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

[Sasha Levin <sashal@kernel.org>]

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 <stable@vger.kernel.org>.

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 '<stable@vger.kernel.org>' --in-reply-to '20261107060610-failed-5.15-97f85204275a@kernel.org' --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 <nb@tipi-net.de>
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: stable@kernel.org
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20261012-fix-pl011-rs485-txe-v2-1-e9bf7d218389@tipi-net.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/serial/amba-pl011.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index cd8cbc9672815..3edd83ec73de1 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -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) {
-- 
2.53.0