serial: Support for RS-485 multipoint addresses

Add support for RS-485 multipoint addressing using 9th bit [*]. The
addressing mode is configured through ->rs485_config().

ADDRB in termios indicates 9th bit addressing mode is enabled. In this
mode, 9th bit is used to indicate an address (byte) within the
communication line. ADDRB can only be enabled/disabled through
->rs485_config() that is also responsible for setting the destination and
receiver (filter) addresses.

Add traps to detect unwanted changes to struct serial_rs485 layout using
static_assert().

[*] Technically, RS485 is just an electronic spec and does not itself
specify the 9th bit addressing mode but 9th bit seems at least
"semi-standard" way to do addressing with RS485.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220624204210.11112-6-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/driver-api/serial/driver.rst b/Documentation/driver-api/serial/driver.rst
index 1e7ab41..ee16798 100644 (file)
--- a/Documentation/driver-api/serial/driver.rst
+++ b/Documentation/driver-api/serial/driver.rst
@@ -261,6 +261,8 @@ hardware.
                        - parity enable
                PARODD
                        - odd parity (when PARENB is in force)
+               ADDRB
+                       - address bit (changed through .rs485_config()).
                CREAD
                        - enable reception of characters (if not set,
                          still receive characters from the port, but

diff --git a/Documentation/driver-api/serial/serial-rs485.rst b/Documentation/driver-api/serial/serial-rs485.rst
index 00b5d33..6ebad75 100644 (file)
--- a/Documentation/driver-api/serial/serial-rs485.rst
+++ b/Documentation/driver-api/serial/serial-rs485.rst
@@ -99,7 +99,31 @@ 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 serial_rs485. 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.
+
+   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
 =============
 
  [1]   include/uapi/linux/serial.h

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 2529153..85ef7ef 100644 (file)
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1288,6 +1288,17 @@ static int uart_check_rs485_flags(struct uart_port *port, struct serial_rs485 *r
        if (flags & ~port->rs485_supported->flags)
                return -EINVAL;
 
+       /* Asking for address w/o addressing mode? */
+       if (!(rs485->flags & SER_RS485_ADDRB) &&
+           (rs485->flags & (SER_RS485_ADDR_RECV|SER_RS485_ADDR_DEST)))
+               return -EINVAL;
+
+       /* Address given but not enabled? */
+       if (!(rs485->flags & SER_RS485_ADDR_RECV) && rs485->addr_recv)
+               return -EINVAL;
+       if (!(rs485->flags & SER_RS485_ADDR_DEST) && rs485->addr_dest)
+               return -EINVAL;
+
        return 0;
 }
 
@@ -1343,7 +1354,8 @@ static void uart_sanitize_serial_rs485(struct uart_port *port, struct serial_rs4
        rs485->flags &= supported_flags;
 
        /* Return clean padding area to userspace */
-       memset(rs485->padding, 0, sizeof(rs485->padding));
+       memset(rs485->padding0, 0, sizeof(rs485->padding0));
+       memset(rs485->padding1, 0, sizeof(rs485->padding1));
 }
 
 int uart_rs485_config(struct uart_port *port)
@@ -3402,5 +3414,13 @@ int uart_get_rs485_mode(struct uart_port *port)
 }
 EXPORT_SYMBOL_GPL(uart_get_rs485_mode);
 
+/* Compile-time assertions for serial_rs485 layout */
+static_assert(offsetof(struct serial_rs485, padding) ==
+              (offsetof(struct serial_rs485, delay_rts_after_send) + sizeof(__u32)));
+static_assert(offsetof(struct serial_rs485, padding1) ==
+             offsetof(struct serial_rs485, padding[1]));
+static_assert((offsetof(struct serial_rs485, padding[4]) + sizeof(__u32)) ==
+             sizeof(struct serial_rs485));
+
 MODULE_DESCRIPTION("Serial driver core");
 MODULE_LICENSE("GPL");

diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
index adae687..2a76b33 100644 (file)
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c
@@ -319,6 +319,8 @@ unsigned char tty_get_frame_size(unsigned int cflag)
                bits++;
        if (cflag & PARENB)
                bits++;
+       if (cflag & ADDRB)
+               bits++;
 
        return bits;
 }
@@ -353,6 +355,8 @@ int tty_set_termios(struct tty_struct *tty, struct ktermios *new_termios)
        old_termios = tty->termios;
        tty->termios = *new_termios;
        unset_locked_termios(tty, &old_termios);
+       /* Reset any ADDRB changes, ADDRB is changed through ->rs485_config() */
+       tty->termios.c_cflag ^= (tty->termios.c_cflag ^ old_termios.c_cflag) & ADDRB;
 
        if (tty->ops->set_termios)
                tty->ops->set_termios(tty, &old_termios);

diff --git a/include/uapi/asm-generic/termbits-common.h b/include/uapi/asm-generic/termbits-common.h
index 4d084fe..4a6a79f 100644 (file)
--- a/include/uapi/asm-generic/termbits-common.h
+++ b/include/uapi/asm-generic/termbits-common.h
@@ -46,6 +46,7 @@ typedef unsigned int  speed_t;
 #define EXTA           B19200
 #define EXTB           B38400
 
+#define ADDRB          0x20000000      /* address bit */
 #define CMSPAR         0x40000000      /* mark or space (stick) parity */
 #define CRTSCTS                0x80000000      /* flow control */
 

diff --git a/include/uapi/linux/serial.h b/include/uapi/linux/serial.h
index fa6b16e..cea0692 100644 (file)
--- a/include/uapi/linux/serial.h
+++ b/include/uapi/linux/serial.h
@@ -126,10 +126,26 @@ struct serial_rs485 {
 #define SER_RS485_TERMINATE_BUS                (1 << 5)        /* Enable bus
                                                           termination
                                                           (if supported) */
+
+/* RS-485 addressing mode */
+#define SER_RS485_ADDRB                        (1 << 6)        /* Enable addressing mode */
+#define SER_RS485_ADDR_RECV            (1 << 7)        /* Receive address filter */
+#define SER_RS485_ADDR_DEST            (1 << 8)        /* Destination address */
+
        __u32   delay_rts_before_send;  /* Delay before send (milliseconds) */
        __u32   delay_rts_after_send;   /* Delay after send (milliseconds) */
-       __u32   padding[5];             /* Memory is cheap, new structs
-                                          are a royal PITA .. */
+
+       /* The fields below are defined by flags */
+       union {
+               __u32   padding[5];             /* Memory is cheap, new structs are a pain */
+
+               struct {
+                       __u8    addr_recv;
+                       __u8    addr_dest;
+                       __u8    padding0[2];
+                       __u32   padding1[4];
+               };
+       };
 };
 
 /*