1Usually, i2c devices are controlled by a kernel driver. But it is also 2possible to access all devices on an adapter from userspace, through 3the /dev interface. You need to load module i2c-dev for this. 4 5Each registered i2c adapter gets a number, counting from 0. You can 6examine /sys/class/i2c-dev/ to see what number corresponds to which adapter. 7Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all 8i2c adapters present on your system at a given time. i2cdetect is part of 9the i2c-tools package. 10 11I2C device files are character device files with major device number 89 12and a minor device number corresponding to the number assigned as 13explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ..., 14i2c-10, ...). All 256 minor device numbers are reserved for i2c. 15 16 17C example 18========= 19 20So let's say you want to access an i2c adapter from a C program. 21First, you need to include these two headers: 22 23 #include <linux/i2c-dev.h> 24 #include <i2c/smbus.h> 25 26Now, you have to decide which adapter you want to access. You should 27inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this. 28Adapter numbers are assigned somewhat dynamically, so you can not 29assume much about them. They can even change from one boot to the next. 30 31Next thing, open the device file, as follows: 32 33 int file; 34 int adapter_nr = 2; /* probably dynamically determined */ 35 char filename[20]; 36 37 snprintf(filename, 19, "/dev/i2c-%d", adapter_nr); 38 file = open(filename, O_RDWR); 39 if (file < 0) { 40 /* ERROR HANDLING; you can check errno to see what went wrong */ 41 exit(1); 42 } 43 44When you have opened the device, you must specify with what device 45address you want to communicate: 46 47 int addr = 0x40; /* The I2C address */ 48 49 if (ioctl(file, I2C_SLAVE, addr) < 0) { 50 /* ERROR HANDLING; you can check errno to see what went wrong */ 51 exit(1); 52 } 53 54Well, you are all set up now. You can now use SMBus commands or plain 55I2C to communicate with your device. SMBus commands are preferred if 56the device supports them. Both are illustrated below. 57 58 __u8 reg = 0x10; /* Device register to access */ 59 __s32 res; 60 char buf[10]; 61 62 /* Using SMBus commands */ 63 res = i2c_smbus_read_word_data(file, reg); 64 if (res < 0) { 65 /* ERROR HANDLING: i2c transaction failed */ 66 } else { 67 /* res contains the read word */ 68 } 69 70 /* 71 * Using I2C Write, equivalent of 72 * i2c_smbus_write_word_data(file, reg, 0x6543) 73 */ 74 buf[0] = reg; 75 buf[1] = 0x43; 76 buf[2] = 0x65; 77 if (write(file, buf, 3) != 3) { 78 /* ERROR HANDLING: i2c transaction failed */ 79 } 80 81 /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */ 82 if (read(file, buf, 1) != 1) { 83 /* ERROR HANDLING: i2c transaction failed */ 84 } else { 85 /* buf[0] contains the read byte */ 86 } 87 88Note that only a subset of the I2C and SMBus protocols can be achieved by 89the means of read() and write() calls. In particular, so-called combined 90transactions (mixing read and write messages in the same transaction) 91aren't supported. For this reason, this interface is almost never used by 92user-space programs. 93 94IMPORTANT: because of the use of inline functions, you *have* to use 95'-O' or some variation when you compile your program! 96 97 98Full interface description 99========================== 100 101The following IOCTLs are defined: 102 103ioctl(file, I2C_SLAVE, long addr) 104 Change slave address. The address is passed in the 7 lower bits of the 105 argument (except for 10 bit addresses, passed in the 10 lower bits in this 106 case). 107 108ioctl(file, I2C_TENBIT, long select) 109 Selects ten bit addresses if select not equals 0, selects normal 7 bit 110 addresses if select equals 0. Default 0. This request is only valid 111 if the adapter has I2C_FUNC_10BIT_ADDR. 112 113ioctl(file, I2C_PEC, long select) 114 Selects SMBus PEC (packet error checking) generation and verification 115 if select not equals 0, disables if select equals 0. Default 0. 116 Used only for SMBus transactions. This request only has an effect if the 117 the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just 118 doesn't have any effect. 119 120ioctl(file, I2C_FUNCS, unsigned long *funcs) 121 Gets the adapter functionality and puts it in *funcs. 122 123ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset) 124 Do combined read/write transaction without stop in between. 125 Only valid if the adapter has I2C_FUNC_I2C. The argument is 126 a pointer to a 127 128 struct i2c_rdwr_ioctl_data { 129 struct i2c_msg *msgs; /* ptr to array of simple messages */ 130 int nmsgs; /* number of messages to exchange */ 131 } 132 133 The msgs[] themselves contain further pointers into data buffers. 134 The function will write or read data to or from that buffers depending 135 on whether the I2C_M_RD flag is set in a particular message or not. 136 The slave address and whether to use ten bit address mode has to be 137 set in each message, overriding the values set with the above ioctl's. 138 139ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args) 140 If possible, use the provided i2c_smbus_* methods described below instead 141 of issuing direct ioctls. 142 143You can do plain i2c transactions by using read(2) and write(2) calls. 144You do not need to pass the address byte; instead, set it through 145ioctl I2C_SLAVE before you try to access the device. 146 147You can do SMBus level transactions (see documentation file smbus-protocol 148for details) through the following functions: 149 __s32 i2c_smbus_write_quick(int file, __u8 value); 150 __s32 i2c_smbus_read_byte(int file); 151 __s32 i2c_smbus_write_byte(int file, __u8 value); 152 __s32 i2c_smbus_read_byte_data(int file, __u8 command); 153 __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value); 154 __s32 i2c_smbus_read_word_data(int file, __u8 command); 155 __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value); 156 __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value); 157 __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values); 158 __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length, 159 __u8 *values); 160All these transactions return -1 on failure; you can read errno to see 161what happened. The 'write' transactions return 0 on success; the 162'read' transactions return the read value, except for read_block, which 163returns the number of values read. The block buffers need not be longer 164than 32 bytes. 165 166The above functions are made available by linking against the libi2c library, 167which is provided by the i2c-tools project. See: 168https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/. 169 170 171Implementation details 172====================== 173 174For the interested, here's the code flow which happens inside the kernel 175when you use the /dev interface to I2C: 176 1771* Your program opens /dev/i2c-N and calls ioctl() on it, as described in 178section "C example" above. 179 1802* These open() and ioctl() calls are handled by the i2c-dev kernel 181driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(), 182respectively. You can think of i2c-dev as a generic I2C chip driver 183that can be programmed from user-space. 184 1853* Some ioctl() calls are for administrative tasks and are handled by 186i2c-dev directly. Examples include I2C_SLAVE (set the address of the 187device you want to access) and I2C_PEC (enable or disable SMBus error 188checking on future transactions.) 189 1904* Other ioctl() calls are converted to in-kernel function calls by 191i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter 192functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which 193performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer(). 194 195The i2c-dev driver is responsible for checking all the parameters that 196come from user-space for validity. After this point, there is no 197difference between these calls that came from user-space through i2c-dev 198and calls that would have been performed by kernel I2C chip drivers 199directly. This means that I2C bus drivers don't need to implement 200anything special to support access from user-space. 201 2025* These i2c.h functions are wrappers to the actual implementation of 203your I2C bus driver. Each adapter must declare callback functions 204implementing these standard calls. i2c.h:i2c_get_functionality() calls 205i2c_adapter.algo->functionality(), while 206i2c-core-smbus.c:i2c_smbus_xfer() calls either 207adapter.algo->smbus_xfer() if it is implemented, or if not, 208i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls 209i2c_adapter.algo->master_xfer(). 210 211After your I2C bus driver has processed these requests, execution runs 212up the call chain, with almost no processing done, except by i2c-dev to 213package the returned data, if any, in suitable format for the ioctl. 214