The UCAN Protocol

UCAN 개요, endpoint와 control setup

UCAN Protocol

UCAN은 Theobroma Systems의 System-on-Module에 통합되어 있고 독립형 USB stick으로도 제공되는 microcontroller 기반 USB-CAN adapter가 사용하는 protocol입니다.

UCAN protocol은 hardware와 독립적으로 설계되었습니다. Linux가 내부적으로 CAN device를 표현하는 방식과 밀접하게 맞춰져 있으며, 모든 multi-byte integer는 Little Endian으로 encode합니다.

이 문서에서 언급하는 모든 structure는 `drivers/net/can/usb/ucan.c`에 정의되어 있습니다.

USB Endpoint

UCAN device는 USB endpoint 세 개를 사용합니다.

• CONTROL endpoint: driver가 device management command를 보냅니다.
• IN endpoint: device가 CAN data frame과 CAN error frame을 보냅니다.
• OUT endpoint: driver가 CAN data frame을 보냅니다.

CONTROL message

UCAN device는 control pipe의 vendor request로 구성합니다. 하나의 USB device에서 여러 CAN interface를 지원할 수 있도록 모든 configuration command는 USB descriptor에 있는 해당 interface를 대상으로 합니다.

Driver는 `ucan_ctrl_command_in/out`과 `ucan_device_request_in`을 사용해 command를 device에 전달합니다.

Setup Packet

오류 처리

Device는 실패한 control command를 pipe stall로 표시합니다.

=================
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. Common ZLP
                     semantics are used.
=================  =====================================================

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

The device indicates failed control commands by stalling the
pipe.

Device·interface command와 정보 조회

Device command

`UCAN_DEVICE_GET_FW_STRING`

*Dev2Host; optional*

Device firmware string을 요청합니다.

Interface command

`UCAN_COMMAND_START`

*Host2Dev; mandatory*

CAN interface를 활성화합니다.

Payload format은 `ucan_ctl_payload_t.cmd_start`이며 `mode`에는 `UCAN_MODE_*` 값의 OR mask를 넣습니다.

`UCAN_COMMAND_STOP`

*Host2Dev; mandatory*

CAN interface를 중지합니다. Payload는 비어 있습니다.

`UCAN_COMMAND_RESET`

*Host2Dev; mandatory*

Error counter를 포함해 CAN controller를 reset합니다. Payload는 비어 있습니다.

`UCAN_COMMAND_GET`

*Host2Dev; mandatory*

Device에서 정보를 가져옵니다.

Subcommand

`UCAN_COMMAND_GET_INFO`

Device information structure인 `ucan_ctl_payload_t.device_info`를 요청합니다. 자세한 내용은 `device_info` field를, `can_bittiming` field 설명은 `uapi/linux/can/netlink.h`를 참조하십시오. Payload format은 `ucan_ctl_payload_t.device_info`입니다.

`UCAN_COMMAND_GET_PROTOCOL_VERSION`

Device protocol version을 `ucan_ctl_payload_t.protocol_version` 형식으로 요청합니다. 현재 protocol version은 3입니다.

참고: 이 command를 구현하지 않은 device는 이전 protocol version 1을 사용합니다.

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

Bit timing, optional command와 상태 전이

`UCAN_COMMAND_SET_BITTIMING`

*Host2Dev; mandatory*

`ucan_ctl_payload_t.cmd_set_bittiming` structure를 보내 bit timing을 설정합니다. 자세한 내용은 `struct bittiming`을 참조하십시오. Payload format은 `ucan_ctl_payload_t.cmd_set_bittiming`입니다.

`UCAN_SLEEP/WAKE`

*Host2Dev; optional*

Sleep과 wake mode를 구성합니다. 아직 driver가 지원하지 않습니다.

`UCAN_FILTER`

*Host2Dev; optional*

Hardware CAN filter를 설정합니다. 아직 driver가 지원하지 않습니다.

허용되는 interface command

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*
==================  ===================  ==================

USB IN message와 수신 flow control

IN Message Format

USB IN endpoint의 data packet은 하나 이상의 `ucan_message_in` 값을 담습니다. USB data packet 하나에 여러 message를 batch했다면 `len` field로 다음 `ucan_message_in` 값으로 이동할 수 있습니다. 이때 실제 data size와 비교하여 `len` 값을 반드시 sanity-check해야 합니다.

`len` field

각 `ucan_message_in`은 data buffer 시작점을 기준으로 4-byte boundary에 정렬되어야 합니다. 따라서 여러 `ucan_message_in` 값 사이에 padding byte가 있을 수 있습니다.

`type` field

`type` field는 message 종류를 지정합니다.

`UCAN_IN_RX`

`subtype`은 0입니다. CAN bus에서 수신한 data(ID + payload)입니다.

`UCAN_IN_TX_COMPLETE`

`subtype`은 0입니다. CAN device가 CAN bus에 message를 보냈다는 응답이며 `<echo-id, flags>` tuple 목록을 전달합니다.

Echo-id는 이전 `UCAN_OUT_TX` message에서 가져온 ID를 echo하여 frame을 식별합니다. Flag는 전송 결과를 나타냅니다. Bit 0이 설정되면 성공이며 나머지 bit는 reserved이므로 0으로 설정합니다.

Flow Control

CAN message를 수신할 때 USB buffer에는 flow control이 없습니다. Driver는 drop을 피할 만큼 빠르게 inbound message를 처리해야 합니다. Device buffer가 overflow하면 해당 상태를 대응하는 error frame으로 보고합니다. :ref:`can_ucan_error_handling`을 참조하십시오.

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`)

USB OUT message와 송신 flow control

OUT Message Format

USB OUT endpoint의 data packet은 하나 이상의 `struct ucan_message_out` 값을 담습니다. Data packet 하나에 여러 message를 batch하면 device는 `len` field를 사용해 다음 `ucan_message_out` 값으로 이동합니다. 각 `ucan_message_out`은 data buffer 시작점을 기준으로 4 byte에 정렬해야 합니다. 동작은 :ref:`can_ucan_in_message_len`에서 설명한 것과 같습니다.

`type` field

Protocol version 3에서는 `UCAN_OUT_TX`만 정의됩니다. 다른 type은 legacy device(protocol version 1)에서만 사용합니다.

`UCAN_OUT_TX`

`subtype`은 `CAN_IN_TX_COMPLETE` message로 응답받을 echo id입니다. `id`와 `data` parameter를 사용해 CAN frame을 전송합니다.

Flow Control

Device의 outbound buffer가 가득 차면 buffer 여유가 생길 때까지 OUT pipe에서 *NAK*를 보내기 시작합니다. 완료되지 않은 OUT packet 수가 일정 threshold에 도달하면 driver는 queue를 중지합니다.

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 error, Bus OFF와 예시 대화

CAN Error Handling

Error reporting을 켜면 device는 error를 CAN error frame으로 encode하고(`uapi/linux/can/error.h` 참조) IN endpoint로 보냅니다. Driver는 error statistic을 갱신한 뒤 frame을 forwarding합니다.

UCAN device는 error frame을 완전히 억제할 수 있지만 Linux driver는 항상 이 정보에 관심이 있습니다. 따라서 device를 시작할 때 항상 `UCAN_MODE_BERR_REPORT`를 설정합니다. User space로 전달할 message를 filtering하는 작업은 driver가 수행합니다.

Bus OFF

• Device는 Bus OFF 상태에서 자동으로 복구하지 않습니다.
• Bus OFF는 error frame으로 표시합니다. `uapi/linux/can/error.h`를 참조하십시오.
• `UCAN_COMMAND_RESTART`로 Bus OFF 복구를 시작합니다.
• Bus OFF 복구가 끝나면 device는 ERROR-ACTIVE 상태임을 나타내는 error frame을 보냅니다.
• Bus OFF 동안 device는 frame을 전송하지 않습니다.
• Bus OFF 동안 host가 보낸 transmission request는 success bit를 설정하지 않은 채 즉시 완료됩니다.

대화 예시

• 1. Device가 USB에 연결됩니다.
• 2. Host가 subcommand 0으로 `UCAN_COMMAND_RESET`을 보냅니다.
• 3. Host가 subcommand `UCAN_COMMAND_GET_INFO`로 `UCAN_COMMAND_GET`을 보냅니다.
• 4. Device가 `UCAN_IN_DEVICE_INFO`를 보냅니다.
• 5. Host가 `UCAN_OUT_SET_BITTIMING`을 보냅니다.
• 6. Host가 subcommand 0, mode `UCAN_MODE_BERR_REPORT`로 `UCAN_COMMAND_START`를 보냅니다.

.. _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``