222 lines
8.8 KiB
ReStructuredText
222 lines
8.8 KiB
ReStructuredText
============================================
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Implementing I2C device drivers in userspace
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============================================
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Usually, I2C devices are controlled by a kernel driver. But it is also
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possible to access all devices on an adapter from userspace, through
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the /dev interface. You need to load module i2c-dev for this.
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Each registered I2C adapter gets a number, counting from 0. You can
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examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
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Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all
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I2C adapters present on your system at a given time. i2cdetect is part of
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the i2c-tools package.
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I2C device files are character device files with major device number 89
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and a minor device number corresponding to the number assigned as
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explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
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i2c-10, ...). All 256 minor device numbers are reserved for I2C.
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C example
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=========
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So let's say you want to access an I2C adapter from a C program.
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First, you need to include these two headers::
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#include <linux/i2c-dev.h>
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#include <i2c/smbus.h>
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Now, you have to decide which adapter you want to access. You should
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inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
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Adapter numbers are assigned somewhat dynamically, so you can not
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assume much about them. They can even change from one boot to the next.
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Next thing, open the device file, as follows::
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int file;
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int adapter_nr = 2; /* probably dynamically determined */
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char filename[20];
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snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
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file = open(filename, O_RDWR);
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if (file < 0) {
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/* ERROR HANDLING; you can check errno to see what went wrong */
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exit(1);
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}
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When you have opened the device, you must specify with what device
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address you want to communicate::
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int addr = 0x40; /* The I2C address */
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if (ioctl(file, I2C_SLAVE, addr) < 0) {
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/* ERROR HANDLING; you can check errno to see what went wrong */
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exit(1);
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}
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Well, you are all set up now. You can now use SMBus commands or plain
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I2C to communicate with your device. SMBus commands are preferred if
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the device supports them. Both are illustrated below::
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__u8 reg = 0x10; /* Device register to access */
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__s32 res;
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char buf[10];
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/* Using SMBus commands */
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res = i2c_smbus_read_word_data(file, reg);
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if (res < 0) {
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/* ERROR HANDLING: I2C transaction failed */
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} else {
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/* res contains the read word */
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}
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/*
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* Using I2C Write, equivalent of
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* i2c_smbus_write_word_data(file, reg, 0x6543)
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*/
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buf[0] = reg;
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buf[1] = 0x43;
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buf[2] = 0x65;
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if (write(file, buf, 3) != 3) {
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/* ERROR HANDLING: I2C transaction failed */
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}
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/* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
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if (read(file, buf, 1) != 1) {
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/* ERROR HANDLING: I2C transaction failed */
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} else {
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/* buf[0] contains the read byte */
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}
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Note that only a subset of the I2C and SMBus protocols can be achieved by
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the means of read() and write() calls. In particular, so-called combined
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transactions (mixing read and write messages in the same transaction)
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aren't supported. For this reason, this interface is almost never used by
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user-space programs.
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IMPORTANT: because of the use of inline functions, you *have* to use
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'-O' or some variation when you compile your program!
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Full interface description
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==========================
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The following IOCTLs are defined:
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``ioctl(file, I2C_SLAVE, long addr)``
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Change slave address. The address is passed in the 7 lower bits of the
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argument (except for 10 bit addresses, passed in the 10 lower bits in this
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case).
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``ioctl(file, I2C_TENBIT, long select)``
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Selects ten bit addresses if select not equals 0, selects normal 7 bit
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addresses if select equals 0. Default 0. This request is only valid
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if the adapter has I2C_FUNC_10BIT_ADDR.
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``ioctl(file, I2C_PEC, long select)``
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Selects SMBus PEC (packet error checking) generation and verification
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if select not equals 0, disables if select equals 0. Default 0.
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Used only for SMBus transactions. This request only has an effect if the
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the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just
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doesn't have any effect.
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``ioctl(file, I2C_FUNCS, unsigned long *funcs)``
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Gets the adapter functionality and puts it in ``*funcs``.
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``ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)``
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Do combined read/write transaction without stop in between.
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Only valid if the adapter has I2C_FUNC_I2C. The argument is
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a pointer to a::
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struct i2c_rdwr_ioctl_data {
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struct i2c_msg *msgs; /* ptr to array of simple messages */
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int nmsgs; /* number of messages to exchange */
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}
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The msgs[] themselves contain further pointers into data buffers.
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The function will write or read data to or from that buffers depending
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on whether the I2C_M_RD flag is set in a particular message or not.
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The slave address and whether to use ten bit address mode has to be
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set in each message, overriding the values set with the above ioctl's.
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``ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)``
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If possible, use the provided ``i2c_smbus_*`` methods described below instead
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of issuing direct ioctls.
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You can do plain I2C transactions by using read(2) and write(2) calls.
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You do not need to pass the address byte; instead, set it through
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ioctl I2C_SLAVE before you try to access the device.
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You can do SMBus level transactions (see documentation file smbus-protocol.rst
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for details) through the following functions::
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__s32 i2c_smbus_write_quick(int file, __u8 value);
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__s32 i2c_smbus_read_byte(int file);
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__s32 i2c_smbus_write_byte(int file, __u8 value);
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__s32 i2c_smbus_read_byte_data(int file, __u8 command);
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__s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
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__s32 i2c_smbus_read_word_data(int file, __u8 command);
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__s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
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__s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
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__s32 i2c_smbus_block_process_call(int file, __u8 command, __u8 length,
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__u8 *values);
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__s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
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__s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,
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__u8 *values);
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All these transactions return -1 on failure; you can read errno to see
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what happened. The 'write' transactions return 0 on success; the
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'read' transactions return the read value, except for read_block, which
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returns the number of values read. The block buffers need not be longer
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than 32 bytes.
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The above functions are made available by linking against the libi2c library,
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which is provided by the i2c-tools project. See:
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https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/.
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Implementation details
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======================
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For the interested, here's the code flow which happens inside the kernel
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when you use the /dev interface to I2C:
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1) Your program opens /dev/i2c-N and calls ioctl() on it, as described in
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section "C example" above.
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2) These open() and ioctl() calls are handled by the i2c-dev kernel
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driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
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respectively. You can think of i2c-dev as a generic I2C chip driver
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that can be programmed from user-space.
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3) Some ioctl() calls are for administrative tasks and are handled by
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i2c-dev directly. Examples include I2C_SLAVE (set the address of the
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device you want to access) and I2C_PEC (enable or disable SMBus error
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checking on future transactions.)
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4) Other ioctl() calls are converted to in-kernel function calls by
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i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
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functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
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performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer().
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The i2c-dev driver is responsible for checking all the parameters that
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come from user-space for validity. After this point, there is no
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difference between these calls that came from user-space through i2c-dev
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and calls that would have been performed by kernel I2C chip drivers
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directly. This means that I2C bus drivers don't need to implement
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anything special to support access from user-space.
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5) These i2c.h functions are wrappers to the actual implementation of
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your I2C bus driver. Each adapter must declare callback functions
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implementing these standard calls. i2c.h:i2c_get_functionality() calls
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i2c_adapter.algo->functionality(), while
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i2c-core-smbus.c:i2c_smbus_xfer() calls either
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adapter.algo->smbus_xfer() if it is implemented, or if not,
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i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls
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i2c_adapter.algo->master_xfer().
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After your I2C bus driver has processed these requests, execution runs
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up the call chain, with almost no processing done, except by i2c-dev to
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package the returned data, if any, in suitable format for the ioctl.
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