1 INTRODUCTION
2 ------------
3
4 Because not every I2C or SMBus adapter implements everything in the
5 I2C specifications, a client can not trust that everything it needs
6 is implemented when it is given the option to attach to an adapter:
7 the client needs some way to check whether an adapter has the needed
8 functionality.
9
10
11 FUNCTIONALITY CONSTANTS
12 -----------------------
13
14 For the most up-to-date list of functionality constants, please check
15 <linux/i2c.h>!
16
17 I2C_FUNC_I2C Plain i2c-level commands (Pure SMBus
18 adapters typically can not do these)
19 I2C_FUNC_10BIT_ADDR Handles the 10-bit address extensions
20 I2C_FUNC_PROTOCOL_MANGLING Knows about the I2C_M_REV_DIR_ADDR,
21 I2C_M_REV_DIR_ADDR and I2C_M_REV_DIR_NOSTART
22 flags (which modify the i2c protocol!)
23 I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command
24 I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command
25 I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command
26 I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command
27 I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command
28 I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command
29 I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command
30 I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command
31 I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command
32 I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command
33 I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command
34 I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command
35
36 A few combinations of the above flags are also defined for your convenience:
37
38 I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte
39 and write_byte commands
40 I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data
41 and write_byte_data commands
42 I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data
43 and write_word_data commands
44 I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data
45 and write_block_data commands
46 I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data
47 and write_i2c_block_data commands
48 I2C_FUNC_SMBUS_EMUL Handles all SMBus commands than can be
49 emulated by a real I2C adapter (using
50 the transparent emulation layer)
51
52
53 ALGORITHM/ADAPTER IMPLEMENTATION
54 --------------------------------
55
56 When you write a new algorithm driver, you will have to implement a
57 function callback `functionality', that gets an i2c_adapter structure
58 pointer as its only parameter:
59
60 struct i2c_algorithm {
61 /* Many other things of course; check <linux/i2c.h>! */
62 u32 (*functionality) (struct i2c_adapter *);
63 }
64
65 A typically implementation is given below, from i2c-algo-bit.c:
66
67 static u32 bit_func(struct i2c_adapter *adap)
68 {
69 return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR |
70 I2C_FUNC_PROTOCOL_MANGLING;
71 }
72
73
74
75 CLIENT CHECKING
76 ---------------
77
78 Before a client tries to attach to an adapter, or even do tests to check
79 whether one of the devices it supports is present on an adapter, it should
80 check whether the needed functionality is present. There are two functions
81 defined which should be used instead of calling the functionality hook
82 in the algorithm structure directly:
83
84 /* Return the functionality mask */
85 extern u32 i2c_get_functionality (struct i2c_adapter *adap);
86
87 /* Return 1 if adapter supports everything we need, 0 if not. */
88 extern int i2c_check_functionality (struct i2c_adapter *adap, u32 func);
89
90 This is a typical way to use these functions (from the writing-clients
91 document):
92 int foo_detect_client(struct i2c_adapter *adapter, int address,
93 unsigned short flags, int kind)
94 {
95 /* Define needed variables */
96
97 /* As the very first action, we check whether the adapter has the
98 needed functionality: we need the SMBus read_word_data,
99 write_word_data and write_byte functions in this example. */
100 if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
101 I2C_FUNC_SMBUS_WRITE_BYTE))
102 goto ERROR0;
103
104 /* Now we can do the real detection */
105
106 ERROR0:
107 /* Return an error */
108 }
109
110
111
112 CHECKING THROUGH /DEV
113 ---------------------
114
115 If you try to access an adapter from a userspace program, you will have
116 to use the /dev interface. You will still have to check whether the
117 functionality you need is supported, of course. This is done using
118 the I2C_FUNCS ioctl. An example, adapted from the lm_sensors i2c_detect
119 program, is below:
120
121 int file;
122 if (file = open("/dev/i2c-0",O_RDWR) < 0) {
123 /* Some kind of error handling */
124 exit(1);
125 }
126 if (ioctl(file,I2C_FUNCS,&funcs) < 0) {
127 /* Some kind of error handling */
128 exit(1);
129 }
130 if (! (funcs & I2C_FUNC_SMBUS_QUICK)) {
131 /* Oops, the needed functionality (SMBus write_quick function) is
132 not available! */
133 exit(1);
134 }
135 /* Now it is safe to use the SMBus write_quick command */
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