Linux read from i2c

Implementing I2C device drivers in userspace¶

Usually, I2C devices are controlled by a kernel driver. But it is also possible to access all devices on an adapter from userspace, through the /dev interface. You need to load module i2c-dev for this.

Each registered I2C adapter gets a number, counting from 0. You can examine /sys/class/i2c-dev/ to see what number corresponds to which adapter. Alternatively, you can run “i2cdetect -l” to obtain a formatted list of all I2C adapters present on your system at a given time. i2cdetect is part of the i2c-tools package.

I2C device files are character device files with major device number 89 and a minor device number corresponding to the number assigned as explained above. They should be called “i2c-%d” (i2c-0, i2c-1, …, i2c-10, …). All 256 minor device numbers are reserved for I2C.

C example¶

So let’s say you want to access an I2C adapter from a C program. First, you need to include these two headers:

Now, you have to decide which adapter you want to access. You should inspect /sys/class/i2c-dev/ or run “i2cdetect -l” to decide this. Adapter numbers are assigned somewhat dynamically, so you can not assume much about them. They can even change from one boot to the next.

Next thing, open the device file, as follows:

When you have opened the device, you must specify with what device address you want to communicate:

Well, you are all set up now. You can now use SMBus commands or plain I2C to communicate with your device. SMBus commands are preferred if the device supports them. Both are illustrated below:

Note that only a subset of the I2C and SMBus protocols can be achieved by the means of read() and write() calls. In particular, so-called combined transactions (mixing read and write messages in the same transaction) aren’t supported. For this reason, this interface is almost never used by user-space programs.

IMPORTANT: because of the use of inline functions, you have to use ‘-O’ or some variation when you compile your program!

Full interface description¶

The following IOCTLs are defined:

ioctl(file, I2C_SLAVE, long addr)

Change slave address. The address is passed in the 7 lower bits of the argument (except for 10 bit addresses, passed in the 10 lower bits in this case).

ioctl(file, I2C_TENBIT, long select)

Selects ten bit addresses if select not equals 0, selects normal 7 bit addresses if select equals 0. Default 0. This request is only valid if the adapter has I2C_FUNC_10BIT_ADDR.

ioctl(file, I2C_PEC, long select)

Selects SMBus PEC (packet error checking) generation and verification if select not equals 0, disables if select equals 0. Default 0. Used only for SMBus transactions. This request only has an effect if the the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just doesn’t have any effect.

ioctl(file, I2C_FUNCS, unsigned long *funcs)

Gets the adapter functionality and puts it in *funcs .

ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)

Do combined read/write transaction without stop in between. Only valid if the adapter has I2C_FUNC_I2C. The argument is a pointer to a:

The msgs[] themselves contain further pointers into data buffers. The function will write or read data to or from that buffers depending on whether the I2C_M_RD flag is set in a particular message or not. The slave address and whether to use ten bit address mode has to be set in each message, overriding the values set with the above ioctl’s.

ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)

If possible, use the provided i2c_smbus_* methods described below instead of issuing direct ioctls.

You can do plain I2C transactions by using read(2) and write(2) calls. You do not need to pass the address byte; instead, set it through ioctl I2C_SLAVE before you try to access the device.

You can do SMBus level transactions (see documentation file smbus-protocol for details) through the following functions:

All these transactions return -1 on failure; you can read errno to see what happened. The ‘write’ transactions return 0 on success; the ‘read’ transactions return the read value, except for read_block, which returns the number of values read. The block buffers need not be longer than 32 bytes.

The above functions are made available by linking against the libi2c library, which is provided by the i2c-tools project. See: https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/.

Implementation details¶

For the interested, here’s the code flow which happens inside the kernel when you use the /dev interface to I2C:

Your program opens /dev/i2c-N and calls ioctl() on it, as described in section “C example” above.

These open() and ioctl() calls are handled by the i2c-dev kernel driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(), respectively. You can think of i2c-dev as a generic I2C chip driver that can be programmed from user-space.

Some ioctl() calls are for administrative tasks and are handled by i2c-dev directly. Examples include I2C_SLAVE (set the address of the device you want to access) and I2C_PEC (enable or disable SMBus error checking on future transactions.)

Other ioctl() calls are converted to in-kernel function calls by i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which performs an SMBus transaction using i2c-core-smbus.c: i2c_smbus_xfer() .

The i2c-dev driver is responsible for checking all the parameters that come from user-space for validity. After this point, there is no difference between these calls that came from user-space through i2c-dev and calls that would have been performed by kernel I2C chip drivers directly. This means that I2C bus drivers don’t need to implement anything special to support access from user-space.

These i2c.h functions are wrappers to the actual implementation of your I2C bus driver. Each adapter must declare callback functions implementing these standard calls. i2c.h:i2c_get_functionality() calls i2c_adapter.algo->functionality(), while i2c-core-smbus.c: i2c_smbus_xfer() calls either adapter.algo->smbus_xfer() if it is implemented, or if not, i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls i2c_adapter.algo->master_xfer().

After your I2C bus driver has processed these requests, execution runs up the call chain, with almost no processing done, except by i2c-dev to package the returned data, if any, in suitable format for the ioctl.

© Copyright The kernel development community.

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Implementing I2C device drivers¶

This is a small guide for those who want to write kernel drivers for I2C or SMBus devices, using Linux as the protocol host/master (not slave).

To set up a driver, you need to do several things. Some are optional, and some things can be done slightly or completely different. Use this as a guide, not as a rule book!

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 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.

The driver structure¶

Usually, you will implement a single driver structure, and instantiate 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.

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 below.

Extra client data¶

Each client structure has a special data field that can point to any structure at all. You should use this to keep device-specific data.

Note that starting with kernel 2.6.34, you don’t have to set the data field to NULL in remove() or if probe() failed anymore. The i2c-core does this automatically on these occasions. Those are also the only times the core will touch this field.

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.

I have found it useful to define foo_read and foo_write functions for this. For some cases, it will be easier to call the I2C functions directly, but many chips have some kind of register-value idea that can easily be encapsulated.

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

Probing and attaching¶

The Linux I2C stack was originally written to support access to hardware monitoring chips on PC motherboards, and thus used to embed some assumptions that were more appropriate to SMBus (and PCs) than to I2C. One of these assumptions was that most adapters and devices drivers support the SMBUS_QUICK protocol to probe device presence. Another was that devices and their drivers can be sufficiently configured using only such probe primitives.

As Linux and its I2C stack became more widely used in embedded systems and complex components such as DVB adapters, those assumptions became more problematic. Drivers for I2C devices that issue interrupts need more (and different) configuration information, as do drivers handling chip variants that can’t be distinguished by protocol probing, or which need some board specific information to operate correctly.

Device/Driver Binding¶

System infrastructure, typically board-specific initialization code or boot firmware, reports what I2C devices exist. For example, there may be a table, in the kernel or from the boot loader, identifying I2C devices and linking them to board-specific configuration information about IRQs and other wiring artifacts, chip type, and so on. That could be used to create i2c_client objects for each I2C device.

I2C device drivers using this binding model work just like any other kind of driver in Linux: they provide a probe() method to bind to those devices, and a remove() method to unbind.

Remember that the i2c_driver does not create those client handles. The handle may be used during foo_probe(). If foo_probe() reports success (zero not a negative status code) it may save the handle and use it until foo_remove() returns. That binding model is used by most Linux drivers.

The probe function is called when an entry in the id_table name field matches the device’s name. It is passed the entry that was matched so the driver knows which one in the table matched.

Device Creation¶

If you know for a fact that an I2C device is connected to a given I2C bus, you can instantiate that device by simply filling an i2c_board_info structure with the device address and driver name, and calling i2c_new_client_device() . This will create the device, then the driver core will take care of finding the right driver and will call its probe() method. If a driver supports different device types, you can specify the type you want using the type field. You can also specify an IRQ and platform data if needed.

Sometimes you know that a device is connected to a given I2C bus, but you don’t know the exact address it uses. This happens on TV adapters for example, where the same driver supports dozens of slightly different models, and I2C device addresses change from one model to the next. In that case, you can use the i2c_new_scanned_device() variant, which is similar to i2c_new_client_device() , except that it takes an additional list of possible I2C addresses to probe. A device is created for the first responsive address in the list. If you expect more than one device to be present in the address range, simply call i2c_new_scanned_device() that many times.

The call to i2c_new_client_device() or i2c_new_scanned_device() typically happens in the I2C bus driver. You may want to save the returned i2c_client reference for later use.

Device Detection¶

Sometimes you do not know in advance which I2C devices are connected to a given I2C bus. This is for example the case of hardware monitoring devices on a PC’s SMBus. In that case, you may want to let your driver detect supported devices automatically. This is how the legacy model was working, and is now available as an extension to the standard driver model.

You simply have to define a detect callback which will attempt to identify supported devices (returning 0 for supported ones and -ENODEV for unsupported ones), a list of addresses to probe, and a device type (or class) so that only I2C buses which may have that type of device connected (and not otherwise enumerated) will be probed. For example, a driver for a hardware monitoring chip for which auto-detection is needed would set its class to I2C_CLASS_HWMON, and only I2C adapters with a class including I2C_CLASS_HWMON would be probed by this driver. Note that the absence of matching classes does not prevent the use of a device of that type on the given I2C adapter. All it prevents is auto-detection; explicit instantiation of devices is still possible.

Note that this mechanism is purely optional and not suitable for all devices. You need some reliable way to identify the supported devices (typically using device-specific, dedicated identification registers), otherwise misdetections are likely to occur and things can get wrong quickly. Keep in mind that the I2C protocol doesn’t include any standard way to detect the presence of a chip at a given address, let alone a standard way to identify devices. Even worse is the lack of semantics associated to bus transfers, which means that the same transfer can be seen as a read operation by a chip and as a write operation by another chip. For these reasons, explicit device instantiation should always be preferred to auto-detection where possible.

Device Deletion¶

Each I2C device which has been created using i2c_new_client_device() or i2c_new_scanned_device() can be unregistered by calling i2c_unregister_device() . If you don’t call it explicitly, it will be 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 driver¶

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.

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.

Driver Information¶

Power Management¶

If your I2C device needs special handling when entering a system low power state – like putting a transceiver into a low power mode, or activating a system wakeup mechanism – do that by implementing the appropriate callbacks for the dev_pm_ops of the driver (like suspend and resume).

These are standard driver model calls, and they work just like they would for any other driver stack. The calls can sleep, and can use I2C messaging to the device being suspended or resumed (since their parent I2C adapter is active when these calls are issued, and IRQs are still enabled).

System Shutdown¶

If your I2C device needs special handling when the system shuts down or reboots (including kexec) – like turning something off – use a shutdown() method.

Again, this is a standard driver model call, working just like it would for any other driver stack: the calls can sleep, and can use I2C messaging.

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.

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
.

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¶

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 to read/write, the third the number of bytes to read/write (must be less than the length of the buffer, also should be less than 64k since msg.len is u16.) Returned is the actual number of bytes read/written.

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 stop condition is issued between transaction. The i2c_msg structure contains 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.

SMBus communication¶

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

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

All these transactions return a negative errno value on failure. The ‘write’ transactions return 0 on success; the ‘read’ transactions return the read value, except for block transactions, which return the number of values read. The block buffers need not be longer than 32 bytes.

You can read the file smbus-protocol for more information about the actual SMBus protocol.

General purpose routines¶

Below all general purpose routines are listed, that were not mentioned before:

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