eefcd75e72
Make the documentation on how to write and port i2c drivers more in line with the current state of things: * i2c-isa is deprecated and soon gone, so stop advertising it. * Drop many sensors-specific references. Most of them were outdated anyway. * Update the example code to reflect the recent and not-so-recent API and coding style preference changes. * Simplify the example init and cleanup functions. This should make things less complex to understand for newcomers. Signed-off-by: Jean Delvare <khali@linux-fr.org>
608 lines
23 KiB
Text
608 lines
23 KiB
Text
This is a small guide for those who want to write kernel drivers for I2C
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or SMBus devices, using Linux as the protocol host/master (not slave).
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To set up a driver, you need to do several things. Some are optional, and
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some things can be done slightly or completely different. Use this as a
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guide, not as a rule book!
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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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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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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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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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static struct i2c_driver foo_driver = {
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.driver = {
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.name = "foo",
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},
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/* iff driver uses driver model ("new style") binding model: */
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.probe = foo_probe,
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.remove = foo_remove,
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/* else, driver uses "legacy" binding model: */
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.attach_adapter = foo_attach_adapter,
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.detach_client = foo_detach_client,
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/* these may be used regardless of the driver binding model */
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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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}
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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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below.
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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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/* store the value */
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void i2c_set_clientdata(struct i2c_client *client, void *data);
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/* retrieve the value */
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void *i2c_get_clientdata(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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I have found it useful to define foo_read and foo_write function 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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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_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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Probing and attaching
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=====================
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The Linux I2C stack was originally written to support access to hardware
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monitoring chips on PC motherboards, and thus it embeds some assumptions
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that are more appropriate to SMBus (and PCs) than to I2C. One of these
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assumptions is that most adapters and devices drivers support the SMBUS_QUICK
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protocol to probe device presence. Another is that devices and their drivers
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can be sufficiently configured using only such probe primitives.
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As Linux and its I2C stack became more widely used in embedded systems
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and complex components such as DVB adapters, those assumptions became more
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problematic. Drivers for I2C devices that issue interrupts need more (and
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different) configuration information, as do drivers handling chip variants
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that can't be distinguished by protocol probing, or which need some board
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specific information to operate correctly.
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Accordingly, the I2C stack now has two models for associating I2C devices
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with their drivers: the original "legacy" model, and a newer one that's
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fully compatible with the Linux 2.6 driver model. These models do not mix,
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since the "legacy" model requires drivers to create "i2c_client" device
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objects after SMBus style probing, while the Linux driver model expects
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drivers to be given such device objects in their probe() routines.
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Standard Driver Model Binding ("New Style")
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-------------------------------------------
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System infrastructure, typically board-specific initialization code or
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boot firmware, reports what I2C devices exist. For example, there may be
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a table, in the kernel or from the boot loader, identifying I2C devices
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and linking them to board-specific configuration information about IRQs
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and other wiring artifacts, chip type, and so on. That could be used to
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create i2c_client objects for each I2C device.
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I2C device drivers using this binding model work just like any other
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kind of driver in Linux: they provide a probe() method to bind to
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those devices, and a remove() method to unbind.
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static int foo_probe(struct i2c_client *client);
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static int foo_remove(struct i2c_client *client);
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Remember that the i2c_driver does not create those client handles. The
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handle may be used during foo_probe(). If foo_probe() reports success
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(zero not a negative status code) it may save the handle and use it until
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foo_remove() returns. That binding model is used by most Linux drivers.
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Drivers match devices when i2c_client.driver_name and the driver name are
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the same; this approach is used in several other busses that don't have
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device typing support in the hardware. The driver and module name should
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match, so hotplug/coldplug mechanisms will modprobe the driver.
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Device Creation (Standard driver model)
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---------------------------------------
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If you know for a fact that an I2C device is connected to a given I2C bus,
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you can instantiate that device by simply filling an i2c_board_info
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structure with the device address and driver name, and calling
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i2c_new_device(). This will create the device, then the driver core will
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take care of finding the right driver and will call its probe() method.
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If a driver supports different device types, you can specify the type you
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want using the type field. You can also specify an IRQ and platform data
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if needed.
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Sometimes you know that a device is connected to a given I2C bus, but you
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don't know the exact address it uses. This happens on TV adapters for
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example, where the same driver supports dozens of slightly different
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models, and I2C device addresses change from one model to the next. In
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that case, you can use the i2c_new_probed_device() variant, which is
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similar to i2c_new_device(), except that it takes an additional list of
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possible I2C addresses to probe. A device is created for the first
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responsive address in the list. If you expect more than one device to be
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present in the address range, simply call i2c_new_probed_device() that
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many times.
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The call to i2c_new_device() or i2c_new_probed_device() typically happens
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in the I2C bus driver. You may want to save the returned i2c_client
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reference for later use.
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Device Deletion (Standard driver model)
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---------------------------------------
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Each I2C device which has been created using i2c_new_device() or
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i2c_new_probed_device() can be unregistered by calling
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i2c_unregister_device(). If you don't call it explicitly, it will be
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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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Legacy Driver Binding Model
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---------------------------
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Most i2c devices can be present on several i2c addresses; for some this
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is determined in hardware (by soldering some chip pins to Vcc or Ground),
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for others this can be changed in software (by writing to specific client
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registers). Some devices are usually on a specific address, but not always;
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and some are even more tricky. So you will probably need to scan several
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i2c addresses for your clients, and do some sort of detection to see
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whether it is actually a device supported by your driver.
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To give the user a maximum of possibilities, some default module parameters
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are defined to help determine what addresses are scanned. Several macros
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are defined in i2c.h to help you support them, as well as a generic
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detection algorithm.
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You do not have to use this parameter interface; but don't try to use
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function i2c_probe() if you don't.
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Probing classes (Legacy model)
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------------------------------
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All parameters are given as lists of unsigned 16-bit integers. Lists are
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terminated by I2C_CLIENT_END.
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The following lists are used internally:
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normal_i2c: filled in by the module writer.
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A list of I2C addresses which should normally be examined.
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probe: insmod parameter.
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A list of pairs. The first value is a bus number (-1 for any I2C bus),
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the second is the address. These addresses are also probed, as if they
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were in the 'normal' list.
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ignore: insmod parameter.
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A list of pairs. The first value is a bus number (-1 for any I2C bus),
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the second is the I2C address. These addresses are never probed.
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This parameter overrules the 'normal_i2c' list only.
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force: insmod parameter.
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A list of pairs. The first value is a bus number (-1 for any I2C bus),
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the second is the I2C address. A device is blindly assumed to be on
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the given address, no probing is done.
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Additionally, kind-specific force lists may optionally be defined if
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the driver supports several chip kinds. They are grouped in a
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NULL-terminated list of pointers named forces, those first element if the
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generic force list mentioned above. Each additional list correspond to an
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insmod parameter of the form force_<kind>.
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Fortunately, as a module writer, you just have to define the `normal_i2c'
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parameter. The complete declaration could look like this:
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/* Scan 0x37, and 0x48 to 0x4f */
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static unsigned short normal_i2c[] = { 0x37, 0x48, 0x49, 0x4a, 0x4b, 0x4c,
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0x4d, 0x4e, 0x4f, I2C_CLIENT_END };
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/* Magic definition of all other variables and things */
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I2C_CLIENT_INSMOD;
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/* Or, if your driver supports, say, 2 kind of devices: */
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I2C_CLIENT_INSMOD_2(foo, bar);
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If you use the multi-kind form, an enum will be defined for you:
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enum chips { any_chip, foo, bar, ... }
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You can then (and certainly should) use it in the driver code.
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Note that you *have* to call the defined variable `normal_i2c',
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without any prefix!
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Attaching to an adapter (Legacy model)
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--------------------------------------
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Whenever a new adapter is inserted, or for all adapters if the driver is
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being registered, the callback attach_adapter() is called. Now is the
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time to determine what devices are present on the adapter, and to register
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a client for each of them.
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The attach_adapter callback is really easy: we just call the generic
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detection function. This function will scan the bus for us, using the
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information as defined in the lists explained above. If a device is
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detected at a specific address, another callback is called.
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int foo_attach_adapter(struct i2c_adapter *adapter)
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{
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return i2c_probe(adapter,&addr_data,&foo_detect_client);
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}
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Remember, structure `addr_data' is defined by the macros explained above,
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so you do not have to define it yourself.
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The i2c_probe function will call the foo_detect_client
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function only for those i2c addresses that actually have a device on
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them (unless a `force' parameter was used). In addition, addresses that
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are already in use (by some other registered client) are skipped.
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The detect client function (Legacy model)
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-----------------------------------------
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The detect client function is called by i2c_probe. The `kind' parameter
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contains -1 for a probed detection, 0 for a forced detection, or a positive
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number for a forced detection with a chip type forced.
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Returning an error different from -ENODEV in a detect function will cause
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the detection to stop: other addresses and adapters won't be scanned.
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This should only be done on fatal or internal errors, such as a memory
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shortage or i2c_attach_client failing.
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For now, you can ignore the `flags' parameter. It is there for future use.
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int foo_detect_client(struct i2c_adapter *adapter, int address,
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int kind)
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{
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int err = 0;
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int i;
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struct i2c_client *client;
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struct foo_data *data;
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const char *name = "";
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/* Let's see whether this adapter can support what we need.
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Please substitute the things you need here! */
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if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
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I2C_FUNC_SMBUS_WRITE_BYTE))
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goto ERROR0;
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/* OK. For now, we presume we have a valid client. We now create the
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client structure, even though we cannot fill it completely yet.
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But it allows us to access several i2c functions safely */
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if (!(data = kzalloc(sizeof(struct foo_data), GFP_KERNEL))) {
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err = -ENOMEM;
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goto ERROR0;
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}
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client = &data->client;
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i2c_set_clientdata(client, data);
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client->addr = address;
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client->adapter = adapter;
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client->driver = &foo_driver;
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/* Now, we do the remaining detection. If no `force' parameter is used. */
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/* First, the generic detection (if any), that is skipped if any force
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parameter was used. */
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if (kind < 0) {
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/* The below is of course bogus */
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if (foo_read(client, FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
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goto ERROR1;
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}
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/* Next, specific detection. This is especially important for `sensors'
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devices. */
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/* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
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was used. */
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if (kind <= 0) {
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i = foo_read(client, FOO_REG_CHIPTYPE);
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if (i == FOO_TYPE_1)
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kind = chip1; /* As defined in the enum */
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else if (i == FOO_TYPE_2)
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kind = chip2;
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else {
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printk("foo: Ignoring 'force' parameter for unknown chip at "
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"adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
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goto ERROR1;
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}
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}
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/* Now set the type and chip names */
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if (kind == chip1) {
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name = "chip1";
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} else if (kind == chip2) {
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name = "chip2";
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}
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/* Fill in the remaining client fields. */
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strlcpy(client->name, name, I2C_NAME_SIZE);
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data->type = kind;
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mutex_init(&data->update_lock); /* Only if you use this field */
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/* Any other initializations in data must be done here too. */
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/* This function can write default values to the client registers, if
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needed. */
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foo_init_client(client);
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/* Tell the i2c layer a new client has arrived */
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if ((err = i2c_attach_client(client)))
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goto ERROR1;
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return 0;
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/* OK, this is not exactly good programming practice, usually. But it is
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very code-efficient in this case. */
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ERROR1:
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kfree(data);
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ERROR0:
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return err;
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}
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Removing the client (Legacy model)
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==================================
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The detach_client call back function is called when a client should be
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removed. It may actually fail, but only when panicking. This code is
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much simpler than the attachment code, fortunately!
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int foo_detach_client(struct i2c_client *client)
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{
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int err;
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/* Try to detach the client from i2c space */
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if ((err = i2c_detach_client(client)))
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return err;
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kfree(i2c_get_clientdata(client));
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return 0;
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}
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Initializing the module or kernel
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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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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 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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/* 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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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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Power Management
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================
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If your I2C device needs special handling when entering a system low
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power state -- like putting a transceiver into a low power mode, or
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activating a system wakeup mechanism -- do that in the suspend() method.
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The resume() method should reverse what the suspend() method does.
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These are standard driver model calls, and they work just like they
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would for any other driver stack. The calls can sleep, and can use
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I2C messaging to the device being suspended or resumed (since their
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parent I2C adapter is active when these calls are issued, and IRQs
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are still enabled).
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System Shutdown
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===============
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If your I2C device needs special handling when the system shuts down
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or reboots (including kexec) -- like turning something off -- use a
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shutdown() method.
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Again, this is a standard driver model call, working just like it
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would for any other driver stack: the calls can sleep, and can use
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I2C messaging.
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Command function
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================
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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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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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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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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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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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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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stop bit is sent between transaction. The i2c_msg structure contains
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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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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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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_write_quick(struct i2c_client * client, u8 value);
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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_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 *values);
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These ones were removed in Linux 2.6.10 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_read_block_data(struct i2c_client * client,
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u8 command, 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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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_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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All these transactions return -1 on failure. The 'write' transactions
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return 0 on success; the 'read' transactions return the read value, except
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for read_block, which returns the number of values read. The block buffers
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need not be longer than 32 bytes.
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You can read the file `smbus-protocol' for more information about the
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actual SMBus protocol.
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General purpose routines
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========================
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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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