[PATCH v9 0/6] Add RS485 9th bit addressing mode support to DW UART

[PATCH v9 0/6] Add RS485 9th bit addressing mode support to DW UART

[Ilpo Järvinen]

This patchset adds RS-485 9th bit addressing mode support to the DW
UART driver and the necessary serial core bits to handle it. The
addressing mode is configured through ->rs485_config() as was requested
during the review of the earlier versions. The line configuration
related ADDRB is still kept in ktermios->c_cflag to be able to take
account the extra addressing bit while calculating timing, etc. but it
is set/cleared by ->rs485_config().

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-api@vger.kernel.org
Cc: linux-arch@vger.kernel.org

v8 -> v9:
- Tweak comments (struct fields info & one callback style correction)
- Add struct layout checks as static_asserts

v7 -> v8:
- Use anonymous union/struct in serial_rs485 to create "v2" of it
- Remove a stray newline change
- Reorder local var declarations
- Put ktermios param before serial_rs485 for rs485_config

v6 -> v7:
- Fixed typos in documentation & comment
- Changes lsr typing from unsigned int to u16

v5 -> v6:
- Reorder remaining patches
- LSR changes are simpler due to helper added by LSR fix series
- Depend on rs485_struct sanitization on catching much of invalid config
- In order to be able to alter ADDRB in termios .c_cflag within
  .rs485_config(), take termios_rwsem and pass ktermios to it.
- Moved addressing mode setup entirely into .rs485_config()
- Use ndelay() instead of udelay() (uart_port->frame_time is in nsecs)


Ilpo Järvinen (6):
  serial: 8250: make saved LSR larger
  serial: 8250: create lsr_save_mask
  serial: 8250_lpss: Use 32-bit reads
  serial: take termios_rwsem for ->rs485_config() & pass termios as
    param
  serial: Support for RS-485 multipoint addresses
  serial: 8250_dwlib: Support for 9th bit multipoint addressing

 Documentation/driver-api/serial/driver.rst    |   2 +
 .../driver-api/serial/serial-rs485.rst        |  26 ++++-
 drivers/tty/serial/8250/8250.h                |   9 +-
 drivers/tty/serial/8250/8250_core.c           |   4 +
 drivers/tty/serial/8250/8250_dw.c             |   2 +-
 drivers/tty/serial/8250/8250_dwlib.c          | 105 +++++++++++++++++-
 drivers/tty/serial/8250/8250_exar.c           |  11 +-
 drivers/tty/serial/8250/8250_fintek.c         |   2 +-
 drivers/tty/serial/8250/8250_fsl.c            |   2 +-
 drivers/tty/serial/8250/8250_ingenic.c        |   2 +-
 drivers/tty/serial/8250/8250_lpc18xx.c        |   2 +-
 drivers/tty/serial/8250/8250_lpss.c           |   2 +-
 drivers/tty/serial/8250/8250_omap.c           |   7 +-
 drivers/tty/serial/8250/8250_pci.c            |   2 +-
 drivers/tty/serial/8250/8250_port.c           |  20 ++--
 drivers/tty/serial/amba-pl011.c               |   2 +-
 drivers/tty/serial/ar933x_uart.c              |   2 +-
 drivers/tty/serial/atmel_serial.c             |   2 +-
 drivers/tty/serial/fsl_lpuart.c               |   4 +-
 drivers/tty/serial/imx.c                      |   2 +-
 drivers/tty/serial/max310x.c                  |   2 +-
 drivers/tty/serial/mcf.c                      |   3 +-
 drivers/tty/serial/omap-serial.c              |   3 +-
 drivers/tty/serial/sc16is7xx.c                |   2 +-
 drivers/tty/serial/serial_core.c              |  36 +++++-
 drivers/tty/serial/stm32-usart.c              |   2 +-
 drivers/tty/tty_ioctl.c                       |   4 +
 include/linux/serial_8250.h                   |   7 +-
 include/linux/serial_core.h                   |   1 +
 include/uapi/asm-generic/termbits-common.h    |   1 +
 include/uapi/linux/serial.h                   |  20 +++-
 31 files changed, 238 insertions(+), 53 deletions(-)

-- 
2.30.2

[PATCH v9 5/6] serial: Support for RS-485 multipoint addresses

[Ilpo Järvinen]

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>

---
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-api@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arch@vger.kernel.org
---
 Documentation/driver-api/serial/driver.rst    |  2 ++
 .../driver-api/serial/serial-rs485.rst        | 26 ++++++++++++++++++-
 drivers/tty/serial/serial_core.c              | 22 +++++++++++++++-
 drivers/tty/tty_ioctl.c                       |  4 +++
 include/uapi/asm-generic/termbits-common.h    |  1 +
 include/uapi/linux/serial.h                   | 20 ++++++++++++--
 6 files changed, 71 insertions(+), 4 deletions(-)

diff --git a/Documentation/driver-api/serial/driver.rst b/Documentation/driver-api/serial/driver.rst
index 7ef83fd3917b..35fb3866bb3d 100644
--- 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 00b5d333acba..6ebad75c74ed 100644
--- 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 44c3785445e3..0aecaf753dea 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1294,6 +1294,17 @@ 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;
 }
 
@@ -1349,7 +1360,8 @@ 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)
@@ -3404,5 +3416,13 @@ 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 adae687f654b..2a76b330e108 100644
--- 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 4d084fe8def5..4a6a79f28b21 100644
--- 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 fa6b16e5fdd8..cea06924b295 100644
--- 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];
+		};
+	};
 };
 
 /*
-- 
2.30.2

Re: [PATCH v9 5/6] serial: Support for RS-485 multipoint addresses

[Lukas Wunner]

On Fri, Jun 24, 2022 at 11:42:09PM +0300, Ilpo Järvinen wrote:
> 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.
[...]
> --- 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 */
>

You may want to consider submitting a patch to the Linux man-pages
project to document the newly introduced bit:

https://www.kernel.org/doc/man-pages/patches.html
https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man3/termios.3

Thanks,

Lukas