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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.
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.
21	First, you need to include these two headers:
23	  #include <linux/i2c-dev.h>
24	  #include <i2c/smbus.h>
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").
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.
36	Next thing, open the device file, as follows:
38	  int file;
39	  int adapter_nr = 2; /* probably dynamically determined */
40	  char filename[20];
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	  }
49	When you have opened the device, you must specify with what device
50	address you want to communicate:
52	  int addr = 0x40; /* The I2C address */
54	  if (ioctl(file, I2C_SLAVE, addr) < 0) {
55	    /* ERROR HANDLING; you can check errno to see what went wrong */
56	    exit(1);
57	  }
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.
63	  __u8 reg = 0x10; /* Device register to access */
64	  __s32 res;
65	  char buf[10];
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	  }
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	  }
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	  }
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.
97	IMPORTANT: because of the use of inline functions, you *have* to use
98	'-O' or some variation when you compile your program!
101	Full interface description
102	==========================
104	The following IOCTLs are defined:
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).
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.
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.
123	ioctl(file, I2C_FUNCS, unsigned long *funcs)
124	  Gets the adapter functionality and puts it in *funcs.
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
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	  }
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.
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.
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.
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.
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.
175	Implementation details
176	======================
178	For the interested, here's the code flow which happens inside the kernel
179	when you use the /dev interface to I2C:
181	1* Your program opens /dev/i2c-N and calls ioctl() on it, as described in
182	section "C example" above.
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.
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.)
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().
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.
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().
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.
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