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i2c: i2c probe() and remove() documented
Update Documentation/i2c to match previous patches updating probe() and remove() logic. Signed-off-by: David Brownell <dbrownell@users.sourceforge.net> Signed-off-by: Jean Delvare <khali@linux-fr.org>
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David Brownell
Jean Delvare
parent
a1d9e6e49f
commit
4298cfc3eb
2 changed files with 92 additions and 19 deletions
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@ -4,17 +4,23 @@ I2C and SMBus
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=============
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I2C (pronounce: I squared C) is a protocol developed by Philips. It is a
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slow two-wire protocol (10-400 kHz), but it suffices for many types of
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devices.
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slow two-wire protocol (variable speed, up to 400 kHz), with a high speed
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extension (3.4 MHz). It provides an inexpensive bus for connecting many
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types of devices with infrequent or low bandwidth communications needs.
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I2C is widely used with embedded systems. Some systems use variants that
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don't meet branding requirements, and so are not advertised as being I2C.
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SMBus (System Management Bus) is a subset of the I2C protocol. Many
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modern mainboards have a System Management Bus. There are a lot of
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devices which can be connected to a SMBus; the most notable are modern
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memory chips with EEPROM memories and chips for hardware monitoring.
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SMBus (System Management Bus) is based on the I2C protocol, and is mostly
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a subset of I2C protocols and signaling. Many I2C devices will work on an
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SMBus, but some SMBus protocols add semantics beyond what is required to
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achieve I2C branding. Modern PC mainboards rely on SMBus. The most common
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devices connected through SMBus are RAM modules configured using I2C EEPROMs,
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and hardware monitoring chips.
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Because the SMBus is just a special case of the generalized I2C bus, we
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can simulate the SMBus protocol on plain I2C busses. The reverse is
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regretfully impossible.
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Because the SMBus is mostly a subset of the generalized I2C bus, we can
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use its protocols on many I2C systems. However, there are systems that don't
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meet both SMBus and I2C electrical constraints; and others which can't
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implement all the common SMBus protocol semantics or messages.
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Terminology
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@ -29,6 +35,7 @@ When we talk about I2C, we use the following terms:
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An Algorithm driver contains general code that can be used for a whole class
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of I2C adapters. Each specific adapter driver depends on one algorithm
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driver.
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A Driver driver (yes, this sounds ridiculous, sorry) contains the general
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code to access some type of device. Each detected device gets its own
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data in the Client structure. Usually, Driver and Client are more closely
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@ -40,6 +47,10 @@ a separate Adapter and Algorithm driver), and drivers for your I2C devices
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in this package. See the lm_sensors project http://www.lm-sensors.nu
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for device drivers.
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At this time, Linux only operates I2C (or SMBus) in master mode; you can't
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use these APIs to make a Linux system behave as a slave/device, either to
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speak a custom protocol or to emulate some other device.
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Included Bus Drivers
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====================
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@ -1,5 +1,5 @@
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This is a small guide for those who want to write kernel drivers for I2C
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or SMBus devices.
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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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@ -29,8 +29,16 @@ 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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@ -40,7 +48,8 @@ static struct i2c_driver foo_driver = {
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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.
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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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@ -141,6 +150,59 @@ Writing is done the same way.
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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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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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@ -162,8 +224,8 @@ NOTE: If you want to write a `sensors' driver, the interface is slightly
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Probing classes
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---------------
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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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@ -210,8 +272,8 @@ 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
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-----------------------
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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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@ -237,8 +299,8 @@ 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
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--------------------------
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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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@ -427,8 +489,8 @@ For now, you can ignore the `flags' parameter. It is there for future use.
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}
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Removing the client
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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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