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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 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.
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.
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.
17	C example
18	=========
20	So let's say you want to access an i2c adapter from a C program. The
21	first thing to do is "#include <linux/i2c-dev.h>". Please note that
22	there are two files named "i2c-dev.h" out there, one is distributed
23	with the Linux kernel and is meant to be included from kernel
24	driver code, the other one is distributed with i2c-tools and is
25	meant to be included from user-space programs. You obviously want
26	the second one here.
28	Now, you have to decide which adapter you want to access. You should
29	inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
30	Adapter numbers are assigned somewhat dynamically, so you can not
31	assume much about them. They can even change from one boot to the next.
33	Next 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	  }
46	When you have opened the device, you must specify with what device
47	address 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	  }
56	Well, you are all set up now. You can now use SMBus commands or plain
57	I2C to communicate with your device. SMBus commands are preferred if
58	the device supports them. Both are illustrated below.
60	  __u8 reg = 0x10; /* Device register to access */
61	  __s32 res;
62	  char buf[10];
64	  /* Using SMBus commands */
65	  res = i2c_smbus_read_word_data(file, reg);
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, reg, 0x6543) */
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	  }
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	  }
88	Note that only a subset of the I2C and SMBus protocols can be achieved by
89	the means of read() and write() calls. In particular, so-called combined
90	transactions (mixing read and write messages in the same transaction)
91	aren't supported. For this reason, this interface is almost never used by
92	user-space programs.
94	IMPORTANT: because of the use of inline functions, you *have* to use
95	'-O' or some variation when you compile your program!
98	Full interface description
99	==========================
101	The following IOCTLs are defined:
103	ioctl(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).
108	ioctl(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.
113	ioctl(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.
120	ioctl(file, I2C_FUNCS, unsigned long *funcs)
121	  Gets the adapter functionality and puts it in *funcs.
123	ioctl(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.
139	ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)
140	  Not meant to be called  directly; instead, use the access functions
141	  below.
143	You can do plain i2c transactions by using read(2) and write(2) calls.
144	You do not need to pass the address byte; instead, set it through
145	ioctl I2C_SLAVE before you try to access the device.
147	You can do SMBus level transactions (see documentation file smbus-protocol 
148	for 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);
160	All these transactions return -1 on failure; you can read errno to see
161	what happened. The 'write' transactions return 0 on success; the
162	'read' transactions return the read value, except for read_block, which
163	returns the number of values read. The block buffers need not be longer
164	than 32 bytes.
166	The above functions are all inline functions, that resolve to calls to
167	the i2c_smbus_access function, that on its turn calls a specific ioctl
168	with the data in a specific format. Read the source code if you
169	want to know what happens behind the screens.
172	Implementation details
173	======================
175	For the interested, here's the code flow which happens inside the kernel
176	when you use the /dev interface to I2C:
178	1* Your program opens /dev/i2c-N and calls ioctl() on it, as described in
179	section "C example" above.
181	2* These open() and ioctl() calls are handled by the i2c-dev kernel
182	driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
183	respectively. You can think of i2c-dev as a generic I2C chip driver
184	that can be programmed from user-space.
186	3* Some ioctl() calls are for administrative tasks and are handled by
187	i2c-dev directly. Examples include I2C_SLAVE (set the address of the
188	device you want to access) and I2C_PEC (enable or disable SMBus error
189	checking on future transactions.)
191	4* Other ioctl() calls are converted to in-kernel function calls by
192	i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
193	functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
194	performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer().
196	The i2c-dev driver is responsible for checking all the parameters that
197	come from user-space for validity. After this point, there is no
198	difference between these calls that came from user-space through i2c-dev
199	and calls that would have been performed by kernel I2C chip drivers
200	directly. This means that I2C bus drivers don't need to implement
201	anything special to support access from user-space.
203	5* These i2c.h functions are wrappers to the actual implementation of
204	your I2C bus driver. Each adapter must declare callback functions
205	implementing these standard calls. i2c.h:i2c_get_functionality() calls
206	i2c_adapter.algo->functionality(), while
207	i2c-core-smbus.c:i2c_smbus_xfer() calls either
208	adapter.algo->smbus_xfer() if it is implemented, or if not,
209	i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls
210	i2c_adapter.algo->master_xfer().
212	After your I2C bus driver has processed these requests, execution runs
213	up the call chain, with almost no processing done, except by i2c-dev to
214	package the returned data, if any, in suitable format for the ioctl.
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