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Diffstat (limited to 'Documentation/driver-api/serial/serial-rs485.rst')
| -rw-r--r-- | Documentation/driver-api/serial/serial-rs485.rst | 58 |
1 files changed, 45 insertions, 13 deletions
diff --git a/Documentation/driver-api/serial/serial-rs485.rst b/Documentation/driver-api/serial/serial-rs485.rst index 6bc824f948f9..dce061ef7647 100644 --- a/Documentation/driver-api/serial/serial-rs485.rst +++ b/Documentation/driver-api/serial/serial-rs485.rst @@ -29,19 +29,28 @@ RS485 Serial Communications 3. Data Structures Already Available in the Kernel ================================================== - The Linux kernel provides the serial_rs485 structure (see [1]) to handle - RS485 communications. This data structure is used to set and configure RS485 + The Linux kernel provides the struct serial_rs485 to handle RS485 + communications. This data structure is used to set and configure RS485 parameters in the platform data and in ioctls. - The device tree can also provide RS485 boot time parameters (see [2] - for bindings). The driver is in charge of filling this data structure from - the values given by the device tree. + The device tree can also provide RS485 boot time parameters + [#DT-bindings]_. The serial core fills the struct serial_rs485 from the + values given by the device tree when the driver calls + uart_get_rs485_mode(). Any driver for devices capable of working both as RS232 and RS485 should - implement the rs485_config callback in the uart_port structure. The - serial_core calls rs485_config to do the device specific part in response - to TIOCSRS485 and TIOCGRS485 ioctls (see below). The rs485_config callback - receives a pointer to struct serial_rs485. + implement the ``rs485_config`` callback and provide ``rs485_supported`` + in the ``struct uart_port``. The serial core calls ``rs485_config`` to do + the device specific part in response to TIOCSRS485 ioctl (see below). The + ``rs485_config`` callback receives a pointer to a sanitizated struct + serial_rs485. The struct serial_rs485 userspace provides is sanitized + before calling ``rs485_config`` using ``rs485_supported`` that indicates + what RS485 features the driver supports for the ``struct uart_port``. + TIOCGRS485 ioctl can be used to read back the struct serial_rs485 + matching to the current configuration. + +.. kernel-doc:: include/uapi/linux/serial.h + :identifiers: serial_rs485 uart_get_rs485_mode 4. Usage from user-level ======================== @@ -95,9 +104,32 @@ RS485 Serial Communications /* Error handling. See errno. */ } -5. References -============= +5. Multipoint Addressing +======================== + + The Linux kernel provides addressing mode for multipoint RS-485 serial + communications line. The addressing mode is enabled with + ``SER_RS485_ADDRB`` flag in struct serial_rs485. The struct serial_rs485 + has two additional flags and fields for enabling receive and destination + addresses. + + Address mode flags: + - ``SER_RS485_ADDRB``: Enabled addressing mode (sets also ADDRB in termios). + - ``SER_RS485_ADDR_RECV``: Receive (filter) address enabled. + - ``SER_RS485_ADDR_DEST``: Set destination address. - [1] include/uapi/linux/serial.h + Address fields (enabled with corresponding ``SER_RS485_ADDR_*`` flag): + - ``addr_recv``: Receive address. + - ``addr_dest``: Destination address. + + Once a receive address is set, the communication can occur only with the + particular device and other peers are filtered out. It is left up to the + receiver side to enforce the filtering. Receive address will be cleared + if ``SER_RS485_ADDR_RECV`` is not set. + + Note: not all devices supporting RS485 support multipoint addressing. + +6. References +============= - [2] Documentation/devicetree/bindings/serial/rs485.txt +.. [#DT-bindings] Documentation/devicetree/bindings/serial/rs485.txt |