1 =======================
2 I2C/SMBus Functionality
3 =======================
4
5 INTRODUCTION
6 ------------
7
8 Because not every I2C or SMBus adapter implements everything in the
9 I2C specifications, a client can not trust that everything it needs
10 is implemented when it is given the option to attach to an adapter:
11 the client needs some way to check whether an adapter has the needed
12 functionality.
13
14
15 FUNCTIONALITY CONSTANTS
16 -----------------------
17
18 For the most up-to-date list of functionality constants, please check
19 <uapi/linux/i2c.h>!
20
21 =============================== ==============================================
22 I2C_FUNC_I2C Plain i2c-level commands (Pure SMBus
23 adapters typically can not do these)
24 I2C_FUNC_10BIT_ADDR Handles the 10-bit address extensions
25 I2C_FUNC_PROTOCOL_MANGLING Knows about the I2C_M_IGNORE_NAK,
26 I2C_M_REV_DIR_ADDR and I2C_M_NO_RD_ACK
27 flags (which modify the I2C protocol!)
28 I2C_FUNC_NOSTART Can skip repeated start sequence
29 I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command
30 I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command
31 I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command
32 I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command
33 I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command
34 I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command
35 I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command
36 I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command
37 I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command
38 I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command
39 I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command
40 I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command
41 =============================== ==============================================
42
43 A few combinations of the above flags are also defined for your convenience:
44
45 ========================= ======================================
46 I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte
47 and write_byte commands
48 I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data
49 and write_byte_data commands
50 I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data
51 and write_word_data commands
52 I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data
53 and write_block_data commands
54 I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data
55 and write_i2c_block_data commands
56 I2C_FUNC_SMBUS_EMUL Handles all SMBus commands that can be
57 emulated by a real I2C adapter (using
58 the transparent emulation layer)
59 ========================= ======================================
60
61 In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as
62 part of I2C_FUNC_PROTOCOL_MANGLING.
63
64
65 ADAPTER IMPLEMENTATION
66 ----------------------
67
68 When you write a new adapter driver, you will have to implement a
69 function callback ``functionality``. Typical implementations are given
70 below.
71
72 A typical SMBus-only adapter would list all the SMBus transactions it
73 supports. This example comes from the i2c-piix4 driver::
74
75 static u32 piix4_func(struct i2c_adapter *adapter)
76 {
77 return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE |
78 I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA |
79 I2C_FUNC_SMBUS_BLOCK_DATA;
80 }
81
82 A typical full-I2C adapter would use the following (from the i2c-pxa
83 driver)::
84
85 static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
86 {
87 return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
88 }
89
90 I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
91 addition of I2C block transactions) which i2c-core can emulate using
92 I2C_FUNC_I2C without any help from the adapter driver. The idea is
93 to let the client drivers check for the support of SMBus functions
94 without having to care whether the said functions are implemented in
95 hardware by the adapter, or emulated in software by i2c-core on top
96 of an I2C adapter.
97
98
99 CLIENT CHECKING
100 ---------------
101
102 Before a client tries to attach to an adapter, or even do tests to check
103 whether one of the devices it supports is present on an adapter, it should
104 check whether the needed functionality is present. The typical way to do
105 this is (from the lm75 driver)::
106
107 static int lm75_detect(...)
108 {
109 (...)
110 if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
111 I2C_FUNC_SMBUS_WORD_DATA))
112 goto exit;
113 (...)
114 }
115
116 Here, the lm75 driver checks if the adapter can do both SMBus byte data
117 and SMBus word data transactions. If not, then the driver won't work on
118 this adapter and there's no point in going on. If the check above is
119 successful, then the driver knows that it can call the following
120 functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
121 i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
122 thumb, the functionality constants you test for with
123 i2c_check_functionality() should match exactly the i2c_smbus_* functions
124 which you driver is calling.
125
126 Note that the check above doesn't tell whether the functionalities are
127 implemented in hardware by the underlying adapter or emulated in
128 software by i2c-core. Client drivers don't have to care about this, as
129 i2c-core will transparently implement SMBus transactions on top of I2C
130 adapters.
131
132
133 CHECKING THROUGH /DEV
134 ---------------------
135
136 If you try to access an adapter from a userspace program, you will have
137 to use the /dev interface. You will still have to check whether the
138 functionality you need is supported, of course. This is done using
139 the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
140 below::
141
142 int file;
143 if (file = open("/dev/i2c-0", O_RDWR) < 0) {
144 /* Some kind of error handling */
145 exit(1);
146 }
147 if (ioctl(file, I2C_FUNCS, &funcs) < 0) {
148 /* Some kind of error handling */
149 exit(1);
150 }
151 if (!(funcs & I2C_FUNC_SMBUS_QUICK)) {
152 /* Oops, the needed functionality (SMBus write_quick function) is
153 not available! */
154 exit(1);
155 }
156 /* Now it is safe to use the SMBus write_quick command */
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