1 ============================================
2 Implementing I2C device drivers in userspace
3 ============================================
4
5 Usually, I2C devices are controlled by a kernel driver. But it is also
6 possible to access all devices on an adapter from userspace, through
7 the /dev interface. You need to load module i2c-dev for this.
8
9 Each registered I2C adapter gets a number, counting from 0. You can
10 examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
11 Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all
12 I2C adapters present on your system at a given time. i2cdetect is part of
13 the i2c-tools package.
14
15 I2C device files are character device files with major device number 89
16 and a minor device number corresponding to the number assigned as
17 explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
18 i2c-10, ...). All 256 minor device numbers are reserved for I2C.
19
20
21 C example
22 =========
23
24 So let's say you want to access an I2C adapter from a C program.
25 First, you need to include these two headers::
26
27 #include <linux/i2c-dev.h>
28 #include <i2c/smbus.h>
29
30 Now, you have to decide which adapter you want to access. You should
31 inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
32 Adapter numbers are assigned somewhat dynamically, so you can not
33 assume much about them. They can even change from one boot to the next.
34
35 Next 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
48 When you have opened the device, you must specify with what device
49 address 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
58 Well, you are all set up now. You can now use SMBus commands or plain
59 I2C to communicate with your device. SMBus commands are preferred if
60 the 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 of
76 * 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
92 Note that only a subset of the I2C and SMBus protocols can be achieved by
93 the means of read() and write() calls. In particular, so-called combined
94 transactions (mixing read and write messages in the same transaction)
95 aren't supported. For this reason, this interface is almost never used by
96 user-space programs.
97
98 IMPORTANT: because of the use of inline functions, you *have* to use
99 '-O' or some variation when you compile your program!
100
101
102 Full interface description
103 ==========================
104
105 The 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 the
109 argument (except for 10 bit addresses, passed in the 10 lower bits in this
110 case).
111
112 ``ioctl(file, I2C_TENBIT, long select)``
113 Selects ten bit addresses if select not equals 0, selects normal 7 bit
114 addresses if select equals 0. Default 0. This request is only valid
115 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 verification
119 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 the
121 the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just
122 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 is
130 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 depending
139 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 be
141 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 instead
145 of issuing direct ioctls.
146
147 You can do plain I2C transactions by using read(2) and write(2) calls.
148 You do not need to pass the address byte; instead, set it through
149 ioctl I2C_SLAVE before you try to access the device.
150
151 You can do SMBus level transactions (see documentation file smbus-protocol.rst
152 for 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
168 All these transactions return -1 on failure; you can read errno to see
169 what happened. The 'write' transactions return 0 on success; the
170 'read' transactions return the read value, except for read_block, which
171 returns the number of values read. The block buffers need not be longer
172 than 32 bytes.
173
174 The above functions are made available by linking against the libi2c library,
175 which is provided by the i2c-tools project. See:
176 https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/.
177
178
179 Implementation details
180 ======================
181
182 For the interested, here's the code flow which happens inside the kernel
183 when you use the /dev interface to I2C:
184
185 1) Your program opens /dev/i2c-N and calls ioctl() on it, as described in
186 section "C example" above.
187
188 2) These open() and ioctl() calls are handled by the i2c-dev kernel
189 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 driver
191 that can be programmed from user-space.
192
193 3) Some ioctl() calls are for administrative tasks and are handled by
194 i2c-dev directly. Examples include I2C_SLAVE (set the address of the
195 device you want to access) and I2C_PEC (enable or disable SMBus error
196 checking on future transactions.)
197
198 4) Other ioctl() calls are converted to in-kernel function calls by
199 i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
200 functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
201 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 that
204 come from user-space for validity. After this point, there is no
205 difference between these calls that came from user-space through i2c-dev
206 and calls that would have been performed by kernel I2C chip drivers
207 directly. This means that I2C bus drivers don't need to implement
208 anything special to support access from user-space.
209
210 5) These i2c.h functions are wrappers to the actual implementation of
211 your I2C bus driver. Each adapter must declare callback functions
212 implementing these standard calls. i2c.h:i2c_get_functionality() calls
213 i2c_adapter.algo->functionality(), while
214 i2c-core-smbus.c:i2c_smbus_xfer() calls either
215 adapter.algo->smbus_xfer() if it is implemented, or if not,
216 i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls
217 i2c_adapter.algo->master_xfer().
218
219 After your I2C bus driver has processed these requests, execution runs
220 up the call chain, with almost no processing done, except by i2c-dev to
221 package the returned data, if any, in suitable format for the ioctl.
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