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.
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 formated list of all
8i2c adapters present on your system at a given time. i2cdetect is part of
9the i2c-tools package.
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.
17C example
20So let's say you want to access an i2c adapter from a C program. The
21first thing to do is "#include <linux/i2c-dev.h>". Please note that
22there are two files named "i2c-dev.h" out there, one is distributed
23with the Linux kernel and is meant to be included from kernel
24driver code, the other one is distributed with i2c-tools and is
25meant to be included from user-space programs. You obviously want
26the second one here.
28Now, you have to decide which adapter you want to access. You should
29inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
30Adapter numbers are assigned somewhat dynamically, so you can not
31assume much about them. They can even change from one boot to the next.
33Next thing, open the device file, as follows:
35  int file;
36  int adapter_nr = 2; /* probably dynamically determined */
37  char filename[20];
39  snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
40  file = open(filename, O_RDWR);
41  if (file < 0) {
42    /* ERROR HANDLING; you can check errno to see what went wrong */
43    exit(1);
44  }
46When you have opened the device, you must specify with what device
47address you want to communicate:
49  int addr = 0x40; /* The I2C address */
51  if (ioctl(file, I2C_SLAVE, addr) < 0) {
52    /* ERROR HANDLING; you can check errno to see what went wrong */
53    exit(1);
54  }
56Well, you are all set up now. You can now use SMBus commands or plain
57I2C to communicate with your device. SMBus commands are preferred if
58the device supports them. Both are illustrated below.
60  __u8 register = 0x10; /* Device register to access */
61  __s32 res;
62  char buf[10];
64  /* Using SMBus commands */
65  res = i2c_smbus_read_word_data(file, register);
66  if (res < 0) {
67    /* ERROR HANDLING: i2c transaction failed */
68  } else {
69    /* res contains the read word */
70  }
72  /* Using I2C Write, equivalent of
73     i2c_smbus_write_word_data(file, register, 0x6543) */
74  buf[0] = register;
75  buf[1] = 0x43;
76  buf[2] = 0x65;
77  if (write(file, buf, 3) ! =3) {
78    /* ERROR HANDLING: i2c transaction failed */
79  }
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  }
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.
94IMPORTANT: because of the use of inline functions, you *have* to use
95'-O' or some variation when you compile your program!
98Full interface description
101The following IOCTLs are defined:
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).
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.
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.
120ioctl(file, I2C_FUNCS, unsigned long *funcs)
121  Gets the adapter functionality and puts it in *funcs.
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
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  }
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.
139ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)
140  Not meant to be called directly; instead, use the access functions
141  below.
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.
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.
166The above functions are all inline functions, that resolve to calls to
167the i2c_smbus_access function, that on its turn calls a specific ioctl
168with the data in a specific format. Read the source code if you
169want to know what happens behind the screens.
172Implementation details
175For the interested, here's the code flow which happens inside the kernel
176when you use the /dev interface to I2C:
1781* Your program opens /dev/i2c-N and calls ioctl() on it, as described in
179section "C example" above.
1812* These open() and ioctl() calls are handled by the i2c-dev kernel
182driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
183respectively. You can think of i2c-dev as a generic I2C chip driver
184that can be programmed from user-space.
1863* Some ioctl() calls are for administrative tasks and are handled by
187i2c-dev directly. Examples include I2C_SLAVE (set the address of the
188device you want to access) and I2C_PEC (enable or disable SMBus error
189checking on future transactions.)
1914* Other ioctl() calls are converted to in-kernel function calls by
192i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
193functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
194performs an SMBus transaction using i2c-core.c:i2c_smbus_xfer().
196The i2c-dev driver is responsible for checking all the parameters that
197come from user-space for validity. After this point, there is no
198difference between these calls that came from user-space through i2c-dev
199and calls that would have been performed by kernel I2C chip drivers
200directly. This means that I2C bus drivers don't need to implement
201anything special to support access from user-space.
2035* These i2c-core.c/i2c.h functions are wrappers to the actual
204implementation of your I2C bus driver. Each adapter must declare
205callback functions implementing these standard calls.
206i2c.h:i2c_get_functionality() calls i2c_adapter.algo->functionality(),
207while i2c-core.c:i2c_smbus_xfer() calls either
208adapter.algo->smbus_xfer() if it is implemented, or if not,
209i2c-core.c:i2c_smbus_xfer_emulated() which in turn calls
212After your I2C bus driver has processed these requests, execution runs
213up the call chain, with almost no processing done, except by i2c-dev to
214package the returned data, if any, in suitable format for the ioctl.

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