path: root/extra/i2c_pseudo/Documentation.rst

=================
i2c-pseudo driver
=================

Usually I2C adapters are implemented in a kernel driver.  It is also possible to
implement an adapter in userspace, through the /dev/i2c-pseudo-controller
interface.  Load module i2c-pseudo for this.

Use cases for this module include:

- Using local I2C device drivers, particularly i2c-dev, with I2C busses on
  remote systems.  For example, interacting with a Device Under Test (DUT)
  connected to a Linux host through a debug interface, or interacting with a
  remote host over a network.

- Implementing I2C device driver tests that are impractical with the i2c-stub
  module.  For example, when simulating an I2C device where its driver might
  issue a sequence of reads and writes without interruption, and the value at a
  certain address must change during the sequence.

This is not intended to replace kernel drivers for actual I2C busses on the
local host machine.


Details
=======

Each time /dev/i2c-pseudo-controller is opened, and the correct initialization
command is written to it (ADAPTER_START), a new I2C adapter is created.  The
adapter will live until its file descriptor is closed.  Multiple pseudo adapters
can co-exist simultaneously, controlled by the same or different userspace
processes.  When an I2C device driver sends an I2C message to a pseudo adapter,
the message becomes readable from its file descriptor.  If a reply is written
before the adapter timeout expires, that reply will be sent back to the I2C
device driver.

Reads and writes are buffered inside i2c-pseudo such that userspace controllers
may split them up into arbitrarily small chunks.  Multiple commands, or portions
of multiple commands, may be read or written together.

Blocking I/O is the default.  Non-blocking I/O is supported as well, enabled by
O_NONBLOCK.  Polling is supported, with or without non-blocking I/O.  A special
command (ADAPTER_SHUTDOWN) is available to unblock any pollers or blocked
reads or writes, as a convenience for a multi-threaded or multi-process program
that wants to exit.

It is safe to access a single controller fd from multiple threads or processes
concurrently, though it is up to the controller to ensure proper ordering, and
to ensure that writes for different commands do not get interleaved.  However,
it is recommended (not required) that controller implementations have only one
reader thread and one writer thread, which may or may not be the same thread.
Avoiding multiple readers and multiple writers greatly simplifies controller
implementation, and there is likely no performance benefit to be gained from
concurrent reads or concurrent writes due to how i2c-pseudo serializes them
internally.  After all, on a real I2C bus only one I2C message can be active at
a time.

Commands are newline-terminated, both those read from the controller device, and
those written to it.


Read Commands
=============

The commands that may be read from a pseudo controller device are:

----

:Read Command: ``I2C_ADAPTER_NUM <num>``
:Example: ``"I2C_ADAPTER_NUM 5\n"``
:Details:
  | This is read in response to the GET_ADAPTER_NUM command being written.
    The number is the I2C adapter number in decimal.  This can only occur after
    ADAPTER_START, because before that the number is not known and cannot be
    predicted reliably.

----

:Read Command: ``I2C_PSEUDO_ID <num>``
:Example: ``"I2C_PSEUDO_ID 98\n"``
:Details:
  | This is read in response to the GET_PSEUDO_ID command being written.
    The number is the pseudo ID in decimal.

----

:Read Command: ``I2C_BEGIN_XFER``
:Example: ``"I2C_BEGIN_XFER\n"``
:Details:
  | This indicates the start of an I2C transaction request, in other words
    the start of the I2C messages from a single invocation of the I2C adapter's
    master_xfer() callback.  This can only occur after ADAPTER_START.

----

:Read Command: ``I2C_XFER_REQ <xfer_id> <msg_id> <addr> <flags> <data_len> [<write_byte>[:...]]``
:Example: ``"I2C_XFER_REQ 3 0 0x0070 0x0000 2 AB:9F\n"``
:Example: ``"I2C_XFER_REQ 3 1 0x0070 0x0001 4\n"``
:Details:
  | This is a single I2C message that a device driver requested be sent on
    the bus, in other words a single struct i2c_msg from master_xfer() msgs arg.
  |
  | The xfer_id is a number representing the whole I2C transaction, thus all
    I2C_XFER_REQ between a I2C_BEGIN_XFER + I2C_COMMIT_XFER pair share an
    xfer_id.  The purpose is to ensure replies from the userspace controller are
    always properly matched to the intended master_xfer() request.  The first
    transaction has xfer_id 0, and it increases by 1 with each transaction,
    however it will eventually wrap back to 0 if enough transactions happen
    during the lifetime of a pseudo adapter.  It is guaranteed to have a large
    enough maximum value such that there can never be multiple outstanding
    transactions with the same ID, due to an internal limit in i2c-pseudo that
    will block master_xfer() calls when the controller is falling behind in its
    replies.
  |
  | The msg_id is a decimal number representing the index of the I2C message
    within its transaction, in other words the index in master_xfer() \*msgs
    array arg.  This starts at 0 after each I2C_BEGIN_XFER.  This is guaranteed
    to not wrap.
  |
  | The addr is the hexadecimal I2C address for this I2C message.  The address
    is right-aligned without any read/write bit.
  |
  | The flags are the same bitmask flags used in struct i2c_msg, in hexadecimal
    form.  Of particular importance to any pseudo controller is the read bit,
    which is guaranteed to be 0x1 per Linux I2C documentation.
  |
  | The data_len is the decimal number of either how many bytes to write that
    will follow, or how many bytes to read and reply with if this is a read
    request.
  |
  | If this is a read, data_len will be the final field in this command.  If
    this is a write, data_len will be followed by the given number of
    colon-separated hexadecimal byte values, in the format shown in the example
    above.

----

:Read Command: ``I2C_COMMIT_XFER``
:Example: ``"I2C_COMMIT_XFER\n"``
:Details:
  | This indicates the end of an I2C transaction request, in other words the
    end of the I2C messages from a single invocation of the I2C adapter's
    master_xfer() callback.  This should be read exactly once after each
    I2C_BEGIN_XFER, with a varying number of I2C_XFER_REQ between them.


Write Commands
==============

The commands that may be written to a pseudo controller device are:


:Write Command: ``SET_ADAPTER_NAME_SUFFIX <suffix>``
:Example: ``"SET_ADAPTER_NAME_SUFFIX My Adapter\n"``
:Details:
  | Sets a suffix to append to the auto-generated I2C adapter name.  Only
    valid before ADAPTER_START.  A space or other separator character will be
    placed between the auto-generated name and the suffix, so there is no need
    to include a leading separator in the suffix.  If the resulting name is too
    long for the I2C adapter name field, it will be quietly truncated.

----

:Write Command: ``SET_ADAPTER_TIMEOUT_MS <ms>``
:Example: ``"SET_ADAPTER_TIMEOUT_MS 2000\n"``
:Details:
  | Sets the timeout in milliseconds for each I2C transaction, in other words
    for each master_xfer() reply.  Only valid before ADAPTER_START.  The I2C
    subsystem will automatically time out transactions based on this setting.
    Set to 0 to use the I2C subsystem default timeout.  The default timeout for
    new pseudo adapters where this command has not been used is configurable at
    i2c-pseudo module load time, and itself has a default independent from the
    I2C subsystem default.  (If the i2c-pseudo module level default is set to 0,
    that has the same meaning as here.)

----

:Write Command: ``ADAPTER_START``
:Example: ``"ADAPTER_START\n"``
:Details:
  | Tells i2c-pseudo to actually create the I2C adapter.  Only valid once per
    open controller fd.

----

:Write Command: ``GET_ADAPTER_NUM``
:Example: ``"GET_ADAPTER_NUM\n"``
:Details:
  | Asks i2c-pseudo for the number assigned to this I2C adapter by the I2C
    subsystem.  Only valid after ADAPTER_START, because before that the number
    is not known and cannot be predicted reliably.

----

:Write Command: ``GET_PSEUDO_ID``
:Example: ``"GET_PSEUDO_ID\n"``
:Details:
  | Asks i2c-pseudo for the pseudo ID of this I2C adapter.  The pseudo ID will
    not be reused for the lifetime of the i2c-pseudo module, unless an internal
    counter wraps.  I2C clients can use this to track specific instances of
    pseudo adapters, even when adapter numbers have been reused.

----

:Write Command: ``I2C_XFER_REPLY <xfer_id> <msg_id> <addr> <flags> <errno> [<read_byte>[:...]]``
:Example: ``"I2C_XFER_REPLY 3 0 0x0070 0x0000 0\n"``
:Example: ``"I2C_XFER_REPLY 3 1 0x0070 0x0001 0 0B:29:02:D9\n"``
:Details:
  | This is how a pseudo controller can reply to I2C_XFER_REQ.  Only valid
    after I2C_XFER_REQ.  A pseudo controller should write one of these for each
    I2C_XFER_REQ it reads, including for failures, so that I2C device drivers
    need not wait for the adapter timeout upon failure (if failure is known
    sooner).
  |
  | The fields in common with I2C_XFER_REQ have their same meanings, and their
    values are expected to exactly match what was read in the I2C_XFER_REQ
    command that this is in reply to.
  |
  | The errno field is how the pseudo controller indicates success or failure
    for this I2C message.  A 0 value indicates success.  A non-zero value
    indicates a failure.  Pseudo controllers are encouraged to use errno values
    to encode some meaning in a failure response, but that is not a requirement,
    and the I2C adapter interface does not provide a way to pass per-message
    errno values to a device driver anyways.
  |
  | Pseudo controllers are encouraged to reply in the same order as messages
    were received, however i2c-pseudo will properly match up out-of-order
    replies with their original requests.

----

:Write Command: ``ADAPTER_SHUTDOWN``
:Example: ``"ADAPTER_SHUTDOWN\n"``
:Details:
  | This tells i2c-pseudo that the pseudo controller wants to shutdown and
    intends to close the controller device fd soon.  Use of this is OPTIONAL, it
    is perfectly valid to close the controller device fd without ever using this
    command.
  |
  | This commands unblocks any blocked controller I/O (reads, writes, or polls),
    and that is its main purpose.
  |
  | Any I2C transactions attempted by a device driver after this command will
    fail, and will not be passed on to the userspace controller.
  |
  | This DOES NOT delete the I2C adapter.  Only closing the fd will do that.
    That MAY CHANGE in the future, such that this does delete the I2C adapter.
    (However this will never be required, it will always be okay to simply close
    the fd.)


Example userspace controller code
=================================

In C, a simple exchange between i2c-pseudo and userspace might look like the
example below.  Note that for brevity this lacks any error checking and
handling, which a real pseudo controller implementation should have.

::

  int fd;
  char buf[1<<12];

  fd = open("/dev/i2c-pseudo-controller", O_RDWR);
  /* Create the I2C adapter. */
  dprintf(fd, "ADAPTER_START\n");

  /*
   * Pretend this I2C adapter number is 5, and the first I2C xfer sent to it was
   * from this command (using its i2c-dev interface):
   * $ i2cset -y 5 0x70 0xC2
   *
   * Then this read would place the following into *buf:
   * "I2C_BEGIN_XFER\n"
   * "I2C_XFER_REQ 0 0 0x0070 0x0000 1 C2\n"
   * "I2C_COMMIT_XFER\n"
   */
  read(fd, buf, sizeof(buf));

  /* This reply would allow the i2cset command above to exit successfully. */
  dprintf(fd, "I2C_XFER_REPLY 0 0 0x0070 0x0000 0\n");

  /*
   * Now pretend the next I2C xfer sent to this adapter was from:
   * $ i2cget -y 5 0x70 0xAB
   *
   * Then this read would place the following into *buf:
   * "I2C_BEGIN_XFER\n"
   * "I2C_XFER_REQ 1 0 0x0070 0x0000 1 AB\n"
   * "I2C_XFER_REQ 1 1 0x0070 0x0001 1\n'"
   * "I2C_COMMIT_XFER\n"
   */
  read(fd, buf, sizeof(buf));

  /*
   * These replies would allow the i2cget command above to print the following to
   * stdout and exit successfully:
   * 0x0b
   *
   * Note that it is also valid to write these together in one write().
   */
  dprintf(fd, "I2C_XFER_REPLY 1 0 0x0070 0x0000 0\n");
  dprintf(fd, "I2C_XFER_REPLY 1 1 0x0070 0x0001 0 0B\n");

  /* Destroy the I2C adapter. */
  close(fd);