instantiate, respectively delete, an I2C device. 239 240File ``new_device`` takes 2 parameters: the name of the I2C device (a 241string) and the address of the I2C device (a number, typically expressed 242in hexadecimal starting with 0x, but can also be expressed in decimal.) 243 244File ``delete_device`` takes a single parameter: the address of the I2C 245device. As no two devices can live at the same address on a given I2C 246segment, the address is sufficient to uniquely identify the device to be 247deleted. 248 249Example:: 250 251 # echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device 252 253While this interface should only be used when in-kernel device declaration 254can'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 267This interface is a replacement for the force_* module parameters some I2C 268drivers implement. Being implemented in i2c-core rather than in each 269device driver individually, it is much more efficient, and also has the 270advantage that you do not have to reload the driver to change a setting. 271You can also instantiate the device before the driver is loaded or even 272available, and you don't need to know what driver the device needs. 273