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1============================================2Implementing I2C device drivers in userspace3============================================4 5Usually, I2C devices are controlled by a kernel driver. But it is also6possible to access all devices on an adapter from userspace, through7the /dev interface. You need to load module i2c-dev for this.8 9Each registered I2C adapter gets a number, counting from 0. You can10examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.11Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all12I2C adapters present on your system at a given time. i2cdetect is part of13the i2c-tools package.14 15I2C device files are character device files with major device number 8916and a minor device number corresponding to the number assigned as17explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,18i2c-10, ...). All 256 minor device numbers are reserved for I2C.19 20 21C example22=========23 24So let's say you want to access an I2C adapter from a C program.25First, you need to include these two headers::26 27 #include <linux/i2c-dev.h>28 #include <i2c/smbus.h>29 30Now, you have to decide which adapter you want to access. You should31inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.32Adapter numbers are assigned somewhat dynamically, so you can not33assume much about them. They can even change from one boot to the next.34 35Next thing, open the device file, as follows::36 37 int file;38 int adapter_nr = 2; /* probably dynamically determined */39 char filename[20];40 41 snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);42 file = open(filename, O_RDWR);43 if (file < 0) {44 /* ERROR HANDLING; you can check errno to see what went wrong */45 exit(1);46 }47 48When you have opened the device, you must specify with what device49address you want to communicate::50 51 int addr = 0x40; /* The I2C address */52 53 if (ioctl(file, I2C_SLAVE, addr) < 0) {54 /* ERROR HANDLING; you can check errno to see what went wrong */55 exit(1);56 }57 58Well, you are all set up now. You can now use SMBus commands or plain59I2C to communicate with your device. SMBus commands are preferred if60the device supports them. Both are illustrated below::61 62 __u8 reg = 0x10; /* Device register to access */63 __s32 res;64 char buf[10];65 66 /* Using SMBus commands */67 res = i2c_smbus_read_word_data(file, reg);68 if (res < 0) {69 /* ERROR HANDLING: I2C transaction failed */70 } else {71 /* res contains the read word */72 }73 74 /*75 * Using I2C Write, equivalent of76 * i2c_smbus_write_word_data(file, reg, 0x6543)77 */78 buf[0] = reg;79 buf[1] = 0x43;80 buf[2] = 0x65;81 if (write(file, buf, 3) != 3) {82 /* ERROR HANDLING: I2C transaction failed */83 }84 85 /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */86 if (read(file, buf, 1) != 1) {87 /* ERROR HANDLING: I2C transaction failed */88 } else {89 /* buf[0] contains the read byte */90 }91 92Note that only a subset of the I2C and SMBus protocols can be achieved by93the means of read() and write() calls. In particular, so-called combined94transactions (mixing read and write messages in the same transaction)95aren't supported. For this reason, this interface is almost never used by96user-space programs.97 98IMPORTANT: because of the use of inline functions, you *have* to use99'-O' or some variation when you compile your program!100 101 102Full interface description103==========================104 105The following IOCTLs are defined:106 107``ioctl(file, I2C_SLAVE, long addr)``108 Change slave address. The address is passed in the 7 lower bits of the109 argument (except for 10 bit addresses, passed in the 10 lower bits in this110 case).111 112``ioctl(file, I2C_TENBIT, long select)``113 Selects ten bit addresses if select not equals 0, selects normal 7 bit114 addresses if select equals 0. Default 0. This request is only valid115 if the adapter has I2C_FUNC_10BIT_ADDR.116 117``ioctl(file, I2C_PEC, long select)``118 Selects SMBus PEC (packet error checking) generation and verification119 if select not equals 0, disables if select equals 0. Default 0.120 Used only for SMBus transactions. This request only has an effect if the121 the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just122 doesn't have any effect.123 124``ioctl(file, I2C_FUNCS, unsigned long *funcs)``125 Gets the adapter functionality and puts it in ``*funcs``.126 127``ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)``128 Do combined read/write transaction without stop in between.129 Only valid if the adapter has I2C_FUNC_I2C. The argument is130 a pointer to a::131 132 struct i2c_rdwr_ioctl_data {133 struct i2c_msg *msgs; /* ptr to array of simple messages */134 int nmsgs; /* number of messages to exchange */135 }136 137 The msgs[] themselves contain further pointers into data buffers.138 The function will write or read data to or from that buffers depending139 on whether the I2C_M_RD flag is set in a particular message or not.140 The slave address and whether to use ten bit address mode has to be141 set in each message, overriding the values set with the above ioctl's.142 143``ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)``144 If possible, use the provided ``i2c_smbus_*`` methods described below instead145 of issuing direct ioctls.146 147You can do plain I2C transactions by using read(2) and write(2) calls.148You do not need to pass the address byte; instead, set it through149ioctl I2C_SLAVE before you try to access the device.150 151You can do SMBus level transactions (see documentation file smbus-protocol.rst152for details) through the following functions::153 154 __s32 i2c_smbus_write_quick(int file, __u8 value);155 __s32 i2c_smbus_read_byte(int file);156 __s32 i2c_smbus_write_byte(int file, __u8 value);157 __s32 i2c_smbus_read_byte_data(int file, __u8 command);158 __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);159 __s32 i2c_smbus_read_word_data(int file, __u8 command);160 __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);161 __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);162 __s32 i2c_smbus_block_process_call(int file, __u8 command, __u8 length,163 __u8 *values);164 __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);165 __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,166 __u8 *values);167 168All these transactions return -1 on failure; you can read errno to see169what happened. The 'write' transactions return 0 on success; the170'read' transactions return the read value, except for read_block, which171returns the number of values read. The block buffers need not be longer172than 32 bytes.173 174The above functions are made available by linking against the libi2c library,175which is provided by the i2c-tools project. See:176https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/.177 178 179Implementation details180======================181 182For the interested, here's the code flow which happens inside the kernel183when you use the /dev interface to I2C:184 1851) Your program opens /dev/i2c-N and calls ioctl() on it, as described in186 section "C example" above.187 1882) These open() and ioctl() calls are handled by the i2c-dev kernel189 driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),190 respectively. You can think of i2c-dev as a generic I2C chip driver191 that can be programmed from user-space.192 1933) Some ioctl() calls are for administrative tasks and are handled by194 i2c-dev directly. Examples include I2C_SLAVE (set the address of the195 device you want to access) and I2C_PEC (enable or disable SMBus error196 checking on future transactions.)197 1984) Other ioctl() calls are converted to in-kernel function calls by199 i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter200 functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which201 performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer().202 203 The i2c-dev driver is responsible for checking all the parameters that204 come from user-space for validity. After this point, there is no205 difference between these calls that came from user-space through i2c-dev206 and calls that would have been performed by kernel I2C chip drivers207 directly. This means that I2C bus drivers don't need to implement208 anything special to support access from user-space.209 2105) These i2c.h functions are wrappers to the actual implementation of211 your I2C bus driver. Each adapter must declare callback functions212 implementing these standard calls. i2c.h:i2c_get_functionality() calls213 i2c_adapter.algo->functionality(), while214 i2c-core-smbus.c:i2c_smbus_xfer() calls either215 adapter.algo->smbus_xfer() if it is implemented, or if not,216 i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls217 i2c_adapter.algo->master_xfer().218 219After your I2C bus driver has processed these requests, execution runs220up the call chain, with almost no processing done, except by i2c-dev to221package the returned data, if any, in suitable format for the ioctl.222