Skip to content

Commit

Permalink
i2c: Update and clean up writing-clients document
Browse files Browse the repository at this point in the history 
* Strip trailing white space.
* Remove out-of-date or irrelevant parts.
* Insist on the fact that command is deprecated.
* Fix spelling mistakes and typos.
* Reformat code examples and function prototypes to comply with the
  kernel coding style.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
  • Loading branch information
Jean Delvare committed Oct 22, 2008
1 parent c0589d4 commit 0e47858
Showing 1 changed file with 97 additions and 129 deletions.
226 changes: 97 additions & 129 deletions Documentation/i2c/writing-clients
View file
Original file line number Diff line number Diff line change
Expand Up @@ -10,17 +10,17 @@ General remarks
===============

Try to keep the kernel namespace as clean as possible. The best way to
do this is to use a unique prefix for all global symbols. This is
do this is to use a unique prefix for all global symbols. This is
especially important for exported symbols, but it is a good idea to do
it for non-exported symbols too. We will use the prefix `foo_' in this
tutorial, and `FOO_' for preprocessor variables.
tutorial.


The driver structure
====================

Usually, you will implement a single driver structure, and instantiate
all clients from it. Remember, a driver structure contains general access
all clients from it. Remember, a driver structure contains general access
routines, and should be zero-initialized except for fields with data you
provide. A client structure holds device-specific information like the
driver model device node, and its I2C address.
Expand Down Expand Up @@ -49,87 +49,64 @@ static struct i2c_driver foo_driver = {
.shutdown = foo_shutdown, /* optional */
.suspend = foo_suspend, /* optional */
.resume = foo_resume, /* optional */
.command = foo_command, /* optional */
.command = foo_command, /* optional, deprecated */
}

The name field is the driver name, and must not contain spaces. It
should match the module name (if the driver can be compiled as a module),
although you can use MODULE_ALIAS (passing "foo" in this example) to add
another name for the module. If the driver name doesn't match the module
name, the module won't be automatically loaded (hotplug/coldplug).

All other fields are for call-back functions which will be explained
All other fields are for call-back functions which will be explained
below.


Extra client data
=================

Each client structure has a special `data' field that can point to any
structure at all. You should use this to keep device-specific data,
especially in drivers that handle multiple I2C or SMBUS devices. You
do not always need this, but especially for `sensors' drivers, it can
be very useful.
structure at all. You should use this to keep device-specific data.

/* store the value */
void i2c_set_clientdata(struct i2c_client *client, void *data);

/* retrieve the value */
void *i2c_get_clientdata(const struct i2c_client *client);

An example structure is below.

struct foo_data {
struct i2c_client *client;
enum chips type; /* To keep the chips type for `sensors' drivers. */

/* Because the i2c bus is slow, it is often useful to cache the read
information of a chip for some time (for example, 1 or 2 seconds).
It depends of course on the device whether this is really worthwhile
or even sensible. */
struct mutex update_lock; /* When we are reading lots of information,
another process should not update the
below information */
char valid; /* != 0 if the following fields are valid. */
unsigned long last_updated; /* In jiffies */
/* Add the read information here too */
};


Accessing the client
====================

Let's say we have a valid client structure. At some time, we will need
to gather information from the client, or write new information to the
client. How we will export this information to user-space is less
important at this moment (perhaps we do not need to do this at all for
some obscure clients). But we need generic reading and writing routines.
client.

I have found it useful to define foo_read and foo_write function for this.
I have found it useful to define foo_read and foo_write functions for this.
For some cases, it will be easier to call the i2c functions directly,
but many chips have some kind of register-value idea that can easily
be encapsulated.

The below functions are simple examples, and should not be copied
literally.

int foo_read_value(struct i2c_client *client, u8 reg)
{
if (reg < 0x10) /* byte-sized register */
return i2c_smbus_read_byte_data(client,reg);
else /* word-sized register */
return i2c_smbus_read_word_data(client,reg);
}

int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
{
if (reg == 0x10) /* Impossible to write - driver error! */ {
return -1;
else if (reg < 0x10) /* byte-sized register */
return i2c_smbus_write_byte_data(client,reg,value);
else /* word-sized register */
return i2c_smbus_write_word_data(client,reg,value);
}
int foo_read_value(struct i2c_client *client, u8 reg)
{
if (reg < 0x10) /* byte-sized register */
return i2c_smbus_read_byte_data(client, reg);
else /* word-sized register */
return i2c_smbus_read_word_data(client, reg);
}

int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
{
if (reg == 0x10) /* Impossible to write - driver error! */
return -EINVAL;
else if (reg < 0x10) /* byte-sized register */
return i2c_smbus_write_byte_data(client, reg, value);
else /* word-sized register */
return i2c_smbus_write_word_data(client, reg, value);
}


Probing and attaching
Expand Down Expand Up @@ -251,42 +228,37 @@ called automatically before the underlying I2C bus itself is removed, as a
device can't survive its parent in the device driver model.


Initializing the module or kernel
=================================
Initializing the driver
=======================

When the kernel is booted, or when your foo driver module is inserted,
you have to do some initializing. Fortunately, just attaching (registering)
the driver module is usually enough.
When the kernel is booted, or when your foo driver module is inserted,
you have to do some initializing. Fortunately, just registering the
driver module is usually enough.

static int __init foo_init(void)
{
int res;

if ((res = i2c_add_driver(&foo_driver))) {
printk("foo: Driver registration failed, module not inserted.\n");
return res;
}
return 0;
}
static int __init foo_init(void)
{
return i2c_add_driver(&foo_driver);
}

static void __exit foo_cleanup(void)
{
i2c_del_driver(&foo_driver);
}
static void __exit foo_cleanup(void)
{
i2c_del_driver(&foo_driver);
}

/* Substitute your own name and email address */
MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
/* Substitute your own name and email address */
MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");

/* a few non-GPL license types are also allowed */
MODULE_LICENSE("GPL");
/* a few non-GPL license types are also allowed */
MODULE_LICENSE("GPL");

module_init(foo_init);
module_exit(foo_cleanup);
module_init(foo_init);
module_exit(foo_cleanup);

Note that some functions are marked by `__init', and some data structures
by `__initdata'. These functions and structures can be removed after
kernel booting (or module loading) is completed.
Note that some functions are marked by `__init'. These functions can
be removed after kernel booting (or module loading) is completed.
Likewise, functions marked by `__exit' are dropped by the compiler when
the code is built into the kernel, as they would never be called.


Power Management
Expand Down Expand Up @@ -321,33 +293,35 @@ Command function

A generic ioctl-like function call back is supported. You will seldom
need this, and its use is deprecated anyway, so newer design should not
use it. Set it to NULL.
use it.


Sending and receiving
=====================

If you want to communicate with your device, there are several functions
to do this. You can find all of them in i2c.h.
to do this. You can find all of them in <linux/i2c.h>.

If you can choose between plain i2c communication and SMBus level
communication, please use the last. All adapters understand SMBus level
commands, but only some of them understand plain i2c!
If you can choose between plain I2C communication and SMBus level
communication, please use the latter. All adapters understand SMBus level
commands, but only some of them understand plain I2C!


Plain i2c communication
Plain I2C communication
-----------------------

extern int i2c_master_send(struct i2c_client *,const char* ,int);
extern int i2c_master_recv(struct i2c_client *,char* ,int);
int i2c_master_send(struct i2c_client *client, const char *buf,
int count);
int i2c_master_recv(struct i2c_client *client, char *buf, int count);

These routines read and write some bytes from/to a client. The client
contains the i2c address, so you do not have to include it. The second
parameter contains the bytes the read/write, the third the length of the
buffer. Returned is the actual number of bytes read/written.

extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
int num);
parameter contains the bytes to read/write, the third the number of bytes
to read/write (must be less than the length of the buffer.) Returned is
the actual number of bytes read/written.

int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
int num);

This sends a series of messages. Each message can be a read or write,
and they can be mixed in any way. The transactions are combined: no
Expand All @@ -356,49 +330,45 @@ for each message the client address, the number of bytes of the message
and the message data itself.

You can read the file `i2c-protocol' for more information about the
actual i2c protocol.
actual I2C protocol.


SMBus communication
-------------------

extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
unsigned short flags,
char read_write, u8 command, int size,
union i2c_smbus_data * data);

This is the generic SMBus function. All functions below are implemented
in terms of it. Never use this function directly!


extern s32 i2c_smbus_read_byte(struct i2c_client * client);
extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
u8 command, u8 value);
extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
u8 command, u16 value);
extern s32 i2c_smbus_process_call(struct i2c_client *client,
u8 command, u16 value);
extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
u8 command, u8 *values);
extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
u8 command, u8 length,
u8 *values);
extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
u8 command, u8 length, u8 *values);
extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
u8 command, u8 length,
u8 *values);
s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
unsigned short flags, char read_write, u8 command,
int size, union i2c_smbus_data *data);

This is the generic SMBus function. All functions below are implemented
in terms of it. Never use this function directly!

s32 i2c_smbus_read_byte(struct i2c_client *client);
s32 i2c_smbus_write_byte(struct i2c_client *client, u8 value);
s32 i2c_smbus_read_byte_data(struct i2c_client *client, u8 command);
s32 i2c_smbus_write_byte_data(struct i2c_client *client,
u8 command, u8 value);
s32 i2c_smbus_read_word_data(struct i2c_client *client, u8 command);
s32 i2c_smbus_write_word_data(struct i2c_client *client,
u8 command, u16 value);
s32 i2c_smbus_process_call(struct i2c_client *client,
u8 command, u16 value);
s32 i2c_smbus_read_block_data(struct i2c_client *client,
u8 command, u8 *values);
s32 i2c_smbus_write_block_data(struct i2c_client *client,
u8 command, u8 length, const u8 *values);
s32 i2c_smbus_read_i2c_block_data(struct i2c_client *client,
u8 command, u8 length, u8 *values);
s32 i2c_smbus_write_i2c_block_data(struct i2c_client *client,
u8 command, u8 length,
const u8 *values);

These ones were removed from i2c-core because they had no users, but could
be added back later if needed:

extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
u8 command, u8 length,
u8 *values)
s32 i2c_smbus_write_quick(struct i2c_client *client, u8 value);
s32 i2c_smbus_block_process_call(struct i2c_client *client,
u8 command, u8 length, u8 *values);

All these transactions return a negative errno value on failure. The 'write'
transactions return 0 on success; the 'read' transactions return the read
Expand All @@ -415,7 +385,5 @@ General purpose routines
Below all general purpose routines are listed, that were not mentioned
before.

/* This call returns a unique low identifier for each registered adapter.
*/
extern int i2c_adapter_id(struct i2c_adapter *adap);

/* Return the adapter number for a specific adapter */
int i2c_adapter_id(struct i2c_adapter *adap);

0 comments on commit 0e47858

Please sign in to comment.