137 lines
5.3 KiB
Plaintext
137 lines
5.3 KiB
Plaintext
INTRODUCTION
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------------
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Because not every I2C or SMBus adapter implements everything in the
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I2C specifications, a client can not trust that everything it needs
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is implemented when it is given the option to attach to an adapter:
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the client needs some way to check whether an adapter has the needed
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functionality.
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FUNCTIONALITY CONSTANTS
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-----------------------
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For the most up-to-date list of functionality constants, please check
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<linux/i2c.h>!
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I2C_FUNC_I2C Plain i2c-level commands (Pure SMBus
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adapters typically can not do these)
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I2C_FUNC_10BIT_ADDR Handles the 10-bit address extensions
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I2C_FUNC_PROTOCOL_MANGLING Knows about the I2C_M_IGNORE_NAK,
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I2C_M_REV_DIR_ADDR, I2C_M_NOSTART and
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I2C_M_NO_RD_ACK flags (which modify the
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I2C protocol!)
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I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command
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I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command
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I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command
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I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command
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I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command
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I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command
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I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command
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I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command
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I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command
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I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command
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I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command
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I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command
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A few combinations of the above flags are also defined for your convenience:
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I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte
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and write_byte commands
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I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data
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and write_byte_data commands
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I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data
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and write_word_data commands
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I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data
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and write_block_data commands
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I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data
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and write_i2c_block_data commands
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I2C_FUNC_SMBUS_EMUL Handles all SMBus commands than can be
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emulated by a real I2C adapter (using
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the transparent emulation layer)
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ALGORITHM/ADAPTER IMPLEMENTATION
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--------------------------------
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When you write a new algorithm driver, you will have to implement a
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function callback `functionality', that gets an i2c_adapter structure
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pointer as its only parameter:
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struct i2c_algorithm {
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/* Many other things of course; check <linux/i2c.h>! */
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u32 (*functionality) (struct i2c_adapter *);
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}
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A typically implementation is given below, from i2c-algo-bit.c:
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static u32 bit_func(struct i2c_adapter *adap)
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{
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return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR |
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I2C_FUNC_PROTOCOL_MANGLING;
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}
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CLIENT CHECKING
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---------------
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Before a client tries to attach to an adapter, or even do tests to check
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whether one of the devices it supports is present on an adapter, it should
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check whether the needed functionality is present. There are two functions
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defined which should be used instead of calling the functionality hook
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in the algorithm structure directly:
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/* Return the functionality mask */
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extern u32 i2c_get_functionality (struct i2c_adapter *adap);
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/* Return 1 if adapter supports everything we need, 0 if not. */
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extern int i2c_check_functionality (struct i2c_adapter *adap, u32 func);
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This is a typical way to use these functions (from the writing-clients
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document):
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int foo_detect_client(struct i2c_adapter *adapter, int address,
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unsigned short flags, int kind)
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{
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/* Define needed variables */
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/* As the very first action, we check whether the adapter has the
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needed functionality: we need the SMBus read_word_data,
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write_word_data and write_byte functions in this example. */
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if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
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I2C_FUNC_SMBUS_WRITE_BYTE))
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goto ERROR0;
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/* Now we can do the real detection */
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ERROR0:
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/* Return an error */
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}
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CHECKING THROUGH /DEV
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---------------------
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If you try to access an adapter from a userspace program, you will have
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to use the /dev interface. You will still have to check whether the
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functionality you need is supported, of course. This is done using
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the I2C_FUNCS ioctl. An example, adapted from the lm_sensors i2cdetect
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program, is below:
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int file;
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if (file = open("/dev/i2c-0",O_RDWR) < 0) {
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/* Some kind of error handling */
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exit(1);
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}
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if (ioctl(file,I2C_FUNCS,&funcs) < 0) {
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/* Some kind of error handling */
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exit(1);
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}
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if (! (funcs & I2C_FUNC_SMBUS_QUICK)) {
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/* Oops, the needed functionality (SMBus write_quick function) is
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not available! */
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exit(1);
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}
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/* Now it is safe to use the SMBus write_quick command */
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