Documentation / networking / can_ucan_protocol.rst


Based on kernel version 5.12. Page generated on 2021-04-27 09:53 EST.

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=================
The UCAN Protocol
=================

UCAN is the protocol used by the microcontroller-based USB-CAN
adapter that is integrated on System-on-Modules from Theobroma Systems
and that is also available as a standalone USB stick.

The UCAN protocol has been designed to be hardware-independent.
It is modeled closely after how Linux represents CAN devices
internally. All multi-byte integers are encoded as Little Endian.

All structures mentioned in this document are defined in
``drivers/net/can/usb/ucan.c``.

USB Endpoints
=============

UCAN devices use three USB endpoints:

CONTROL endpoint
  The driver sends device management commands on this endpoint

IN endpoint
  The device sends CAN data frames and CAN error frames

OUT endpoint
  The driver sends CAN data frames on the out endpoint


CONTROL Messages
================

UCAN devices are configured using vendor requests on the control pipe.

To support multiple CAN interfaces in a single USB device all
configuration commands target the corresponding interface in the USB
descriptor.

The driver uses ``ucan_ctrl_command_in/out`` and
``ucan_device_request_in`` to deliver commands to the device.

Setup Packet
------------

=================  =====================================================
``bmRequestType``  Direction | Vendor | (Interface or Device)
``bRequest``       Command Number
``wValue``         Subcommand Number (16 Bit) or 0 if not used
``wIndex``         USB Interface Index (0 for device commands)
``wLength``        * Host to Device - Number of bytes to transmit
                   * Device to Host - Maximum Number of bytes to
                     receive. If the device send less. Commom ZLP
                     semantics are used.
=================  =====================================================

Error Handling
--------------

The device indicates failed control commands by stalling the
pipe.

Device Commands
---------------

UCAN_DEVICE_GET_FW_STRING
~~~~~~~~~~~~~~~~~~~~~~~~~

*Dev2Host; optional*

Request the device firmware string.


Interface Commands
------------------

UCAN_COMMAND_START
~~~~~~~~~~~~~~~~~~

*Host2Dev; mandatory*

Bring the CAN interface up.

Payload Format
  ``ucan_ctl_payload_t.cmd_start``

====  ============================
mode  or mask of ``UCAN_MODE_*``
====  ============================

UCAN_COMMAND_STOP
~~~~~~~~~~~~~~~~~~

*Host2Dev; mandatory*

Stop the CAN interface

Payload Format
  *empty*

UCAN_COMMAND_RESET
~~~~~~~~~~~~~~~~~~

*Host2Dev; mandatory*

Reset the CAN controller (including error counters)

Payload Format
  *empty*

UCAN_COMMAND_GET
~~~~~~~~~~~~~~~~

*Host2Dev; mandatory*

Get Information from the Device

Subcommands
^^^^^^^^^^^

UCAN_COMMAND_GET_INFO
  Request the device information structure ``ucan_ctl_payload_t.device_info``.

  See the ``device_info`` field for details, and
  ``uapi/linux/can/netlink.h`` for an explanation of the
  ``can_bittiming fields``.

  Payload Format
    ``ucan_ctl_payload_t.device_info``

UCAN_COMMAND_GET_PROTOCOL_VERSION

  Request the device protocol version
  ``ucan_ctl_payload_t.protocol_version``. The current protocol version is 3.

  Payload Format
    ``ucan_ctl_payload_t.protocol_version``

.. note:: Devices that do not implement this command use the old
          protocol version 1

UCAN_COMMAND_SET_BITTIMING
~~~~~~~~~~~~~~~~~~~~~~~~~~

*Host2Dev; mandatory*

Setup bittiming by sending the structure
``ucan_ctl_payload_t.cmd_set_bittiming`` (see ``struct bittiming`` for
details)

Payload Format
  ``ucan_ctl_payload_t.cmd_set_bittiming``.

UCAN_SLEEP/WAKE
~~~~~~~~~~~~~~~

*Host2Dev; optional*

Configure sleep and wake modes. Not yet supported by the driver.

UCAN_FILTER
~~~~~~~~~~~

*Host2Dev; optional*

Setup hardware CAN filters. Not yet supported by the driver.

Allowed interface commands
--------------------------

==================  ===================  ==================
Legal Device State  Command              New Device State
==================  ===================  ==================
stopped             SET_BITTIMING        stopped
stopped             START                started
started             STOP or RESET        stopped
stopped             STOP or RESET        stopped
started             RESTART              started
any                 GET                  *no change*
==================  ===================  ==================

IN Message Format
=================

A data packet on the USB IN endpoint contains one or more
``ucan_message_in`` values. If multiple messages are batched in a USB
data packet, the ``len`` field can be used to jump to the next
``ucan_message_in`` value (take care to sanity-check the ``len`` value
against the actual data size).

.. _can_ucan_in_message_len:

``len`` field
-------------

Each ``ucan_message_in`` must be aligned to a 4-byte boundary (relative
to the start of the start of the data buffer). That means that there
may be padding bytes between multiple ``ucan_message_in`` values:

.. code::

    +----------------------------+ < 0
    |                            |
    |   struct ucan_message_in   |
    |                            |
    +----------------------------+ < len
              [padding]
    +----------------------------+ < round_up(len, 4)
    |                            |
    |   struct ucan_message_in   |
    |                            |
    +----------------------------+
                [...]

``type`` field
--------------

The ``type`` field specifies the type of the message.

UCAN_IN_RX
~~~~~~~~~~

``subtype``
  zero

Data received from the CAN bus (ID + payload).

UCAN_IN_TX_COMPLETE
~~~~~~~~~~~~~~~~~~~

``subtype``
  zero

The CAN device has sent a message to the CAN bus. It answers with a
list of tuples <echo-ids, flags>.

The echo-id identifies the frame from (echos the id from a previous
UCAN_OUT_TX message). The flag indicates the result of the
transmission. Whereas a set Bit 0 indicates success. All other bits
are reserved and set to zero.

Flow Control
------------

When receiving CAN messages there is no flow control on the USB
buffer. The driver has to handle inbound message quickly enough to
avoid drops. I case the device buffer overflow the condition is
reported by sending corresponding error frames (see
:ref:`can_ucan_error_handling`)


OUT Message Format
==================

A data packet on the USB OUT endpoint contains one or more ``struct
ucan_message_out`` values. If multiple messages are batched into one
data packet, the device uses the ``len`` field to jump to the next
ucan_message_out value. Each ucan_message_out must be aligned to 4
bytes (relative to the start of the data buffer). The mechanism is
same as described in :ref:`can_ucan_in_message_len`.

.. code::

    +----------------------------+ < 0
    |                            |
    |   struct ucan_message_out  |
    |                            |
    +----------------------------+ < len
              [padding]
    +----------------------------+ < round_up(len, 4)
    |                            |
    |   struct ucan_message_out  |
    |                            |
    +----------------------------+
                [...]

``type`` field
--------------

In protocol version 3 only ``UCAN_OUT_TX`` is defined, others are used
only by legacy devices (protocol version 1).

UCAN_OUT_TX
~~~~~~~~~~~
``subtype``
  echo id to be replied within a CAN_IN_TX_COMPLETE message

Transmit a CAN frame. (parameters: ``id``, ``data``)

Flow Control
------------

When the device outbound buffers are full it starts sending *NAKs* on
the *OUT* pipe until more buffers are available. The driver stops the
queue when a certain threshold of out packets are incomplete.

.. _can_ucan_error_handling:

CAN Error Handling
==================

If error reporting is turned on the device encodes errors into CAN
error frames (see ``uapi/linux/can/error.h``) and sends it using the
IN endpoint. The driver updates its error statistics and forwards
it.

Although UCAN devices can suppress error frames completely, in Linux
the driver is always interested. Hence, the device is always started with
the ``UCAN_MODE_BERR_REPORT`` set. Filtering those messages for the
user space is done by the driver.

Bus OFF
-------

- The device does not recover from bus of automatically.
- Bus OFF is indicated by an error frame (see ``uapi/linux/can/error.h``)
- Bus OFF recovery is started by ``UCAN_COMMAND_RESTART``
- Once Bus OFF recover is completed the device sends an error frame
  indicating that it is on ERROR-ACTIVE state.
- During Bus OFF no frames are sent by the device.
- During Bus OFF transmission requests from the host are completed
  immediately with the success bit left unset.

Example Conversation
====================

#) Device is connected to USB
#) Host sends command ``UCAN_COMMAND_RESET``, subcmd 0
#) Host sends command ``UCAN_COMMAND_GET``, subcmd ``UCAN_COMMAND_GET_INFO``
#) Device sends ``UCAN_IN_DEVICE_INFO``
#) Host sends command ``UCAN_OUT_SET_BITTIMING``
#) Host sends command ``UCAN_COMMAND_START``, subcmd 0, mode ``UCAN_MODE_BERR_REPORT``