linux/Documentation/i2c/i2c-stub.rst
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   1========
   2i2c-stub
   3========
   4
   5Description
   6===========
   7
   8This module is a very simple fake I2C/SMBus driver.  It implements six
   9types of SMBus commands: write quick, (r/w) byte, (r/w) byte data, (r/w)
  10word data, (r/w) I2C block data, and (r/w) SMBus block data.
  11
  12You need to provide chip addresses as a module parameter when loading this
  13driver, which will then only react to SMBus commands to these addresses.
  14
  15No hardware is needed nor associated with this module.  It will accept write
  16quick commands to the specified addresses; it will respond to the other
  17commands (also to the specified addresses) by reading from or writing to
  18arrays in memory.  It will also spam the kernel logs for every command it
  19handles.
  20
  21A pointer register with auto-increment is implemented for all byte
  22operations.  This allows for continuous byte reads like those supported by
  23EEPROMs, among others.
  24
  25SMBus block command support is disabled by default, and must be enabled
  26explicitly by setting the respective bits (0x03000000) in the functionality
  27module parameter.
  28
  29SMBus block commands must be written to configure an SMBus command for
  30SMBus block operations. Writes can be partial. Block read commands always
  31return the number of bytes selected with the largest write so far.
  32
  33The typical use-case is like this:
  34
  35        1. load this module
  36        2. use i2cset (from the i2c-tools project) to pre-load some data
  37        3. load the target chip driver module
  38        4. observe its behavior in the kernel log
  39
  40There's a script named i2c-stub-from-dump in the i2c-tools package which
  41can load register values automatically from a chip dump.
  42
  43Parameters
  44==========
  45
  46int chip_addr[10]:
  47        The SMBus addresses to emulate chips at.
  48
  49unsigned long functionality:
  50        Functionality override, to disable some commands. See I2C_FUNC_*
  51        constants in <linux/i2c.h> for the suitable values. For example,
  52        value 0x1f0000 would only enable the quick, byte and byte data
  53        commands.
  54
  55u8 bank_reg[10], u8 bank_mask[10], u8 bank_start[10], u8 bank_end[10]:
  56        Optional bank settings. They tell which bits in which register
  57        select the active bank, as well as the range of banked registers.
  58
  59Caveats
  60=======
  61
  62If your target driver polls some byte or word waiting for it to change, the
  63stub could lock it up.  Use i2cset to unlock it.
  64
  65If you spam it hard enough, printk can be lossy.  This module really wants
  66something like relayfs.
  67