Commit | Line | Data |
---|---|---|
1da177e4 LT |
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 | |
8421d4fe | 15 | <uapi/linux/i2c.h>! |
1da177e4 LT |
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 | |
d057c96c | 20 | I2C_FUNC_PROTOCOL_MANGLING Knows about the I2C_M_IGNORE_NAK, |
14674e70 MB |
21 | I2C_M_REV_DIR_ADDR and I2C_M_NO_RD_ACK |
22 | flags (which modify the I2C protocol!) | |
23 | I2C_FUNC_NOSTART Can skip repeated start sequence | |
1da177e4 LT |
24 | I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command |
25 | I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command | |
26 | I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command | |
27 | I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command | |
28 | I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command | |
29 | I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command | |
30 | I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command | |
31 | I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command | |
32 | I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command | |
33 | I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command | |
34 | I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command | |
35 | I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command | |
36 | ||
37 | A few combinations of the above flags are also defined for your convenience: | |
38 | ||
39 | I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte | |
40 | and write_byte commands | |
41 | I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data | |
42 | and write_byte_data commands | |
43 | I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data | |
44 | and write_word_data commands | |
45 | I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data | |
46 | and write_block_data commands | |
47 | I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data | |
48 | and write_i2c_block_data commands | |
6808b002 | 49 | I2C_FUNC_SMBUS_EMUL Handles all SMBus commands that can be |
1da177e4 LT |
50 | emulated by a real I2C adapter (using |
51 | the transparent emulation layer) | |
52 | ||
14674e70 MB |
53 | In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as |
54 | part of I2C_FUNC_PROTOCOL_MANGLING. | |
55 | ||
1da177e4 | 56 | |
88b28328 JD |
57 | ADAPTER IMPLEMENTATION |
58 | ---------------------- | |
1da177e4 | 59 | |
88b28328 JD |
60 | When you write a new adapter driver, you will have to implement a |
61 | function callback `functionality'. Typical implementations are given | |
62 | below. | |
1da177e4 | 63 | |
88b28328 JD |
64 | A typical SMBus-only adapter would list all the SMBus transactions it |
65 | supports. This example comes from the i2c-piix4 driver: | |
66 | ||
67 | static u32 piix4_func(struct i2c_adapter *adapter) | |
68 | { | |
69 | return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE | | |
70 | I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA | | |
71 | I2C_FUNC_SMBUS_BLOCK_DATA; | |
1da177e4 LT |
72 | } |
73 | ||
88b28328 JD |
74 | A typical full-I2C adapter would use the following (from the i2c-pxa |
75 | driver): | |
1da177e4 | 76 | |
88b28328 | 77 | static u32 i2c_pxa_functionality(struct i2c_adapter *adap) |
1da177e4 | 78 | { |
88b28328 | 79 | return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL; |
1da177e4 LT |
80 | } |
81 | ||
88b28328 JD |
82 | I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the |
83 | addition of I2C block transactions) which i2c-core can emulate using | |
84 | I2C_FUNC_I2C without any help from the adapter driver. The idea is | |
85 | to let the client drivers check for the support of SMBus functions | |
86 | without having to care whether the said functions are implemented in | |
87 | hardware by the adapter, or emulated in software by i2c-core on top | |
88 | of an I2C adapter. | |
1da177e4 LT |
89 | |
90 | ||
91 | CLIENT CHECKING | |
92 | --------------- | |
93 | ||
94 | Before a client tries to attach to an adapter, or even do tests to check | |
95 | whether one of the devices it supports is present on an adapter, it should | |
88b28328 JD |
96 | check whether the needed functionality is present. The typical way to do |
97 | this is (from the lm75 driver): | |
1da177e4 | 98 | |
88b28328 | 99 | static int lm75_detect(...) |
1da177e4 | 100 | { |
88b28328 JD |
101 | (...) |
102 | if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA | | |
103 | I2C_FUNC_SMBUS_WORD_DATA)) | |
104 | goto exit; | |
105 | (...) | |
1da177e4 LT |
106 | } |
107 | ||
88b28328 JD |
108 | Here, the lm75 driver checks if the adapter can do both SMBus byte data |
109 | and SMBus word data transactions. If not, then the driver won't work on | |
110 | this adapter and there's no point in going on. If the check above is | |
111 | successful, then the driver knows that it can call the following | |
112 | functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(), | |
113 | i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of | |
114 | thumb, the functionality constants you test for with | |
115 | i2c_check_functionality() should match exactly the i2c_smbus_* functions | |
116 | which you driver is calling. | |
117 | ||
118 | Note that the check above doesn't tell whether the functionalities are | |
119 | implemented in hardware by the underlying adapter or emulated in | |
120 | software by i2c-core. Client drivers don't have to care about this, as | |
121 | i2c-core will transparently implement SMBus transactions on top of I2C | |
122 | adapters. | |
1da177e4 LT |
123 | |
124 | ||
125 | CHECKING THROUGH /DEV | |
126 | --------------------- | |
127 | ||
128 | If you try to access an adapter from a userspace program, you will have | |
129 | to use the /dev interface. You will still have to check whether the | |
130 | functionality you need is supported, of course. This is done using | |
88b28328 JD |
131 | the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is |
132 | below: | |
1da177e4 LT |
133 | |
134 | int file; | |
88b28328 | 135 | if (file = open("/dev/i2c-0", O_RDWR) < 0) { |
1da177e4 LT |
136 | /* Some kind of error handling */ |
137 | exit(1); | |
138 | } | |
88b28328 | 139 | if (ioctl(file, I2C_FUNCS, &funcs) < 0) { |
1da177e4 LT |
140 | /* Some kind of error handling */ |
141 | exit(1); | |
142 | } | |
88b28328 | 143 | if (!(funcs & I2C_FUNC_SMBUS_QUICK)) { |
1da177e4 LT |
144 | /* Oops, the needed functionality (SMBus write_quick function) is |
145 | not available! */ | |
146 | exit(1); | |
147 | } | |
148 | /* Now it is safe to use the SMBus write_quick command */ |