FAILED: patch "[PATCH] serial: amba-pl011: keep transmitter enabled when disabling RS485" failed to apply to 5.15-stable tree

PATCH New Sasha Levin 1 participant 3 messages Nov 7 – Nov 11

Subsystems (3)

Stable branch TTY layer and serial drivers Arm primecell UART PL010 and PL011 drivers

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Sasha Levin

author maintainer

2026-11-07 11:06 UTC #0 lore raw

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 97f85204275ae4d7fb530e451f3acc65c1ec1155
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '[email protected]' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 97f85204275ae4d7fb530e451f3acc65c1ec1155 Mon Sep 17 00:00:00 2001
From: Nicolai Buchwitz <[email protected]>
Date: Mon, 12 Oct 2026 16:23:50 +0200
Subject: [PATCH] serial: amba-pl011: keep transmitter enabled when disabling
RS485

Disabling RS485 on an open port calls pl011_rs485_tx_stop(), which turns off the transmitter. Only the RS485 TX path turns it back on, so any later output stays queued and tcdrain() hangs until the port is closed.

pl011_rs485_tx_stop() also checks only the hardware FIFO. If a DMA transfer has started but has not filled the FIFO yet, the transmitter is turned off under the transfer and the DMA channel never recovers.

When leaving RS485 mode, cancel pending deferred starts and stops, leave the transmitter enabled and start any TX that was still waiting for delay_rts_before_send.

Fixes 8d479237727c ("serial: amba-pl011: add RS485 support")

Cc [email protected]

Signed-off-by Nicolai Buchwitz <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Greg Kroah-Hartman <[email protected]>

---
 drivers/tty/serial/amba-pl011.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

drivers/tty/serial/amba-pl011.c +24 -1

index cd8cbc9672815..3edd83ec73de1 100644
@@ -2398,8 +2398,31 @@ 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);
 
-	if (port->rs485.flags & SER_RS485_ENABLED)
+	if ((port->rs485.flags & SER_RS485_ENABLED) &&
+	    !(rs485->flags & SER_RS485_ENABLED)) {
+		bool tx_pending = uap->rs485_tx_state == WAIT_AFTER_RTS;
+		u32 cr = pl011_read(uap, REG_CR);
+
+		/*
+		 * Leaving RS485 mode, which only enables the transmitter while
+		 * sending. Don't stop it, as that stalls a DMA transfer still
+		 * in flight. Drop pending deferred starts and stops and keep
+		 * it enabled.
+		 */
+		hrtimer_try_to_cancel(&uap->trigger_start_tx);
+		hrtimer_try_to_cancel(&uap->trigger_stop_tx);
+		uap->rs485_tx_state = OFF;
+
+		cr |= UART011_CR_TXE | UART011_CR_RXE;
+		pl011_write(cr, uap, REG_CR);
+
+		/* Start TX that was still waiting for delay_rts_before_send */
+		if (tx_pending && !uart_tx_stopped(port) &&
+		    !pl011_dma_tx_start(uap))
+			pl011_start_tx_pio(uap);
+	} else if (port->rs485.flags & SER_RS485_ENABLED) {
 		pl011_rs485_tx_stop(uap);
+	}
 
 	/* Make sure auto RTS is disabled */
 	if (rs485->flags & SER_RS485_ENABLED) {

Signature · 1 line

2.53.0

Sasha Levin

author maintainer

2026-11-11 15:13 UTC #1 lore raw

PATCH 1/2 serial: amba-pl011: remove redundant code in rs485_config

From: Lino Sanfilippo <[email protected]>

[ Upstream commit a9efa452486ed59b38f15c2287d3e7e129f6e7fc ]

In uart_set_rs485_config() the serial core already

- ensures that only one of both options RTS on send or RTS after send is set

- nullifies the padding field of the passed serial_rs485 struct

- clamps the RTS delays

- assigns the passed serial_rs485 struct to the uart port

So remove these tasks from the code of the drivers rs485_config() function to avoid redundancy.

Signed-off-by Lino Sanfilippo <[email protected]>

Link https://lore.kernel.org/r/[email protected] open here

Signed-off-by Greg Kroah-Hartman <[email protected]>

Stable-dep-of: 97f85204275a ("serial: amba-pl011: keep transmitter enabled when disabling RS485")

Signed-off-by Sasha Levin <[email protected]>

---
 drivers/tty/serial/amba-pl011.c | 16 +---------------
 1 file changed, 1 insertion(+), 15 deletions(-)

drivers/tty/serial/amba-pl011.c +1 ~~-15~~

index a5340c5e11b10..7b21907cd14dd 100644
@@ -2241,25 +2241,11 @@ static int pl011_rs485_config(struct uart_port *port,
 	struct uart_amba_port *uap =
 		container_of(port, struct uart_amba_port, port);
 
-	/* pick sane settings if the user hasn't */
-	if (!(rs485->flags & SER_RS485_RTS_ON_SEND) ==
-	    !(rs485->flags & SER_RS485_RTS_AFTER_SEND)) {
-		rs485->flags |= SER_RS485_RTS_ON_SEND;
-		rs485->flags &= ~SER_RS485_RTS_AFTER_SEND;
-	}
-	/* clamp the delays to [0, 100ms] */
-	rs485->delay_rts_before_send = min(rs485->delay_rts_before_send, 100U);
-	rs485->delay_rts_after_send = min(rs485->delay_rts_after_send, 100U);
-	memset(rs485->padding, 0, sizeof(rs485->padding));
-
 	if (port->rs485.flags & SER_RS485_ENABLED)
 		pl011_rs485_tx_stop(uap);
 
-	/* Set new configuration */
-	port->rs485 = *rs485;
-
 	/* Make sure auto RTS is disabled */
-	if (port->rs485.flags & SER_RS485_ENABLED) {
+	if (rs485->flags & SER_RS485_ENABLED) {
 		u32 cr = pl011_read(uap, REG_CR);
 
 		cr &= ~UART011_CR_RTSEN;

Signature · 1 line

2.53.0

Sasha Levin

author maintainer

2026-11-11 15:13 UTC #2 lore raw

PATCH 2/2 serial: amba-pl011: keep transmitter enabled when disabling RS485

From: Nicolai Buchwitz <[email protected]>

[ Upstream commit 97f85204275ae4d7fb530e451f3acc65c1ec1155 ]

When leaving RS485 mode, cancel pending deferred starts and stops, leave the transmitter enabled and start any TX that was still waiting for delay_rts_before_send.

Fixes 8d479237727c ("serial: amba-pl011: add RS485 support")

Cc [email protected]

Signed-off-by Nicolai Buchwitz <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Greg Kroah-Hartman <[email protected]>

[ sashal: Adapted RS485 handling to the existing rs485_tx_started boolean and synchronous delays, omitting unavailable timer and deferred-start logic. ]

Signed-off-by Sasha Levin <[email protected]>

---
 drivers/tty/serial/amba-pl011.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

drivers/tty/serial/amba-pl011.c +15 -1

index 7b21907cd14dd..3f74b96e7745c 100644
@@ -2241,8 +2241,22 @@ static int pl011_rs485_config(struct uart_port *port,
 	struct uart_amba_port *uap =
 		container_of(port, struct uart_amba_port, port);
 
-	if (port->rs485.flags & SER_RS485_ENABLED)
+	if ((port->rs485.flags & SER_RS485_ENABLED) &&
+	    !(rs485->flags & SER_RS485_ENABLED)) {
+		u32 cr = pl011_read(uap, REG_CR);
+
+		/*
+		 * Leaving RS485 mode, which only enables the transmitter while
+		 * sending. Don't stop it, as that stalls a DMA transfer still
+		 * in flight. Keep it enabled.
+		 */
+		uap->rs485_tx_started = false;
+
+		cr |= UART011_CR_TXE | UART011_CR_RXE;
+		pl011_write(cr, uap, REG_CR);
+	} else if (port->rs485.flags & SER_RS485_ENABLED) {
 		pl011_rs485_tx_stop(uap);
+	}
 
 	/* Make sure auto RTS is disabled */
 	if (rs485->flags & SER_RS485_ENABLED) {

Signature · 1 line

2.53.0