[PATCH v2] serial: amba-pl011: drive the bus during break in RS485 mode

[PATCH v2] serial: amba-pl011: drive the bus during break in RS485 mode

[Nicolai Buchwitz]

In RS485 mode the transceiver is enabled only while data is sent. A
break set with TIOCSBRK therefore never reaches the bus, and protocols
that start each frame with a break, like DMX512, do not work.

Handle an active break like a transmission. Enable the transceiver
before setting the break, after delay_rts_before_send if one is
configured. When the break ends, stop transmitting unless the TX path
is still sending data.

Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
---
Noticed while implementing a DMX master on a CM5 with its PL011.
Tested on that board, driving a DMX512 fixture with RS485 mode left
enabled.

The 8250 em485 code probably has the same issue. I have no hardware to
test it, but I'm happy to send a patch if someone is willing to.
---
Changes in v2:
- Rebase onto tty-testing, which has the RS485 TXE and
  delay_rts_before_send fixes now
- Link to v1: https://patch.msgid.link/20261011-devel-pl011-rs485-break-v1-1-62ff402378c6@tipi-net.de

To: Russell King <linux@armlinux.org.uk>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Jiri Slaby <jirislaby@kernel.org>
Cc: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Cc: linux-kernel@vger.kernel.org
Cc: linux-serial@vger.kernel.org
---
 drivers/tty/serial/amba-pl011.c | 65 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 64 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 3edd83ec73de..79776a882ae3 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -327,6 +327,7 @@ struct uart_amba_port {
 	char			type[12];
 	ktime_t			rs485_tx_drain_interval; /* nano */
 	enum pl011_rs485_tx_state	rs485_tx_state;
+	bool			rs485_break;
 	struct hrtimer		trigger_start_tx;
 	struct hrtimer		trigger_stop_tx;
 	bool			console_line_ended;
@@ -1279,6 +1280,11 @@ static inline bool pl011_dma_rx_running(struct uart_amba_port *uap)
 	return uap->using_rx_dma && uap->dmarx.running;
 }
 
+static inline bool pl011_dma_tx_queued(struct uart_amba_port *uap)
+{
+	return uap->dmatx.queued;
+}
+
 #else
 /* Blank functions if the DMA engine is not available */
 static inline void pl011_dma_remove(struct uart_amba_port *uap)
@@ -1330,6 +1336,11 @@ static inline bool pl011_dma_rx_running(struct uart_amba_port *uap)
 	return false;
 }
 
+static inline bool pl011_dma_tx_queued(struct uart_amba_port *uap)
+{
+	return false;
+}
+
 #define pl011_dma_flush_buffer	NULL
 #endif
 
@@ -1357,6 +1368,10 @@ static void pl011_rs485_tx_stop(struct uart_amba_port *uap)
 {
 	struct uart_port *port = &uap->port;
 
+	/* pl011_break_ctl() stops the transmission when the break ends */
+	if (uap->rs485_break)
+		return;
+
 	if (uap->rs485_tx_state == SEND)
 		uap->rs485_tx_state = WAIT_AFTER_SEND;
 
@@ -1479,7 +1494,9 @@ static enum hrtimer_restart pl011_trigger_start_tx(struct hrtimer *t)
 	uart_port_lock_irqsave(&uap->port, &flags);
 	if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
 		uap->rs485_tx_state = SEND;
-		pl011_start_tx(&uap->port);
+		/* A break can arm the timer while TX is stopped */
+		if (!uart_tx_stopped(&uap->port))
+			pl011_start_tx(&uap->port);
 	}
 	uart_port_unlock_irqrestore(&uap->port, flags);
 
@@ -1772,16 +1789,60 @@ static void pl011_break_ctl(struct uart_port *port, int break_state)
 {
 	struct uart_amba_port *uap =
 	    container_of(port, struct uart_amba_port, port);
+	struct tty_port *tport = &port->state->port;
+	bool rs485 = port->rs485.flags & SER_RS485_ENABLED;
 	unsigned long flags;
 	unsigned int lcr_h;
 
 	uart_port_lock_irqsave(&uap->port, &flags);
+
+	/*
+	 * In RS485 mode the transceiver only drives the bus while
+	 * transmitting. Treat a break as a transmission.
+	 */
+	if (rs485 && break_state == -1 && !uap->rs485_break) {
+		uap->rs485_break = true;
+
+		/* Pending data already enabled the transceiver */
+		if (uap->rs485_tx_state != SEND &&
+		    uap->rs485_tx_state != WAIT_AFTER_RTS)
+			pl011_rs485_tx_start(uap);
+
+		/*
+		 * Wait out delay_rts_before_send before starting the break.
+		 * The state stays WAIT_AFTER_RTS, so the TX path behaves as
+		 * without a break. Start it here if trigger_start_tx is late.
+		 */
+		if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
+			uart_port_unlock_irqrestore(&uap->port, flags);
+			msleep(port->rs485.delay_rts_before_send);
+			uart_port_lock_irqsave(&uap->port, &flags);
+
+			if (uap->rs485_tx_state == WAIT_AFTER_RTS) {
+				hrtimer_try_to_cancel(&uap->trigger_start_tx);
+				uap->rs485_tx_state = SEND;
+				if (!uart_tx_stopped(port))
+					pl011_start_tx(port);
+			}
+		}
+	}
+
 	lcr_h = pl011_read(uap, REG_LCRH_TX);
 	if (break_state == -1)
 		lcr_h |= UART01x_LCRH_BRK;
 	else
 		lcr_h &= ~UART01x_LCRH_BRK;
 	pl011_write(lcr_h, uap, REG_LCRH_TX);
+
+	if (break_state == 0 && uap->rs485_break) {
+		uap->rs485_break = false;
+		/* An active TX path stops the transmission when it is done */
+		if (rs485 && (uart_tx_stopped(port) ||
+			      (!pl011_dma_tx_queued(uap) &&
+			       kfifo_is_empty(&tport->xmit_fifo))))
+			pl011_rs485_tx_stop(uap);
+	}
+
 	uart_port_unlock_irqrestore(&uap->port, flags);
 }
 
@@ -2102,6 +2163,7 @@ static void pl011_shutdown(struct uart_port *port)
 	hrtimer_cancel(&uap->trigger_stop_tx);
 
 	uart_port_lock_irq(port);
+	uap->rs485_break = false;
 	if (uap->rs485_tx_state != OFF)
 		pl011_rs485_tx_stop_now(uap);
 	uart_port_unlock_irq(port);
@@ -2398,6 +2460,7 @@ static int pl011_rs485_config(struct uart_port *port, struct ktermios *termios,
 	struct uart_amba_port *uap =
 		container_of(port, struct uart_amba_port, port);
 
+	uap->rs485_break = false;
 	if ((port->rs485.flags & SER_RS485_ENABLED) &&
 	    !(rs485->flags & SER_RS485_ENABLED)) {
 		bool tx_pending = uap->rs485_tx_state == WAIT_AFTER_RTS;

---
base-commit: 979e9ad7178dcbd131dafdbb2f9a8c5bb0b7a26f
change-id: 20261011-devel-pl011-rs485-break-bec3b979a420

Best regards,
--  
Nicolai Buchwitz <nb@tipi-net.de>

Re: [PATCH v2] serial: amba-pl011: drive the bus during break in RS485 mode

[sashiko-bot]

> In RS485 mode the transceiver is enabled only while data is sent. A
> break set with TIOCSBRK therefore never reaches the bus, and protocols
> that start each frame with a break, like DMX512, do not work.
> 
> Handle an active break like a transmission. Enable the transceiver
> before setting the break, after delay_rts_before_send if one is
> configured. When the break ends, stop transmitting unless the TX path
> is still sending data.
> 
> Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261013-devel-pl011-rs485-break-v2-1-31c9f80d3e3d@tipi-net.de?part=1