88b283281f
Attempt to make the documentation about the I2C/SMBus functionality checking API clearer. Signed-off-by: Jean Delvare <khali@linux-fr.org>
146 lines
5.9 KiB
Plaintext
146 lines
5.9 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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ADAPTER IMPLEMENTATION
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----------------------
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When you write a new adapter driver, you will have to implement a
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function callback `functionality'. Typical implementations are given
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below.
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A typical SMBus-only adapter would list all the SMBus transactions it
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supports. This example comes from the i2c-piix4 driver:
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static u32 piix4_func(struct i2c_adapter *adapter)
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{
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return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE |
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I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA |
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I2C_FUNC_SMBUS_BLOCK_DATA;
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}
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A typical full-I2C adapter would use the following (from the i2c-pxa
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driver):
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static u32 i2c_pxa_functionality(struct i2c_adapter *adap)
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{
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return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
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}
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I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
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addition of I2C block transactions) which i2c-core can emulate using
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I2C_FUNC_I2C without any help from the adapter driver. The idea is
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to let the client drivers check for the support of SMBus functions
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without having to care whether the said functions are implemented in
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hardware by the adapter, or emulated in software by i2c-core on top
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of an I2C adapter.
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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. The typical way to do
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this is (from the lm75 driver):
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static int lm75_detect(...)
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{
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(...)
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if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
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I2C_FUNC_SMBUS_WORD_DATA))
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goto exit;
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(...)
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}
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Here, the lm75 driver checks if the adapter can do both SMBus byte data
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and SMBus word data transactions. If not, then the driver won't work on
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this adapter and there's no point in going on. If the check above is
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successful, then the driver knows that it can call the following
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functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
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i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
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thumb, the functionality constants you test for with
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i2c_check_functionality() should match exactly the i2c_smbus_* functions
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which you driver is calling.
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Note that the check above doesn't tell whether the functionalities are
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implemented in hardware by the underlying adapter or emulated in
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software by i2c-core. Client drivers don't have to care about this, as
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i2c-core will transparently implement SMBus transactions on top of I2C
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adapters.
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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 i2cdetect program, is
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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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