333 lines
8.9 KiB
ReStructuredText
333 lines
8.9 KiB
ReStructuredText
=================
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The UCAN Protocol
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=================
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UCAN is the protocol used by the microcontroller-based USB-CAN
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adapter that is integrated on System-on-Modules from Theobroma Systems
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and that is also available as a standalone USB stick.
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The UCAN protocol has been designed to be hardware-independent.
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It is modeled closely after how Linux represents CAN devices
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internally. All multi-byte integers are encoded as Little Endian.
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All structures mentioned in this document are defined in
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``drivers/net/can/usb/ucan.c``.
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USB Endpoints
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=============
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UCAN devices use three USB endpoints:
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CONTROL endpoint
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The driver sends device management commands on this endpoint
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IN endpoint
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The device sends CAN data frames and CAN error frames
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OUT endpoint
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The driver sends CAN data frames on the out endpoint
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CONTROL Messages
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================
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UCAN devices are configured using vendor requests on the control pipe.
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To support multiple CAN interfaces in a single USB device all
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configuration commands target the corresponding interface in the USB
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descriptor.
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The driver uses ``ucan_ctrl_command_in/out`` and
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``ucan_device_request_in`` to deliver commands to the device.
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Setup Packet
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------------
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================= =====================================================
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``bmRequestType`` Direction | Vendor | (Interface or Device)
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``bRequest`` Command Number
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``wValue`` Subcommand Number (16 Bit) or 0 if not used
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``wIndex`` USB Interface Index (0 for device commands)
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``wLength`` * Host to Device - Number of bytes to transmit
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* Device to Host - Maximum Number of bytes to
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receive. If the device send less. Commom ZLP
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semantics are used.
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================= =====================================================
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Error Handling
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--------------
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The device indicates failed control commands by stalling the
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pipe.
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Device Commands
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---------------
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UCAN_DEVICE_GET_FW_STRING
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~~~~~~~~~~~~~~~~~~~~~~~~~
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*Dev2Host; optional*
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Request the device firmware string.
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Interface Commands
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------------------
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UCAN_COMMAND_START
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~~~~~~~~~~~~~~~~~~
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*Host2Dev; mandatory*
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Bring the CAN interface up.
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Payload Format
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``ucan_ctl_payload_t.cmd_start``
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==== ============================
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mode or mask of ``UCAN_MODE_*``
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==== ============================
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UCAN_COMMAND_STOP
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~~~~~~~~~~~~~~~~~~
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*Host2Dev; mandatory*
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Stop the CAN interface
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Payload Format
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*empty*
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UCAN_COMMAND_RESET
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~~~~~~~~~~~~~~~~~~
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*Host2Dev; mandatory*
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Reset the CAN controller (including error counters)
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Payload Format
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*empty*
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UCAN_COMMAND_GET
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~~~~~~~~~~~~~~~~
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*Host2Dev; mandatory*
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Get Information from the Device
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Subcommands
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^^^^^^^^^^^
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UCAN_COMMAND_GET_INFO
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Request the device information structure ``ucan_ctl_payload_t.device_info``.
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See the ``device_info`` field for details, and
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``uapi/linux/can/netlink.h`` for an explanation of the
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``can_bittiming fields``.
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Payload Format
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``ucan_ctl_payload_t.device_info``
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UCAN_COMMAND_GET_PROTOCOL_VERSION
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Request the device protocol version
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``ucan_ctl_payload_t.protocol_version``. The current protocol version is 3.
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Payload Format
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``ucan_ctl_payload_t.protocol_version``
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.. note:: Devices that do not implement this command use the old
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protocol version 1
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UCAN_COMMAND_SET_BITTIMING
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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*Host2Dev; mandatory*
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Setup bittiming by sending the the structure
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``ucan_ctl_payload_t.cmd_set_bittiming`` (see ``struct bittiming`` for
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details)
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Payload Format
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``ucan_ctl_payload_t.cmd_set_bittiming``.
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UCAN_SLEEP/WAKE
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~~~~~~~~~~~~~~~
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*Host2Dev; optional*
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Configure sleep and wake modes. Not yet supported by the driver.
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UCAN_FILTER
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~~~~~~~~~~~
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*Host2Dev; optional*
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Setup hardware CAN filters. Not yet supported by the driver.
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Allowed interface commands
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--------------------------
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================== =================== ==================
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Legal Device State Command New Device State
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================== =================== ==================
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stopped SET_BITTIMING stopped
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stopped START started
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started STOP or RESET stopped
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stopped STOP or RESET stopped
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started RESTART started
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any GET *no change*
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================== =================== ==================
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IN Message Format
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=================
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A data packet on the USB IN endpoint contains one or more
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``ucan_message_in`` values. If multiple messages are batched in a USB
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data packet, the ``len`` field can be used to jump to the next
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``ucan_message_in`` value (take care to sanity-check the ``len`` value
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against the actual data size).
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.. _can_ucan_in_message_len:
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``len`` field
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-------------
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Each ``ucan_message_in`` must be aligned to a 4-byte boundary (relative
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to the start of the start of the data buffer). That means that there
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may be padding bytes between multiple ``ucan_message_in`` values:
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.. code::
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+----------------------------+ < 0
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| struct ucan_message_in |
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+----------------------------+ < len
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[padding]
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+----------------------------+ < round_up(len, 4)
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| struct ucan_message_in |
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+----------------------------+
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[...]
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``type`` field
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--------------
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The ``type`` field specifies the type of the message.
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UCAN_IN_RX
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~~~~~~~~~~
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``subtype``
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zero
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Data received from the CAN bus (ID + payload).
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UCAN_IN_TX_COMPLETE
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~~~~~~~~~~~~~~~~~~~
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``subtype``
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zero
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The CAN device has sent a message to the CAN bus. It answers with a
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list of of tuples <echo-ids, flags>.
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The echo-id identifies the frame from (echos the id from a previous
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UCAN_OUT_TX message). The flag indicates the result of the
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transmission. Whereas a set Bit 0 indicates success. All other bits
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are reserved and set to zero.
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Flow Control
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------------
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When receiving CAN messages there is no flow control on the USB
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buffer. The driver has to handle inbound message quickly enough to
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avoid drops. I case the device buffer overflow the condition is
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reported by sending corresponding error frames (see
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:ref:`can_ucan_error_handling`)
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OUT Message Format
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==================
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A data packet on the USB OUT endpoint contains one or more ``struct
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ucan_message_out`` values. If multiple messages are batched into one
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data packet, the device uses the ``len`` field to jump to the next
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ucan_message_out value. Each ucan_message_out must be aligned to 4
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bytes (relative to the start of the data buffer). The mechanism is
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same as described in :ref:`can_ucan_in_message_len`.
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.. code::
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+----------------------------+ < 0
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| struct ucan_message_out |
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+----------------------------+ < len
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[padding]
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+----------------------------+ < round_up(len, 4)
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| struct ucan_message_out |
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+----------------------------+
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[...]
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``type`` field
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--------------
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In protocol version 3 only ``UCAN_OUT_TX`` is defined, others are used
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only by legacy devices (protocol version 1).
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UCAN_OUT_TX
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~~~~~~~~~~~
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``subtype``
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echo id to be replied within a CAN_IN_TX_COMPLETE message
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Transmit a CAN frame. (parameters: ``id``, ``data``)
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Flow Control
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------------
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When the device outbound buffers are full it starts sending *NAKs* on
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the *OUT* pipe until more buffers are available. The driver stops the
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queue when a certain threshold of out packets are incomplete.
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.. _can_ucan_error_handling:
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CAN Error Handling
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==================
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If error reporting is turned on the device encodes errors into CAN
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error frames (see ``uapi/linux/can/error.h``) and sends it using the
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IN endpoint. The driver updates its error statistics and forwards
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it.
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Although UCAN devices can suppress error frames completely, in Linux
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the driver is always interested. Hence, the device is always started with
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the ``UCAN_MODE_BERR_REPORT`` set. Filtering those messages for the
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user space is done by the driver.
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Bus OFF
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-------
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- The device does not recover from bus of automatically.
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- Bus OFF is indicated by an error frame (see ``uapi/linux/can/error.h``)
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- Bus OFF recovery is started by ``UCAN_COMMAND_RESTART``
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- Once Bus OFF recover is completed the device sends an error frame
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indicating that it is on ERROR-ACTIVE state.
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- During Bus OFF no frames are sent by the device.
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- During Bus OFF transmission requests from the host are completed
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immediately with the success bit left unset.
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Example Conversation
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====================
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#) Device is connected to USB
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#) Host sends command ``UCAN_COMMAND_RESET``, subcmd 0
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#) Host sends command ``UCAN_COMMAND_GET``, subcmd ``UCAN_COMMAND_GET_INFO``
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#) Device sends ``UCAN_IN_DEVICE_INFO``
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#) Host sends command ``UCAN_OUT_SET_BITTIMING``
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#) Host sends command ``UCAN_COMMAND_START``, subcmd 0, mode ``UCAN_MODE_BERR_REPORT``
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