Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 Usually, i2c devices are controlled by a kernel driver. But it is also 2 possible to access all devices on an adapter from userspace, through 3 the /dev interface. You need to load module i2c-dev for this. 4 5 Each registered i2c adapter gets a number, counting from 0. You can 6 examine /sys/class/i2c-dev/ to see what number corresponds to which adapter. 7 Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all 8 i2c adapters present on your system at a given time. i2cdetect is part of 9 the i2c-tools package. 10 11 I2C device files are character device files with major device number 89 12 and a minor device number corresponding to the number assigned as 13 explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ..., 14 i2c-10, ...). All 256 minor device numbers are reserved for i2c. 15 16 17 C example 18 ========= 19 20 So let's say you want to access an i2c adapter from a C program. 21 First, you need to include these two headers: 22 23 #include <linux/i2c-dev.h> 24 #include <i2c/smbus.h> 25 26 (Please note that there are two files named "i2c-dev.h" out there. One is 27 distributed with the Linux kernel and the other one is included in the 28 source tree of i2c-tools. They used to be different in content but since 2012 29 they're identical. You should use "linux/i2c-dev.h"). 30 31 Now, you have to decide which adapter you want to access. You should 32 inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this. 33 Adapter numbers are assigned somewhat dynamically, so you can not 34 assume much about them. They can even change from one boot to the next. 35 36 Next thing, open the device file, as follows: 37 38 int file; 39 int adapter_nr = 2; /* probably dynamically determined */ 40 char filename[20]; 41 42 snprintf(filename, 19, "/dev/i2c-%d", adapter_nr); 43 file = open(filename, O_RDWR); 44 if (file < 0) { 45 /* ERROR HANDLING; you can check errno to see what went wrong */ 46 exit(1); 47 } 48 49 When you have opened the device, you must specify with what device 50 address you want to communicate: 51 52 int addr = 0x40; /* The I2C address */ 53 54 if (ioctl(file, I2C_SLAVE, addr) < 0) { 55 /* ERROR HANDLING; you can check errno to see what went wrong */ 56 exit(1); 57 } 58 59 Well, you are all set up now. You can now use SMBus commands or plain 60 I2C to communicate with your device. SMBus commands are preferred if 61 the device supports them. Both are illustrated below. 62 63 __u8 reg = 0x10; /* Device register to access */ 64 __s32 res; 65 char buf[10]; 66 67 /* Using SMBus commands */ 68 res = i2c_smbus_read_word_data(file, reg); 69 if (res < 0) { 70 /* ERROR HANDLING: i2c transaction failed */ 71 } else { 72 /* res contains the read word */ 73 } 74 75 /* Using I2C Write, equivalent of 76 i2c_smbus_write_word_data(file, reg, 0x6543) */ 77 buf[0] = reg; 78 buf[1] = 0x43; 79 buf[2] = 0x65; 80 if (write(file, buf, 3) != 3) { 81 /* ERROR HANDLING: i2c transaction failed */ 82 } 83 84 /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */ 85 if (read(file, buf, 1) != 1) { 86 /* ERROR HANDLING: i2c transaction failed */ 87 } else { 88 /* buf[0] contains the read byte */ 89 } 90 91 Note that only a subset of the I2C and SMBus protocols can be achieved by 92 the means of read() and write() calls. In particular, so-called combined 93 transactions (mixing read and write messages in the same transaction) 94 aren't supported. For this reason, this interface is almost never used by 95 user-space programs. 96 97 IMPORTANT: because of the use of inline functions, you *have* to use 98 '-O' or some variation when you compile your program! 99 100 101 Full interface description 102 ========================== 103 104 The following IOCTLs are defined: 105 106 ioctl(file, I2C_SLAVE, long addr) 107 Change slave address. The address is passed in the 7 lower bits of the 108 argument (except for 10 bit addresses, passed in the 10 lower bits in this 109 case). 110 111 ioctl(file, I2C_TENBIT, long select) 112 Selects ten bit addresses if select not equals 0, selects normal 7 bit 113 addresses if select equals 0. Default 0. This request is only valid 114 if the adapter has I2C_FUNC_10BIT_ADDR. 115 116 ioctl(file, I2C_PEC, long select) 117 Selects SMBus PEC (packet error checking) generation and verification 118 if select not equals 0, disables if select equals 0. Default 0. 119 Used only for SMBus transactions. This request only has an effect if the 120 the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just 121 doesn't have any effect. 122 123 ioctl(file, I2C_FUNCS, unsigned long *funcs) 124 Gets the adapter functionality and puts it in *funcs. 125 126 ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset) 127 Do combined read/write transaction without stop in between. 128 Only valid if the adapter has I2C_FUNC_I2C. The argument is 129 a pointer to a 130 131 struct i2c_rdwr_ioctl_data { 132 struct i2c_msg *msgs; /* ptr to array of simple messages */ 133 int nmsgs; /* number of messages to exchange */ 134 } 135 136 The msgs[] themselves contain further pointers into data buffers. 137 The function will write or read data to or from that buffers depending 138 on whether the I2C_M_RD flag is set in a particular message or not. 139 The slave address and whether to use ten bit address mode has to be 140 set in each message, overriding the values set with the above ioctl's. 141 142 ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args) 143 Not meant to be called directly; instead, use the access functions 144 below. 145 146 You can do plain i2c transactions by using read(2) and write(2) calls. 147 You do not need to pass the address byte; instead, set it through 148 ioctl I2C_SLAVE before you try to access the device. 149 150 You can do SMBus level transactions (see documentation file smbus-protocol 151 for details) through the following functions: 152 __s32 i2c_smbus_write_quick(int file, __u8 value); 153 __s32 i2c_smbus_read_byte(int file); 154 __s32 i2c_smbus_write_byte(int file, __u8 value); 155 __s32 i2c_smbus_read_byte_data(int file, __u8 command); 156 __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value); 157 __s32 i2c_smbus_read_word_data(int file, __u8 command); 158 __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value); 159 __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value); 160 __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values); 161 __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length, 162 __u8 *values); 163 All these transactions return -1 on failure; you can read errno to see 164 what happened. The 'write' transactions return 0 on success; the 165 'read' transactions return the read value, except for read_block, which 166 returns the number of values read. The block buffers need not be longer 167 than 32 bytes. 168 169 The above functions are all inline functions, that resolve to calls to 170 the i2c_smbus_access function, that on its turn calls a specific ioctl 171 with the data in a specific format. Read the source code if you 172 want to know what happens behind the screens. 173 174 175 Implementation details 176 ====================== 177 178 For the interested, here's the code flow which happens inside the kernel 179 when you use the /dev interface to I2C: 180 181 1* Your program opens /dev/i2c-N and calls ioctl() on it, as described in 182 section "C example" above. 183 184 2* These open() and ioctl() calls are handled by the i2c-dev kernel 185 driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(), 186 respectively. You can think of i2c-dev as a generic I2C chip driver 187 that can be programmed from user-space. 188 189 3* Some ioctl() calls are for administrative tasks and are handled by 190 i2c-dev directly. Examples include I2C_SLAVE (set the address of the 191 device you want to access) and I2C_PEC (enable or disable SMBus error 192 checking on future transactions.) 193 194 4* Other ioctl() calls are converted to in-kernel function calls by 195 i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter 196 functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which 197 performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer(). 198 199 The i2c-dev driver is responsible for checking all the parameters that 200 come from user-space for validity. After this point, there is no 201 difference between these calls that came from user-space through i2c-dev 202 and calls that would have been performed by kernel I2C chip drivers 203 directly. This means that I2C bus drivers don't need to implement 204 anything special to support access from user-space. 205 206 5* These i2c.h functions are wrappers to the actual implementation of 207 your I2C bus driver. Each adapter must declare callback functions 208 implementing these standard calls. i2c.h:i2c_get_functionality() calls 209 i2c_adapter.algo->functionality(), while 210 i2c-core-smbus.c:i2c_smbus_xfer() calls either 211 adapter.algo->smbus_xfer() if it is implemented, or if not, 212 i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls 213 i2c_adapter.algo->master_xfer(). 214 215 After your I2C bus driver has processed these requests, execution runs 216 up the call chain, with almost no processing done, except by i2c-dev to 217 package the returned data, if any, in suitable format for the ioctl.