Documentation/i2c/dev-interface.rst

   1 ============================================
   2 Implementing I2C device drivers in userspace
   3 ============================================
   4
   5 Usually, I2C devices are controlled by a kernel driver. But it is also
   6 possible to access all devices on an adapter from userspace, through
   7 the /dev interface. You need to load module i2c-dev for this.
   8
   9 Each registered I2C adapter gets a number, counting from 0. You can
  10 examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
  11 Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all
  12 I2C adapters present on your system at a given time. i2cdetect is part of
  13 the i2c-tools package.
  14
  15 I2C device files are character device files with major device number 89
  16 and a minor device number corresponding to the number assigned as
  17 explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
  18 i2c-10, ...). All 256 minor device numbers are reserved for I2C.
  19
  20
  21 C example
  22 =========
  23
  24 So let's say you want to access an I2C adapter from a C program.
  25 First, you need to include these two headers::
  26
  27   #include <linux/i2c-dev.h>
  28   #include <i2c/smbus.h>
  29
  30 Now, you have to decide which adapter you want to access. You should
  31 inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
  32 Adapter numbers are assigned somewhat dynamically, so you can not
  33 assume much about them. They can even change from one boot to the next.
  34
  35 Next thing, open the device file, as follows::
  36
  37   int file;
  38   int adapter_nr = 2; /* probably dynamically determined */
  39   char filename[20];
  40
  41   snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
  42   file = open(filename, O_RDWR);
  43   if (file < 0) {
  44     /* ERROR HANDLING; you can check errno to see what went wrong */
  45     exit(1);
  46   }
  47
  48 When you have opened the device, you must specify with what device
  49 address you want to communicate::
  50
  51   int addr = 0x40; /* The I2C address */
  52
  53   if (ioctl(file, I2C_SLAVE, addr) < 0) {
  54     /* ERROR HANDLING; you can check errno to see what went wrong */
  55     exit(1);
  56   }
  57
  58 Well, you are all set up now. You can now use SMBus commands or plain
  59 I2C to communicate with your device. SMBus commands are preferred if
  60 the device supports them. Both are illustrated below::
  61
  62   __u8 reg = 0x10; /* Device register to access */
  63   __s32 res;
  64   char buf[10];
  65
  66   /* Using SMBus commands */
  67   res = i2c_smbus_read_word_data(file, reg);
  68   if (res < 0) {
  69     /* ERROR HANDLING: I2C transaction failed */
  70   } else {
  71     /* res contains the read word */
  72   }
  73
  74   /*
  75    * Using I2C Write, equivalent of
  76    * i2c_smbus_write_word_data(file, reg, 0x6543)
  77    */
  78   buf[0] = reg;
  79   buf[1] = 0x43;
  80   buf[2] = 0x65;
  81   if (write(file, buf, 3) != 3) {
  82     /* ERROR HANDLING: I2C transaction failed */
  83   }
  84
  85   /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
  86   if (read(file, buf, 1) != 1) {
  87     /* ERROR HANDLING: I2C transaction failed */
  88   } else {
  89     /* buf[0] contains the read byte */
  90   }
  91
  92 Note that only a subset of the I2C and SMBus protocols can be achieved by
  93 the means of read() and write() calls. In particular, so-called combined
  94 transactions (mixing read and write messages in the same transaction)
  95 aren't supported. For this reason, this interface is almost never used by
  96 user-space programs.
  97
  98 IMPORTANT: because of the use of inline functions, you *have* to use
  99 '-O' or some variation when you compile your program!
 100
 101
 102 Full interface description
 103 ==========================
 104
 105 The following IOCTLs are defined:
 106
 107 ``ioctl(file, I2C_SLAVE, long addr)``
 108   Change slave address. The address is passed in the 7 lower bits of the
 109   argument (except for 10 bit addresses, passed in the 10 lower bits in this
 110   case).
 111
 112 ``ioctl(file, I2C_TENBIT, long select)``
 113   Selects ten bit addresses if select not equals 0, selects normal 7 bit
 114   addresses if select equals 0. Default 0.  This request is only valid
 115   if the adapter has I2C_FUNC_10BIT_ADDR.
 116
 117 ``ioctl(file, I2C_PEC, long select)``
 118   Selects SMBus PEC (packet error checking) generation and verification
 119   if select not equals 0, disables if select equals 0. Default 0.
 120   Used only for SMBus transactions.  This request only has an effect if the
 121   the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just
 122   doesn't have any effect.
 123
 124 ``ioctl(file, I2C_FUNCS, unsigned long *funcs)``
 125   Gets the adapter functionality and puts it in ``*funcs``.
 126
 127 ``ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)``
 128   Do combined read/write transaction without stop in between.
 129   Only valid if the adapter has I2C_FUNC_I2C.  The argument is
 130   a pointer to a::
 131
 132     struct i2c_rdwr_ioctl_data {
 133       struct i2c_msg *msgs;  /* ptr to array of simple messages */
 134       int nmsgs;             /* number of messages to exchange */
 135     }
 136
 137   The msgs[] themselves contain further pointers into data buffers.
 138   The function will write or read data to or from that buffers depending
 139   on whether the I2C_M_RD flag is set in a particular message or not.
 140   The slave address and whether to use ten bit address mode has to be
 141   set in each message, overriding the values set with the above ioctl's.
 142
 143 ``ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)``
 144   If possible, use the provided ``i2c_smbus_*`` methods described below instead
 145   of issuing direct ioctls.
 146
 147 You can do plain I2C transactions by using read(2) and write(2) calls.
 148 You do not need to pass the address byte; instead, set it through
 149 ioctl I2C_SLAVE before you try to access the device.
 150
 151 You can do SMBus level transactions (see documentation file smbus-protocol
 152 for details) through the following functions::
 153
 154   __s32 i2c_smbus_write_quick(int file, __u8 value);
 155   __s32 i2c_smbus_read_byte(int file);
 156   __s32 i2c_smbus_write_byte(int file, __u8 value);
 157   __s32 i2c_smbus_read_byte_data(int file, __u8 command);
 158   __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
 159   __s32 i2c_smbus_read_word_data(int file, __u8 command);
 160   __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
 161   __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
 162   __s32 i2c_smbus_block_process_call(int file, __u8 command, __u8 length,
 163                                      __u8 *values);
 164   __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
 165   __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,
 166                                    __u8 *values);
 167
 168 All these transactions return -1 on failure; you can read errno to see
 169 what happened. The 'write' transactions return 0 on success; the
 170 'read' transactions return the read value, except for read_block, which
 171 returns the number of values read. The block buffers need not be longer
 172 than 32 bytes.
 173
 174 The above functions are made available by linking against the libi2c library,
 175 which is provided by the i2c-tools project.  See:
 176 https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/.
 177
 178
 179 Implementation details
 180 ======================
 181
 182 For the interested, here's the code flow which happens inside the kernel
 183 when you use the /dev interface to I2C:
 184
 185 1) Your program opens /dev/i2c-N and calls ioctl() on it, as described in
 186    section "C example" above.
 187
 188 2) These open() and ioctl() calls are handled by the i2c-dev kernel
 189    driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
 190    respectively. You can think of i2c-dev as a generic I2C chip driver
 191    that can be programmed from user-space.
 192
 193 3) Some ioctl() calls are for administrative tasks and are handled by
 194    i2c-dev directly. Examples include I2C_SLAVE (set the address of the
 195    device you want to access) and I2C_PEC (enable or disable SMBus error
 196    checking on future transactions.)
 197
 198 4) Other ioctl() calls are converted to in-kernel function calls by
 199    i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
 200    functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
 201    performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer().
 202
 203    The i2c-dev driver is responsible for checking all the parameters that
 204    come from user-space for validity. After this point, there is no
 205    difference between these calls that came from user-space through i2c-dev
 206    and calls that would have been performed by kernel I2C chip drivers
 207    directly. This means that I2C bus drivers don't need to implement
 208    anything special to support access from user-space.
 209
 210 5) These i2c.h functions are wrappers to the actual implementation of
 211    your I2C bus driver. Each adapter must declare callback functions
 212    implementing these standard calls. i2c.h:i2c_get_functionality() calls
 213    i2c_adapter.algo->functionality(), while
 214    i2c-core-smbus.c:i2c_smbus_xfer() calls either
 215    adapter.algo->smbus_xfer() if it is implemented, or if not,
 216    i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls
 217    i2c_adapter.algo->master_xfer().
 218
 219 After your I2C bus driver has processed these requests, execution runs
 220 up the call chain, with almost no processing done, except by i2c-dev to
 221 package the returned data, if any, in suitable format for the ioctl.