CAN-FD¶

CAN-FD is supported. A bus opened in FD mode uses the kernel's CAN-FD frame format (CAN_RAW_FD_FRAMES), can carry payloads up to 64 bytes, and preserves the bit-rate-switch (BRS) and error-state (ESI) flags. Classical CAN remains the default and is unchanged when FD is off.

Enabling FD¶

The interface itself must be CAN-FD-capable. For a virtual interface:

sudo ip link add dev vcanfd0 type vcan
sudo ip link set vcanfd0 mtu 72        # 72 = sizeof(struct canfd_frame)
sudo ip link set up vcanfd0

TOML config — set fd = true on the bus (see configs/openarm_canfd.toml):

[bus.main]
kind      = "socketcan"
interface = "canfd0"
fd        = true
vendor    = "damiao"

Rust:

let bus = can_motor_control::SocketCanBus::open("canfd0", /* fd_enabled = */ true)?;
assert!(bus.capabilities().supports_fd);

Python:

import can_motor_control
bus = can_motor_control.SocketCanBus("canfd0", fd=True)

How format is chosen¶

A bus advertises [BusCapabilities]: classical() (8-byte cap) or fd() (64-byte cap). This is the single source of truth — upper layers (Robot, Arm, Gripper, MotorGroup) never branch on frame format.

Layer	Behavior
`SocketCanBus`	Opened with `fd=true` → sets `CAN_RAW_FD_FRAMES`, advertises `fd()`. Sends a frame as `canfd_frame` iff its `FD_FORMAT` flag is set, otherwise as a classical `can_frame` — an FD socket carries both.
`validate_send`	Rejects an FD frame on a classical bus (`FdFrameOnNonFdBus`).
`MotorCodec::bind_to_bus(caps)`	The codec keys decode/emit decisions off the bound capabilities.
`DamiaoCodec`	Conservative: emits classical 8-byte frames on any bus (those are valid CAN-FD frames). When bound to an FD bus it additionally accepts FD-format state frames on decode; bound classical, it reproduces the v1 byte layout exactly.

Scope¶

Enabling FD unlocks the FD transport path and FD-frame decode. It does not, by itself, widen Damiao command payloads beyond 8 bytes — Damiao's v1 command/state protocol is 8-byte. A vendor protocol that genuinely needs

8-byte payloads can now build on a working FD transport.

BRS data-phase bitrate is taken from the interface's configured dbitrate; the BRS flag is preserved on frames but not tuned by this library.

Testing without FD hardware¶

MockCanBus::new_fd(name) (Rust) advertises FD capabilities and loops FD frames back, so the FD send/receive and codec-gating paths are exercisable in CI without an FD-capable interface. The SocketCAN FD path itself is validated against a real or virtual FD interface during hardware bring-up.