[PATCH 5.15.y 2/2] serial: amba-pl011: keep transmitter enabled when disabling RS485

[Sasha Levin <sashal@kernel.org>]

From: Nicolai Buchwitz <nb@tipi-net.de>

[ Upstream commit 97f85204275ae4d7fb530e451f3acc65c1ec1155 ]

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>
[ 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 <sashal@kernel.org>
---
 drivers/tty/serial/amba-pl011.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 7b21907cd14dd..3f74b96e7745c 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -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) {
-- 
2.53.0