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diff --git a/Documentation/i2c/functionality.rst b/Documentation/i2c/functionality.rst new file mode 100644 index 0000000000..377507c561 --- /dev/null +++ b/Documentation/i2c/functionality.rst @@ -0,0 +1,156 @@ +======================= +I2C/SMBus Functionality +======================= + +INTRODUCTION +------------ + +Because not every I2C or SMBus adapter implements everything in the +I2C specifications, a client can not trust that everything it needs +is implemented when it is given the option to attach to an adapter: +the client needs some way to check whether an adapter has the needed +functionality. + + +FUNCTIONALITY CONSTANTS +----------------------- + +For the most up-to-date list of functionality constants, please check +<uapi/linux/i2c.h>! + + =============================== ============================================== + I2C_FUNC_I2C Plain i2c-level commands (Pure SMBus + adapters typically can not do these) + I2C_FUNC_10BIT_ADDR Handles the 10-bit address extensions + I2C_FUNC_PROTOCOL_MANGLING Knows about the I2C_M_IGNORE_NAK, + I2C_M_REV_DIR_ADDR and I2C_M_NO_RD_ACK + flags (which modify the I2C protocol!) + I2C_FUNC_NOSTART Can skip repeated start sequence + I2C_FUNC_SMBUS_QUICK Handles the SMBus write_quick command + I2C_FUNC_SMBUS_READ_BYTE Handles the SMBus read_byte command + I2C_FUNC_SMBUS_WRITE_BYTE Handles the SMBus write_byte command + I2C_FUNC_SMBUS_READ_BYTE_DATA Handles the SMBus read_byte_data command + I2C_FUNC_SMBUS_WRITE_BYTE_DATA Handles the SMBus write_byte_data command + I2C_FUNC_SMBUS_READ_WORD_DATA Handles the SMBus read_word_data command + I2C_FUNC_SMBUS_WRITE_WORD_DATA Handles the SMBus write_byte_data command + I2C_FUNC_SMBUS_PROC_CALL Handles the SMBus process_call command + I2C_FUNC_SMBUS_READ_BLOCK_DATA Handles the SMBus read_block_data command + I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command + I2C_FUNC_SMBUS_READ_I2C_BLOCK Handles the SMBus read_i2c_block_data command + I2C_FUNC_SMBUS_WRITE_I2C_BLOCK Handles the SMBus write_i2c_block_data command + =============================== ============================================== + +A few combinations of the above flags are also defined for your convenience: + + ========================= ====================================== + I2C_FUNC_SMBUS_BYTE Handles the SMBus read_byte + and write_byte commands + I2C_FUNC_SMBUS_BYTE_DATA Handles the SMBus read_byte_data + and write_byte_data commands + I2C_FUNC_SMBUS_WORD_DATA Handles the SMBus read_word_data + and write_word_data commands + I2C_FUNC_SMBUS_BLOCK_DATA Handles the SMBus read_block_data + and write_block_data commands + I2C_FUNC_SMBUS_I2C_BLOCK Handles the SMBus read_i2c_block_data + and write_i2c_block_data commands + I2C_FUNC_SMBUS_EMUL Handles all SMBus commands that can be + emulated by a real I2C adapter (using + the transparent emulation layer) + ========================= ====================================== + +In kernel versions prior to 3.5 I2C_FUNC_NOSTART was implemented as +part of I2C_FUNC_PROTOCOL_MANGLING. + + +ADAPTER IMPLEMENTATION +---------------------- + +When you write a new adapter driver, you will have to implement a +function callback ``functionality``. Typical implementations are given +below. + +A typical SMBus-only adapter would list all the SMBus transactions it +supports. This example comes from the i2c-piix4 driver:: + + static u32 piix4_func(struct i2c_adapter *adapter) + { + return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE | + I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA | + I2C_FUNC_SMBUS_BLOCK_DATA; + } + +A typical full-I2C adapter would use the following (from the i2c-pxa +driver):: + + static u32 i2c_pxa_functionality(struct i2c_adapter *adap) + { + return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL; + } + +I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the +addition of I2C block transactions) which i2c-core can emulate using +I2C_FUNC_I2C without any help from the adapter driver. The idea is +to let the client drivers check for the support of SMBus functions +without having to care whether the said functions are implemented in +hardware by the adapter, or emulated in software by i2c-core on top +of an I2C adapter. + + +CLIENT CHECKING +--------------- + +Before a client tries to attach to an adapter, or even do tests to check +whether one of the devices it supports is present on an adapter, it should +check whether the needed functionality is present. The typical way to do +this is (from the lm75 driver):: + + static int lm75_detect(...) + { + (...) + if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA | + I2C_FUNC_SMBUS_WORD_DATA)) + goto exit; + (...) + } + +Here, the lm75 driver checks if the adapter can do both SMBus byte data +and SMBus word data transactions. If not, then the driver won't work on +this adapter and there's no point in going on. If the check above is +successful, then the driver knows that it can call the following +functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(), +i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of +thumb, the functionality constants you test for with +i2c_check_functionality() should match exactly the i2c_smbus_* functions +which you driver is calling. + +Note that the check above doesn't tell whether the functionalities are +implemented in hardware by the underlying adapter or emulated in +software by i2c-core. Client drivers don't have to care about this, as +i2c-core will transparently implement SMBus transactions on top of I2C +adapters. + + +CHECKING THROUGH /DEV +--------------------- + +If you try to access an adapter from a userspace program, you will have +to use the /dev interface. You will still have to check whether the +functionality you need is supported, of course. This is done using +the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is +below:: + + int file; + if (file = open("/dev/i2c-0", O_RDWR) < 0) { + /* Some kind of error handling */ + exit(1); + } + if (ioctl(file, I2C_FUNCS, &funcs) < 0) { + /* Some kind of error handling */ + exit(1); + } + if (!(funcs & I2C_FUNC_SMBUS_QUICK)) { + /* Oops, the needed functionality (SMBus write_quick function) is + not available! */ + exit(1); + } + /* Now it is safe to use the SMBus write_quick command */ |