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i2c: Improve the functionality documentation

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Attempt to make the documentation about the I2C/SMBus functionality
checking API clearer.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
This commit is contained in:
Jean Delvare 2008-05-11 20:37:05 +02:00 committed by Jean Delvare
parent 1a31a88f4f
commit 88b283281f
1 changed files with 54 additions and 45 deletions
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99
Documentation/i2c/functionality
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@ -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:
A typically implementation is given below, from i2c-algo-bit.c:
static u32 bit_func(struct i2c_adapter *adap)
static u32 piix4_func(struct i2c_adapter *adapter)
{
return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_10BIT_ADDR |
I2C_FUNC_PROTOCOL_MANGLING;
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
@ -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:
check whether the needed functionality is present. The typical way to do
this is (from the lm75 driver):
/* 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)
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);