1 ============================================ 1 ============================================
2 Implementing I2C device drivers in userspace 2 Implementing I2C device drivers in userspace
3 ============================================ 3 ============================================
4 4
5 Usually, I2C devices are controlled by a kerne 5 Usually, I2C devices are controlled by a kernel driver. But it is also
6 possible to access all devices on an adapter f 6 possible to access all devices on an adapter from userspace, through
7 the /dev interface. You need to load module i2 7 the /dev interface. You need to load module i2c-dev for this.
8 8
9 Each registered I2C adapter gets a number, cou 9 Each registered I2C adapter gets a number, counting from 0. You can
10 examine /sys/class/i2c-dev/ to see what number 10 examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
11 Alternatively, you can run "i2cdetect -l" to o 11 Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all
12 I2C adapters present on your system at a given 12 I2C adapters present on your system at a given time. i2cdetect is part of
13 the i2c-tools package. 13 the i2c-tools package.
14 14
15 I2C device files are character device files wi 15 I2C device files are character device files with major device number 89
16 and a minor device number corresponding to the 16 and a minor device number corresponding to the number assigned as
17 explained above. They should be called "i2c-%d 17 explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
18 i2c-10, ...). All 256 minor device numbers are 18 i2c-10, ...). All 256 minor device numbers are reserved for I2C.
19 19
20 20
21 C example 21 C example
22 ========= 22 =========
23 23
24 So let's say you want to access an I2C adapter 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:: 25 First, you need to include these two headers::
26 26
27 #include <linux/i2c-dev.h> 27 #include <linux/i2c-dev.h>
28 #include <i2c/smbus.h> 28 #include <i2c/smbus.h>
29 29
30 Now, you have to decide which adapter you want 30 Now, you have to decide which adapter you want to access. You should
31 inspect /sys/class/i2c-dev/ or run "i2cdetect 31 inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
32 Adapter numbers are assigned somewhat dynamica 32 Adapter numbers are assigned somewhat dynamically, so you can not
33 assume much about them. They can even change f 33 assume much about them. They can even change from one boot to the next.
34 34
35 Next thing, open the device file, as follows:: 35 Next thing, open the device file, as follows::
36 36
37 int file; 37 int file;
38 int adapter_nr = 2; /* probably dynamically 38 int adapter_nr = 2; /* probably dynamically determined */
39 char filename[20]; 39 char filename[20];
40 40
41 snprintf(filename, 19, "/dev/i2c-%d", adapte 41 snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
42 file = open(filename, O_RDWR); 42 file = open(filename, O_RDWR);
43 if (file < 0) { 43 if (file < 0) {
44 /* ERROR HANDLING; you can check errno to 44 /* ERROR HANDLING; you can check errno to see what went wrong */
45 exit(1); 45 exit(1);
46 } 46 }
47 47
48 When you have opened the device, you must spec 48 When you have opened the device, you must specify with what device
49 address you want to communicate:: 49 address you want to communicate::
50 50
51 int addr = 0x40; /* The I2C address */ 51 int addr = 0x40; /* The I2C address */
52 52
53 if (ioctl(file, I2C_SLAVE, addr) < 0) { 53 if (ioctl(file, I2C_SLAVE, addr) < 0) {
54 /* ERROR HANDLING; you can check errno to 54 /* ERROR HANDLING; you can check errno to see what went wrong */
55 exit(1); 55 exit(1);
56 } 56 }
57 57
58 Well, you are all set up now. You can now use 58 Well, you are all set up now. You can now use SMBus commands or plain
59 I2C to communicate with your device. SMBus com 59 I2C to communicate with your device. SMBus commands are preferred if
60 the device supports them. Both are illustrated 60 the device supports them. Both are illustrated below::
61 61
62 __u8 reg = 0x10; /* Device register to acces 62 __u8 reg = 0x10; /* Device register to access */
63 __s32 res; 63 __s32 res;
64 char buf[10]; 64 char buf[10];
65 65
66 /* Using SMBus commands */ 66 /* Using SMBus commands */
67 res = i2c_smbus_read_word_data(file, reg); 67 res = i2c_smbus_read_word_data(file, reg);
68 if (res < 0) { 68 if (res < 0) {
69 /* ERROR HANDLING: I2C transaction failed 69 /* ERROR HANDLING: I2C transaction failed */
70 } else { 70 } else {
71 /* res contains the read word */ 71 /* res contains the read word */
72 } 72 }
73 73
74 /* 74 /*
75 * Using I2C Write, equivalent of 75 * Using I2C Write, equivalent of
76 * i2c_smbus_write_word_data(file, reg, 0x65 76 * i2c_smbus_write_word_data(file, reg, 0x6543)
77 */ 77 */
78 buf[0] = reg; 78 buf[0] = reg;
79 buf[1] = 0x43; 79 buf[1] = 0x43;
80 buf[2] = 0x65; 80 buf[2] = 0x65;
81 if (write(file, buf, 3) != 3) { 81 if (write(file, buf, 3) != 3) {
82 /* ERROR HANDLING: I2C transaction failed 82 /* ERROR HANDLING: I2C transaction failed */
83 } 83 }
84 84
85 /* Using I2C Read, equivalent of i2c_smbus_r 85 /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
86 if (read(file, buf, 1) != 1) { 86 if (read(file, buf, 1) != 1) {
87 /* ERROR HANDLING: I2C transaction failed 87 /* ERROR HANDLING: I2C transaction failed */
88 } else { 88 } else {
89 /* buf[0] contains the read byte */ 89 /* buf[0] contains the read byte */
90 } 90 }
91 91
92 Note that only a subset of the I2C and SMBus p 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 part 93 the means of read() and write() calls. In particular, so-called combined
94 transactions (mixing read and write messages i 94 transactions (mixing read and write messages in the same transaction)
95 aren't supported. For this reason, this interf 95 aren't supported. For this reason, this interface is almost never used by
96 user-space programs. 96 user-space programs.
97 97
98 IMPORTANT: because of the use of inline functi 98 IMPORTANT: because of the use of inline functions, you *have* to use
99 '-O' or some variation when you compile your p 99 '-O' or some variation when you compile your program!
100 100
101 101
102 Full interface description 102 Full interface description
103 ========================== 103 ==========================
104 104
105 The following IOCTLs are defined: 105 The following IOCTLs are defined:
106 106
107 ``ioctl(file, I2C_SLAVE, long addr)`` 107 ``ioctl(file, I2C_SLAVE, long addr)``
108 Change slave address. The address is passed 108 Change slave address. The address is passed in the 7 lower bits of the
109 argument (except for 10 bit addresses, passe 109 argument (except for 10 bit addresses, passed in the 10 lower bits in this
110 case). 110 case).
111 111
112 ``ioctl(file, I2C_TENBIT, long select)`` 112 ``ioctl(file, I2C_TENBIT, long select)``
113 Selects ten bit addresses if select not equa 113 Selects ten bit addresses if select not equals 0, selects normal 7 bit
114 addresses if select equals 0. Default 0. Th 114 addresses if select equals 0. Default 0. This request is only valid
115 if the adapter has I2C_FUNC_10BIT_ADDR. 115 if the adapter has I2C_FUNC_10BIT_ADDR.
116 116
117 ``ioctl(file, I2C_PEC, long select)`` 117 ``ioctl(file, I2C_PEC, long select)``
118 Selects SMBus PEC (packet error checking) ge 118 Selects SMBus PEC (packet error checking) generation and verification
119 if select not equals 0, disables if select e 119 if select not equals 0, disables if select equals 0. Default 0.
120 Used only for SMBus transactions. This requ 120 Used only for SMBus transactions. This request only has an effect if the
121 the adapter has I2C_FUNC_SMBUS_PEC; it is st 121 the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just
122 doesn't have any effect. 122 doesn't have any effect.
123 123
124 ``ioctl(file, I2C_FUNCS, unsigned long *funcs) 124 ``ioctl(file, I2C_FUNCS, unsigned long *funcs)``
125 Gets the adapter functionality and puts it i 125 Gets the adapter functionality and puts it in ``*funcs``.
126 126
127 ``ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_ 127 ``ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)``
128 Do combined read/write transaction without s 128 Do combined read/write transaction without stop in between.
129 Only valid if the adapter has I2C_FUNC_I2C. 129 Only valid if the adapter has I2C_FUNC_I2C. The argument is
130 a pointer to a:: 130 a pointer to a::
131 131
132 struct i2c_rdwr_ioctl_data { 132 struct i2c_rdwr_ioctl_data {
133 struct i2c_msg *msgs; /* ptr to array o 133 struct i2c_msg *msgs; /* ptr to array of simple messages */
134 int nmsgs; /* number of mess 134 int nmsgs; /* number of messages to exchange */
135 } 135 }
136 136
137 The msgs[] themselves contain further pointe 137 The msgs[] themselves contain further pointers into data buffers.
138 The function will write or read data to or f 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 par 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 140 The slave address and whether to use ten bit address mode has to be
141 set in each message, overriding the values s 141 set in each message, overriding the values set with the above ioctl's.
142 142
143 ``ioctl(file, I2C_SMBUS, struct i2c_smbus_ioct 143 ``ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)``
144 If possible, use the provided ``i2c_smbus_*` 144 If possible, use the provided ``i2c_smbus_*`` methods described below instead
145 of issuing direct ioctls. 145 of issuing direct ioctls.
146 146
147 You can do plain I2C transactions by using rea 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; inst 148 You do not need to pass the address byte; instead, set it through
149 ioctl I2C_SLAVE before you try to access the d 149 ioctl I2C_SLAVE before you try to access the device.
150 150
151 You can do SMBus level transactions (see docum 151 You can do SMBus level transactions (see documentation file smbus-protocol.rst
152 for details) through the following functions:: 152 for details) through the following functions::
153 153
154 __s32 i2c_smbus_write_quick(int file, __u8 v 154 __s32 i2c_smbus_write_quick(int file, __u8 value);
155 __s32 i2c_smbus_read_byte(int file); 155 __s32 i2c_smbus_read_byte(int file);
156 __s32 i2c_smbus_write_byte(int file, __u8 va 156 __s32 i2c_smbus_write_byte(int file, __u8 value);
157 __s32 i2c_smbus_read_byte_data(int file, __u 157 __s32 i2c_smbus_read_byte_data(int file, __u8 command);
158 __s32 i2c_smbus_write_byte_data(int file, __ 158 __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
159 __s32 i2c_smbus_read_word_data(int file, __u 159 __s32 i2c_smbus_read_word_data(int file, __u8 command);
160 __s32 i2c_smbus_write_word_data(int file, __ 160 __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
161 __s32 i2c_smbus_process_call(int file, __u8 161 __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
162 __s32 i2c_smbus_block_process_call(int file, 162 __s32 i2c_smbus_block_process_call(int file, __u8 command, __u8 length,
163 __u8 *val 163 __u8 *values);
164 __s32 i2c_smbus_read_block_data(int file, __ 164 __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
165 __s32 i2c_smbus_write_block_data(int file, _ 165 __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,
166 __u8 *value 166 __u8 *values);
167 167
168 All these transactions return -1 on failure; y 168 All these transactions return -1 on failure; you can read errno to see
169 what happened. The 'write' transactions return 169 what happened. The 'write' transactions return 0 on success; the
170 'read' transactions return the read value, exc 170 'read' transactions return the read value, except for read_block, which
171 returns the number of values read. The block b 171 returns the number of values read. The block buffers need not be longer
172 than 32 bytes. 172 than 32 bytes.
173 173
174 The above functions are made available by link 174 The above functions are made available by linking against the libi2c library,
175 which is provided by the i2c-tools project. S 175 which is provided by the i2c-tools project. See:
176 https://git.kernel.org/pub/scm/utils/i2c-tools 176 https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/.
177 177
178 178
179 Implementation details 179 Implementation details
180 ====================== 180 ======================
181 181
182 For the interested, here's the code flow which 182 For the interested, here's the code flow which happens inside the kernel
183 when you use the /dev interface to I2C: 183 when you use the /dev interface to I2C:
184 184
185 1) Your program opens /dev/i2c-N and calls ioc 185 1) Your program opens /dev/i2c-N and calls ioctl() on it, as described in
186 section "C example" above. 186 section "C example" above.
187 187
188 2) These open() and ioctl() calls are handled 188 2) These open() and ioctl() calls are handled by the i2c-dev kernel
189 driver: see i2c-dev.c:i2cdev_open() and i2c 189 driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
190 respectively. You can think of i2c-dev as a 190 respectively. You can think of i2c-dev as a generic I2C chip driver
191 that can be programmed from user-space. 191 that can be programmed from user-space.
192 192
193 3) Some ioctl() calls are for administrative t 193 3) Some ioctl() calls are for administrative tasks and are handled by
194 i2c-dev directly. Examples include I2C_SLAV 194 i2c-dev directly. Examples include I2C_SLAVE (set the address of the
195 device you want to access) and I2C_PEC (ena 195 device you want to access) and I2C_PEC (enable or disable SMBus error
196 checking on future transactions.) 196 checking on future transactions.)
197 197
198 4) Other ioctl() calls are converted to in-ker 198 4) Other ioctl() calls are converted to in-kernel function calls by
199 i2c-dev. Examples include I2C_FUNCS, which 199 i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
200 functionality using i2c.h:i2c_get_functiona 200 functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
201 performs an SMBus transaction using i2c-cor 201 performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer().
202 202
203 The i2c-dev driver is responsible for check 203 The i2c-dev driver is responsible for checking all the parameters that
204 come from user-space for validity. After th 204 come from user-space for validity. After this point, there is no
205 difference between these calls that came fr 205 difference between these calls that came from user-space through i2c-dev
206 and calls that would have been performed by 206 and calls that would have been performed by kernel I2C chip drivers
207 directly. This means that I2C bus drivers d 207 directly. This means that I2C bus drivers don't need to implement
208 anything special to support access from use 208 anything special to support access from user-space.
209 209
210 5) These i2c.h functions are wrappers to the a 210 5) These i2c.h functions are wrappers to the actual implementation of
211 your I2C bus driver. Each adapter must decl 211 your I2C bus driver. Each adapter must declare callback functions
212 implementing these standard calls. i2c.h:i2 212 implementing these standard calls. i2c.h:i2c_get_functionality() calls
213 i2c_adapter.algo->functionality(), while 213 i2c_adapter.algo->functionality(), while
214 i2c-core-smbus.c:i2c_smbus_xfer() calls eit 214 i2c-core-smbus.c:i2c_smbus_xfer() calls either
215 adapter.algo->smbus_xfer() if it is impleme 215 adapter.algo->smbus_xfer() if it is implemented, or if not,
216 i2c-core-smbus.c:i2c_smbus_xfer_emulated() 216 i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls
217 i2c_adapter.algo->master_xfer(). 217 i2c_adapter.algo->master_xfer().
218 218
219 After your I2C bus driver has processed these 219 After your I2C bus driver has processed these requests, execution runs
220 up the call chain, with almost no processing d 220 up the call chain, with almost no processing done, except by i2c-dev to
221 package the returned data, if any, in suitable 221 package the returned data, if any, in suitable format for the ioctl.
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