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_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
36A 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
53ALGORITHM/ADAPTER IMPLEMENTATION
54--------------------------------
55
56When you write a new algorithm driver, you will have to implement a
57function callback `functionality', that gets an i2c_adapter structure
58pointer 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
65A 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
75CLIENT CHECKING
76---------------
77
78Before a client tries to attach to an adapter, or even do tests to check
79whether one of the devices it supports is present on an adapter, it should
80check whether the needed functionality is present. There are two functions
81defined which should be used instead of calling the functionality hook
82in 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
90This is a typical way to use these functions (from the writing-clients
91document):
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
112CHECKING THROUGH /DEV
113---------------------
114
115If you try to access an adapter from a userspace program, you will have
116to use the /dev interface. You will still have to check whether the
117functionality you need is supported, of course. This is done using
118the I2C_FUNCS ioctl. An example, adapted from the lm_sensors i2c_detect
119program, 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 */
136