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| -rw-r--r-- | Documentation/i2c/writing-clients | 226 |
1 files changed, 97 insertions, 129 deletions
diff --git a/Documentation/i2c/writing-clients b/Documentation/i2c/writing-clients index c3e1885..6b9af7d 100644 --- a/Documentation/i2c/writing-clients +++ b/Documentation/i2c/writing-clients @@ -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. @@ -49,16 +49,16 @@ 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. @@ -66,10 +66,7 @@ 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); @@ -77,35 +74,15 @@ be very useful. /* 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. @@ -113,23 +90,23 @@ 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 @@ -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 @@ -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 @@ -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 @@ -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); |
