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+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:
+
+  #include <linux/i2c-dev.h>
+  #include <i2c/smbus.h>
+
+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:
+
+  int file;
+  int adapter_nr = 2; /* probably dynamically determined */
+  char filename[20];
+
+  snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
+  file = open(filename, O_RDWR);
+  if (file < 0) {
+    /* ERROR HANDLING; you can check errno to see what went wrong */
+    exit(1);
+  }
+
+When you have opened the device, you must specify with what device
+address you want to communicate:
+
+  int addr = 0x40; /* The I2C address */
+
+  if (ioctl(file, I2C_SLAVE, addr) < 0) {
+    /* ERROR HANDLING; you can check errno to see what went wrong */
+    exit(1);
+  }
+
+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.
+
+  __u8 reg = 0x10; /* Device register to access */
+  __s32 res;
+  char buf[10];
+
+  /* Using SMBus commands */
+  res = i2c_smbus_read_word_data(file, reg);
+  if (res < 0) {
+    /* ERROR HANDLING: i2c transaction failed */
+  } else {
+    /* res contains the read word */
+  }
+
+  /*
+   * Using I2C Write, equivalent of
+   * i2c_smbus_write_word_data(file, reg, 0x6543)
+   */
+  buf[0] = reg;
+  buf[1] = 0x43;
+  buf[2] = 0x65;
+  if (write(file, buf, 3) != 3) {
+    /* ERROR HANDLING: i2c transaction failed */
+  }
+
+  /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
+  if (read(file, buf, 1) != 1) {
+    /* ERROR HANDLING: i2c transaction failed */
+  } else {
+    /* buf[0] contains the read byte */
+  }
+
+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
+
+  struct i2c_rdwr_ioctl_data {
+      struct i2c_msg *msgs;  /* ptr to array of simple messages */
+      int nmsgs;             /* number of messages to exchange */
+  }
+
+  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:
+  __s32 i2c_smbus_write_quick(int file, __u8 value);
+  __s32 i2c_smbus_read_byte(int file);
+  __s32 i2c_smbus_write_byte(int file, __u8 value);
+  __s32 i2c_smbus_read_byte_data(int file, __u8 command);
+  __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
+  __s32 i2c_smbus_read_word_data(int file, __u8 command);
+  __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
+  __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
+  __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
+  __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,
+                                   __u8 *values);
+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:
+
+1* Your program opens /dev/i2c-N and calls ioctl() on it, as described in
+section "C example" above.
+
+2* 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.
+
+3* 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.)
+
+4* 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.
+
+5* 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.