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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
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tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/i2c/functionality
downloadlinux-stericsson-1da177e4c3f41524e886b7f1b8a0c1fc7321cac2.tar.gz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+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
+<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_REV_DIR_ADDR,
+ I2C_M_REV_DIR_ADDR and I2C_M_REV_DIR_NOSTART
+ flags (which modify the i2c protocol!)
+ 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 than can be
+ emulated by a real I2C adapter (using
+ the transparent emulation layer)
+
+
+ALGORITHM/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:
+
+ struct i2c_algorithm {
+ /* Many other things of course; check <linux/i2c.h>! */
+ u32 (*functionality) (struct i2c_adapter *);
+ }
+
+A typically implementation is given below, from i2c-algo-bit.c:
+
+ static u32 bit_func(struct i2c_adapter *adap)
+ {
+ return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR |
+ I2C_FUNC_PROTOCOL_MANGLING;
+ }
+
+
+
+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);
+
+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)
+ {
+ /* 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 */
+ }
+
+
+
+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 i2c_detect
+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 */