Root/Documentation/i2c/functionality

1INTRODUCTION
2------------
3
4Because not every I2C or SMBus adapter implements everything in the
5I2C specifications, a client can not trust that everything it needs
6is implemented when it is given the option to attach to an adapter:
7the client needs some way to check whether an adapter has the needed
8functionality.
9
10
11FUNCTIONALITY CONSTANTS
12-----------------------
13
14For 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_IGNORE_NAK,
21                                  I2C_M_REV_DIR_ADDR, I2C_M_NOSTART and
22                                  I2C_M_NO_RD_ACK flags (which modify the
23                                  I2C protocol!)
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
37A 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
49  I2C_FUNC_SMBUS_EMUL Handles all SMBus commands than can be
50                                  emulated by a real I2C adapter (using
51                                  the transparent emulation layer)
52
53
54ADAPTER IMPLEMENTATION
55----------------------
56
57When you write a new adapter driver, you will have to implement a
58function callback `functionality'. Typical implementations are given
59below.
60
61A typical SMBus-only adapter would list all the SMBus transactions it
62supports. This example comes from the i2c-piix4 driver:
63
64  static u32 piix4_func(struct i2c_adapter *adapter)
65  {
66    return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE |
67           I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA |
68           I2C_FUNC_SMBUS_BLOCK_DATA;
69  }
70
71A typical full-I2C adapter would use the following (from the i2c-pxa
72driver):
73
74  static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
75  {
76    return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
77  }
78
79I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
80addition of I2C block transactions) which i2c-core can emulate using
81I2C_FUNC_I2C without any help from the adapter driver. The idea is
82to let the client drivers check for the support of SMBus functions
83without having to care whether the said functions are implemented in
84hardware by the adapter, or emulated in software by i2c-core on top
85of an I2C adapter.
86
87
88CLIENT CHECKING
89---------------
90
91Before a client tries to attach to an adapter, or even do tests to check
92whether one of the devices it supports is present on an adapter, it should
93check whether the needed functionality is present. The typical way to do
94this is (from the lm75 driver):
95
96  static int lm75_detect(...)
97  {
98    (...)
99    if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
100                     I2C_FUNC_SMBUS_WORD_DATA))
101        goto exit;
102    (...)
103  }
104
105Here, the lm75 driver checks if the adapter can do both SMBus byte data
106and SMBus word data transactions. If not, then the driver won't work on
107this adapter and there's no point in going on. If the check above is
108successful, then the driver knows that it can call the following
109functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
110i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
111thumb, the functionality constants you test for with
112i2c_check_functionality() should match exactly the i2c_smbus_* functions
113which you driver is calling.
114
115Note that the check above doesn't tell whether the functionalities are
116implemented in hardware by the underlying adapter or emulated in
117software by i2c-core. Client drivers don't have to care about this, as
118i2c-core will transparently implement SMBus transactions on top of I2C
119adapters.
120
121
122CHECKING THROUGH /DEV
123---------------------
124
125If you try to access an adapter from a userspace program, you will have
126to use the /dev interface. You will still have to check whether the
127functionality you need is supported, of course. This is done using
128the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
129below:
130
131  int file;
132  if (file = open("/dev/i2c-0", O_RDWR) < 0) {
133    /* Some kind of error handling */
134    exit(1);
135  }
136  if (ioctl(file, I2C_FUNCS, &funcs) < 0) {
137    /* Some kind of error handling */
138    exit(1);
139  }
140  if (!(funcs & I2C_FUNC_SMBUS_QUICK)) {
141    /* Oops, the needed functionality (SMBus write_quick function) is
142           not available! */
143    exit(1);
144  }
145  /* Now it is safe to use the SMBus write_quick command */
146

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