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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

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_IGNORE_NAK,
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
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
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	In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as
54	part of I2C_FUNC_PROTOCOL_MANGLING.
55	
56	
57	ADAPTER IMPLEMENTATION
58	----------------------
59	
60	When you write a new adapter driver, you will have to implement a
61	function callback `functionality'. Typical implementations are given
62	below.
63	
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;
72	  }
73	
74	A typical full-I2C adapter would use the following (from the i2c-pxa
75	driver):
76	
77	  static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
78	  {
79		return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
80	  }
81	
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.
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
96	check whether the needed functionality is present. The typical way to do
97	this is (from the lm75 driver):
98	
99	  static int lm75_detect(...)
100	  {
101		(...)
102		if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
103					     I2C_FUNC_SMBUS_WORD_DATA))
104			goto exit;
105		(...)
106	  }
107	
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.
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
131	the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
132	below:
133	
134	  int file;
135	  if (file = open("/dev/i2c-0", O_RDWR) < 0) {
136		/* Some kind of error handling */
137		exit(1);
138	  }
139	  if (ioctl(file, I2C_FUNCS, &funcs) < 0) {
140		/* Some kind of error handling */
141		exit(1);
142	  }
143	  if (!(funcs & I2C_FUNC_SMBUS_QUICK)) {
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 */
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