diff options
| -rw-r--r-- | Documentation/i2c/functionality | 95 |
1 files changed, 52 insertions, 43 deletions
diff --git a/Documentation/i2c/functionality b/Documentation/i2c/functionality index 60cca24..42c17c1 100644 --- a/Documentation/i2c/functionality +++ b/Documentation/i2c/functionality @@ -51,26 +51,38 @@ A few combinations of the above flags are also defined for your convenience: the transparent emulation layer) -ALGORITHM/ADAPTER IMPLEMENTATION --------------------------------- +ADAPTER IMPLEMENTATION +---------------------- -When you write a new algorithm driver, you will have to implement a -function callback `functionality', that gets an i2c_adapter structure -pointer as its only parameter: +When you write a new adapter driver, you will have to implement a +function callback `functionality'. Typical implementations are given +below. - struct i2c_algorithm { - /* Many other things of course; check <linux/i2c.h>! */ - u32 (*functionality) (struct i2c_adapter *); +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 typically implementation is given below, from i2c-algo-bit.c: +A typical full-I2C adapter would use the following (from the i2c-pxa +driver): - static u32 bit_func(struct i2c_adapter *adap) + static u32 i2c_pxa_functionality(struct i2c_adapter *adap) { - return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR | - I2C_FUNC_PROTOCOL_MANGLING; + 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 @@ -78,36 +90,33 @@ 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. There are two functions -defined which should be used instead of calling the functionality hook -in the algorithm structure directly: - - /* Return the functionality mask */ - extern u32 i2c_get_functionality (struct i2c_adapter *adap); - - /* Return 1 if adapter supports everything we need, 0 if not. */ - extern int i2c_check_functionality (struct i2c_adapter *adap, u32 func); +check whether the needed functionality is present. The typical way to do +this is (from the lm75 driver): -This is a typical way to use these functions (from the writing-clients -document): - int foo_detect_client(struct i2c_adapter *adapter, int address, - unsigned short flags, int kind) + static int lm75_detect(...) { - /* Define needed variables */ - - /* As the very first action, we check whether the adapter has the - needed functionality: we need the SMBus read_word_data, - write_word_data and write_byte functions in this example. */ - if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA | - I2C_FUNC_SMBUS_WRITE_BYTE)) - goto ERROR0; - - /* Now we can do the real detection */ - - ERROR0: - /* Return an error */ + (...) + 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 @@ -116,19 +125,19 @@ 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 lm_sensors i2cdetect -program, is below: +the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is +below: int file; - if (file = open("/dev/i2c-0",O_RDWR) < 0) { + if (file = open("/dev/i2c-0", O_RDWR) < 0) { /* Some kind of error handling */ exit(1); } - if (ioctl(file,I2C_FUNCS,&funcs) < 0) { + if (ioctl(file, I2C_FUNCS, &funcs) < 0) { /* Some kind of error handling */ exit(1); } - if (! (funcs & I2C_FUNC_SMBUS_QUICK)) { + if (!(funcs & I2C_FUNC_SMBUS_QUICK)) { /* Oops, the needed functionality (SMBus write_quick function) is not available! */ exit(1); |