can: ucan: add driver for Theobroma Systems UCAN devices

=================

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 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 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``

   af_xdp

   batman-adv

   can

   can_ucan_protocol

   dpaa2/index

   e100

   e1000

	  This driver supports the CAN BUS Analyzer interface

	  from Microchip (http://www.microchip.com/development-tools/).

config CAN_UCAN

	tristate "Theobroma Systems UCAN interface"

	---help---

	  This driver supports the Theobroma Systems

	  UCAN USB-CAN interface.

	  The UCAN driver supports the microcontroller-based USB/CAN

	  adapters from Theobroma Systems. There are two form-factors

	  that run essentially the same firmware:

	  * Seal: standalone USB stick

	          https://www.theobroma-systems.com/seal)

	  * Mule: integrated on the PCB of various System-on-Modules

	          from Theobroma Systems like the A31-µQ7 and the RK3399-Q7

	          (https://www.theobroma-systems.com/rk3399-q7)

endmenu

obj-$(CONFIG_CAN_KVASER_USB) += kvaser_usb.o

obj-$(CONFIG_CAN_MCBA_USB) += mcba_usb.o

obj-$(CONFIG_CAN_PEAK_USB) += peak_usb/

obj-$(CONFIG_CAN_UCAN) += ucan.o