SocketLIN (sllin)

sllin is a TTY line discipline that allows a Linux system to act as a LIN master and, to some extent, as a LIN slave.

Communication to userspace is exposed as a CAN netdevice (sllin0, sllin1, ...), so you can use standard tools like ip, candump, and cangen.

Features:

• Slave Mode Answer to RTR Frames (via Cache response)
• Slave Mode Listen only
• Master Mode Send RTR Frames (Break,Sync + ID) and wait for slave answer (Data + CRC)
• Master Mode Send Complete LIN Frames

Requirements

• Linux with matching kernel headers/sources for the currently running kernel
• UART interface (for example /dev/ttyS0, /dev/ttyUSB0)
• LIN level shifter/transceiver (UART ↔ LIN)
• can-utils (for example candump, cangen)
• Root privileges for insmod, ip link, ldattach

Optional:

• slcan_attach or project-specific attach tools/scripts

Build

Inside the sllin directory:

make

This builds the kernel module sllin.ko.

Load the module

sudo insmod ./sllin.ko

With parameters (example):

sudo insmod ./sllin.ko master=1 baudrate=19200 maxdev=10

Important parameters:

• master: 1 = master, 0 = slave
• baudrate: LIN baud rate (driver default: LIN_DEFAULT_BAUDRATE, usually 19200)
• maxdev: maximum number of dynamic interfaces (currently defaults to 10 in this driver)
• break_by_baud: generate break via temporary baud-rate switch (0/1)

Check status:

dmesg | tail -n 20

Attach a TTY to sllin

Option A: ldattach

sudo ldattach <ldisc_number> /dev/ttyS0

Note: historically, 28 (master) and 29 (slave) were often used. On modern kernels, these numbers are already assigned (N_MCTP, N_DEVELOPMENT). For out-of-tree drivers, the line discipline number registered by the driver must match the running kernel setup.

The kernel reports assignemnt of the line discipline numbers by proc entry /proc/tty/ldiscs. The number assigned to the sllin master can be obtained by

sed -n -e 's/^[n_]*sllin[ \t]*\([0-9]*\)$/\1/p' /proc/tty/ldiscs

for the slave

sed -n -e 's/^[n_]*sllin-slave[ \t]*\([0-9]*\)$/\1/p' /proc/tty/ldiscs

The line discipline numbers can be adjusted in the sllin.c source file

#define N_SLLIN                 28
#define N_SLLIN_SLAVE           29

Option B: attach helper (slcan_attach-style)

If a compatible helper tool is available (need to be patched first with patch under slin/canutils-patches) :

sudo slcan_attach -w /dev/ttyS0

Bring the interface up

ip link show dev sllin0
sudo ip link set sllin0 up
ip link show dev sllin0

state UNKNOWN is expected for this interface type.

Cache runtime behavior

• LIN ID is always in the lower 6 bits (can_id & 0x3f)
• EFF (CAN_EFF_FLAG) marks a control/configuration frame for sllin frame cache
• Important control bits (above LIN ID):
  • LIN_CACHE_RESPONSE (1 << 6 = 0x40): enable cached response for this LIN ID
  • LIN_CHECKSUM_EXTENDED (1 << 7 = 0x80): use enhanced checksum
  • LIN_SINGLE_RESPONSE (1 << 8 = 0x100): one-shot cached response

One-shot cache response (LIN ID 0x10):

# Configure one-shot cache response for LIN ID 0x10
# can_id = 0x10 (ID) + 0x40 (CACHE_RESPONSE) + 0x100 (SINGLE_RESPONSE) = 0x150
cangen sllin0 -e -I 0x150 -n 1 -L 2 -D a1b2

# Trigger with RTR
cangen sllin0 -R -I 0x10 -n 1 -L 0

cache response (LIN ID 0x10):

# Configure cache response for LIN ID 0x10
# can_id = 0x10 (ID) + 0x40 (CACHE_RESPONSE) = 0x50
cangen sllin0 -e -I 0x50 -n 1 -L 2 -D a1b2

# Trigger with RTR
cangen sllin0 -R -I 0x10 -n 3 -L 0

# clear cache response
# can_id = 0x10 (ID) + 0x40 (CACHE_RESPONSE) = 0x50
# BUT DLC = 0 -> Disable cache response
cangen sllin0 -e -I 0x50 -n 1 -L 0

Practical examples

Request LIN header/response (RTR)

cangen sllin0 -R -I 1 -n 1 -L 0

Monitor in parallel:

candump sllin0

Demonstrate timeout behavior

cangen sllin0 -R -I 8 -n 1 -L 0
ip -s link show dev sllin0

If no slave responds, RX error counters increase.

Configure frame cache

cangen sllin0 -e -I 0x848 -n 1 -L 2 -D beef
cangen sllin0 -R -I 8 -n 1 -L 0

Send non-RTR frame

cangen sllin0 -I 7 -n 1 -L 2 -D f00f