i2c: Update and clean up writing-clients document
* 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 * Remove out-of-date or irrelevant parts.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>
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Jean Delvare
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@ -10,17 +10,17 @@ General remarks
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===============
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Try to keep the kernel namespace as clean as possible. The best way to
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do this is to use a unique prefix for all global symbols. This is
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do this is to use a unique prefix for all global symbols. This is
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especially important for exported symbols, but it is a good idea to do
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it for non-exported symbols too. We will use the prefix `foo_' in this
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tutorial, and `FOO_' for preprocessor variables.
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tutorial.
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The driver structure
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====================
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Usually, you will implement a single driver structure, and instantiate
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all clients from it. Remember, a driver structure contains general access
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all clients from it. Remember, a driver structure contains general access
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routines, and should be zero-initialized except for fields with data you
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provide. A client structure holds device-specific information like the
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driver model device node, and its I2C address.
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@ -49,16 +49,16 @@ static struct i2c_driver foo_driver = {
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.shutdown = foo_shutdown, /* optional */
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.suspend = foo_suspend, /* optional */
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.resume = foo_resume, /* optional */
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.command = foo_command, /* optional */
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.command = foo_command, /* optional, deprecated */
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}
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The name field is the driver name, and must not contain spaces. It
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should match the module name (if the driver can be compiled as a module),
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although you can use MODULE_ALIAS (passing "foo" in this example) to add
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another name for the module. If the driver name doesn't match the module
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name, the module won't be automatically loaded (hotplug/coldplug).
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All other fields are for call-back functions which will be explained
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All other fields are for call-back functions which will be explained
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below.
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@ -66,10 +66,7 @@ Extra client data
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=================
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Each client structure has a special `data' field that can point to any
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structure at all. You should use this to keep device-specific data,
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especially in drivers that handle multiple I2C or SMBUS devices. You
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do not always need this, but especially for `sensors' drivers, it can
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be very useful.
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structure at all. You should use this to keep device-specific data.
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/* store the value */
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void i2c_set_clientdata(struct i2c_client *client, void *data);
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@ -77,35 +74,15 @@ be very useful.
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/* retrieve the value */
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void *i2c_get_clientdata(const struct i2c_client *client);
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An example structure is below.
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struct foo_data {
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struct i2c_client *client;
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enum chips type; /* To keep the chips type for `sensors' drivers. */
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/* Because the i2c bus is slow, it is often useful to cache the read
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information of a chip for some time (for example, 1 or 2 seconds).
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It depends of course on the device whether this is really worthwhile
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or even sensible. */
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struct mutex update_lock; /* When we are reading lots of information,
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another process should not update the
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below information */
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char valid; /* != 0 if the following fields are valid. */
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unsigned long last_updated; /* In jiffies */
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/* Add the read information here too */
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};
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Accessing the client
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====================
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Let's say we have a valid client structure. At some time, we will need
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to gather information from the client, or write new information to the
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client. How we will export this information to user-space is less
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important at this moment (perhaps we do not need to do this at all for
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some obscure clients). But we need generic reading and writing routines.
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client.
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I have found it useful to define foo_read and foo_write function for this.
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I have found it useful to define foo_read and foo_write functions for this.
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For some cases, it will be easier to call the i2c functions directly,
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but many chips have some kind of register-value idea that can easily
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be encapsulated.
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@ -113,23 +90,23 @@ be encapsulated.
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The below functions are simple examples, and should not be copied
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literally.
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int foo_read_value(struct i2c_client *client, u8 reg)
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{
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if (reg < 0x10) /* byte-sized register */
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return i2c_smbus_read_byte_data(client,reg);
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else /* word-sized register */
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return i2c_smbus_read_word_data(client,reg);
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}
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int foo_read_value(struct i2c_client *client, u8 reg)
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{
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if (reg < 0x10) /* byte-sized register */
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return i2c_smbus_read_byte_data(client, reg);
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else /* word-sized register */
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return i2c_smbus_read_word_data(client, reg);
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}
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int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
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{
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if (reg == 0x10) /* Impossible to write - driver error! */ {
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return -1;
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else if (reg < 0x10) /* byte-sized register */
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return i2c_smbus_write_byte_data(client,reg,value);
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else /* word-sized register */
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return i2c_smbus_write_word_data(client,reg,value);
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}
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int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
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{
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if (reg == 0x10) /* Impossible to write - driver error! */
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return -EINVAL;
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else if (reg < 0x10) /* byte-sized register */
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return i2c_smbus_write_byte_data(client, reg, value);
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else /* word-sized register */
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return i2c_smbus_write_word_data(client, reg, value);
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}
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Probing and attaching
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@ -251,42 +228,37 @@ called automatically before the underlying I2C bus itself is removed, as a
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device can't survive its parent in the device driver model.
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Initializing the module or kernel
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=================================
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Initializing the driver
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=======================
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When the kernel is booted, or when your foo driver module is inserted,
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you have to do some initializing. Fortunately, just attaching (registering)
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the driver module is usually enough.
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When the kernel is booted, or when your foo driver module is inserted,
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you have to do some initializing. Fortunately, just registering the
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driver module is usually enough.
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static int __init foo_init(void)
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{
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int res;
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if ((res = i2c_add_driver(&foo_driver))) {
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printk("foo: Driver registration failed, module not inserted.\n");
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return res;
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}
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return 0;
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}
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static int __init foo_init(void)
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{
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return i2c_add_driver(&foo_driver);
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}
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static void __exit foo_cleanup(void)
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{
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i2c_del_driver(&foo_driver);
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}
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static void __exit foo_cleanup(void)
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{
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i2c_del_driver(&foo_driver);
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}
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/* Substitute your own name and email address */
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MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
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MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
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/* Substitute your own name and email address */
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MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
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MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
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/* a few non-GPL license types are also allowed */
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MODULE_LICENSE("GPL");
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/* a few non-GPL license types are also allowed */
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MODULE_LICENSE("GPL");
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module_init(foo_init);
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module_exit(foo_cleanup);
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module_init(foo_init);
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module_exit(foo_cleanup);
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Note that some functions are marked by `__init', and some data structures
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by `__initdata'. These functions and structures can be removed after
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kernel booting (or module loading) is completed.
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Note that some functions are marked by `__init'. These functions can
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be removed after kernel booting (or module loading) is completed.
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Likewise, functions marked by `__exit' are dropped by the compiler when
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the code is built into the kernel, as they would never be called.
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Power Management
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@ -321,33 +293,35 @@ Command function
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A generic ioctl-like function call back is supported. You will seldom
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need this, and its use is deprecated anyway, so newer design should not
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use it. Set it to NULL.
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use it.
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Sending and receiving
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=====================
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If you want to communicate with your device, there are several functions
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to do this. You can find all of them in i2c.h.
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to do this. You can find all of them in <linux/i2c.h>.
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If you can choose between plain i2c communication and SMBus level
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communication, please use the last. All adapters understand SMBus level
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commands, but only some of them understand plain i2c!
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If you can choose between plain I2C communication and SMBus level
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communication, please use the latter. All adapters understand SMBus level
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commands, but only some of them understand plain I2C!
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Plain i2c communication
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Plain I2C communication
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-----------------------
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extern int i2c_master_send(struct i2c_client *,const char* ,int);
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extern int i2c_master_recv(struct i2c_client *,char* ,int);
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int i2c_master_send(struct i2c_client *client, const char *buf,
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int count);
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int i2c_master_recv(struct i2c_client *client, char *buf, int count);
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These routines read and write some bytes from/to a client. The client
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contains the i2c address, so you do not have to include it. The second
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parameter contains the bytes the read/write, the third the length of the
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buffer. Returned is the actual number of bytes read/written.
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extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
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int num);
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parameter contains the bytes to read/write, the third the number of bytes
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to read/write (must be less than the length of the buffer.) Returned is
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the actual number of bytes read/written.
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int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
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int num);
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This sends a series of messages. Each message can be a read or write,
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and they can be mixed in any way. The transactions are combined: no
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@ -356,49 +330,45 @@ for each message the client address, the number of bytes of the message
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and the message data itself.
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You can read the file `i2c-protocol' for more information about the
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actual i2c protocol.
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actual I2C protocol.
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SMBus communication
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-------------------
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extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
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unsigned short flags,
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char read_write, u8 command, int size,
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union i2c_smbus_data * data);
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s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
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unsigned short flags, char read_write, u8 command,
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int size, union i2c_smbus_data *data);
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This is the generic SMBus function. All functions below are implemented
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in terms of it. Never use this function directly!
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This is the generic SMBus function. All functions below are implemented
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in terms of it. Never use this function directly!
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extern s32 i2c_smbus_read_byte(struct i2c_client * client);
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extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
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extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
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extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
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u8 command, u8 value);
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extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
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extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
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u8 command, u16 value);
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extern s32 i2c_smbus_process_call(struct i2c_client *client,
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u8 command, u16 value);
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extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
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u8 command, u8 *values);
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extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
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u8 command, u8 length,
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u8 *values);
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extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
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u8 command, u8 length, u8 *values);
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extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
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u8 command, u8 length,
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u8 *values);
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s32 i2c_smbus_read_byte(struct i2c_client *client);
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s32 i2c_smbus_write_byte(struct i2c_client *client, u8 value);
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s32 i2c_smbus_read_byte_data(struct i2c_client *client, u8 command);
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s32 i2c_smbus_write_byte_data(struct i2c_client *client,
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u8 command, u8 value);
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s32 i2c_smbus_read_word_data(struct i2c_client *client, u8 command);
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s32 i2c_smbus_write_word_data(struct i2c_client *client,
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u8 command, u16 value);
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s32 i2c_smbus_process_call(struct i2c_client *client,
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u8 command, u16 value);
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s32 i2c_smbus_read_block_data(struct i2c_client *client,
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u8 command, u8 *values);
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s32 i2c_smbus_write_block_data(struct i2c_client *client,
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u8 command, u8 length, const u8 *values);
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s32 i2c_smbus_read_i2c_block_data(struct i2c_client *client,
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u8 command, u8 length, u8 *values);
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s32 i2c_smbus_write_i2c_block_data(struct i2c_client *client,
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u8 command, u8 length,
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const u8 *values);
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These ones were removed from i2c-core because they had no users, but could
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be added back later if needed:
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extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
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extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
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u8 command, u8 length,
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u8 *values)
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s32 i2c_smbus_write_quick(struct i2c_client *client, u8 value);
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s32 i2c_smbus_block_process_call(struct i2c_client *client,
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u8 command, u8 length, u8 *values);
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All these transactions return a negative errno value on failure. The 'write'
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transactions return 0 on success; the 'read' transactions return the read
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@ -415,7 +385,5 @@ General purpose routines
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Below all general purpose routines are listed, that were not mentioned
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before.
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/* This call returns a unique low identifier for each registered adapter.
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*/
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extern int i2c_adapter_id(struct i2c_adapter *adap);
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/* Return the adapter number for a specific adapter */
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int i2c_adapter_id(struct i2c_adapter *adap);
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