[PATCH 6.1.y 3/3] tty: serial: samsung: Remove redundant port lock acquisition in rx helpers

[Sasha Levin <sashal@kernel.org>]

From: Tudor Ambarus <tudor.ambarus@linaro.org>

[ Upstream commit a3bb136bff5e6a5e48cdd813246c9c4686feaaa9 ]

Sashiko identified a deadlock when the console flow is engaged [1].

When console flow control is enabled (UPF_CONS_FLOW),
s3c24xx_serial_stop_tx() calls s3c24xx_serial_rx_enable() and
s3c24xx_serial_start_tx() calls s3c24xx_serial_rx_disable().

The serial core framework invokes the .stop_tx() and .start_tx()
callbacks with the port->lock spinlock already held. Furthermore, all
internal driver paths that invoke stop_tx (such as the DMA TX
completion handler s3c24xx_serial_tx_dma_complete() or the PIO TX IRQ
handler s3c24xx_serial_tx_irq()) also acquire port->lock prior to
calling it. (Note that s3c24xx_serial_start_tx() is only invoked by the
serial core).

However, s3c24xx_serial_rx_enable() and s3c24xx_serial_rx_disable()
unconditionally attempt to acquire port->lock again using
uart_port_lock_irqsave(). Since spinlocks are not recursive, this
causes a deadlock on the same CPU when console flow control is engaged.

Remove the redundant lock acquisition from both rx helper functions.

Cc: stable <stable@kernel.org>
Fixes: b497549a035c ("[ARM] S3C24XX: Split serial driver into core and per-cpu drivers")
Reported-by: John Ogness <john.ogness@linutronix.de>
Closes: https://sashiko.dev/#/patchset/20260506121606.5805-1-john.ogness%40linutronix.de [1]
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://patch.msgid.link/20260515-samsung-tty-flow-control-deadlock-v1-1-93255edbc9bc@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/tty/serial/samsung_tty.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
index 9d825dd8b6cd4e..b8756576699c05 100644
--- a/drivers/tty/serial/samsung_tty.c
+++ b/drivers/tty/serial/samsung_tty.c
@@ -247,12 +247,9 @@ static int s3c24xx_serial_txempty_nofifo(const struct uart_port *port)
 static void s3c24xx_serial_rx_enable(struct uart_port *port)
 {
 	struct s3c24xx_uart_port *ourport = to_ourport(port);
-	unsigned long flags;
 	int count = 10000;
 	u32 ucon, ufcon;
 
-	uart_port_lock_irqsave(port, &flags);
-
 	while (--count && !s3c24xx_serial_txempty_nofifo(port))
 		udelay(100);
 
@@ -265,23 +262,18 @@ static void s3c24xx_serial_rx_enable(struct uart_port *port)
 	wr_regl(port, S3C2410_UCON, ucon);
 
 	ourport->rx_enabled = 1;
-	uart_port_unlock_irqrestore(port, flags);
 }
 
 static void s3c24xx_serial_rx_disable(struct uart_port *port)
 {
 	struct s3c24xx_uart_port *ourport = to_ourport(port);
-	unsigned long flags;
 	u32 ucon;
 
-	uart_port_lock_irqsave(port, &flags);
-
 	ucon = rd_regl(port, S3C2410_UCON);
 	ucon &= ~S3C2410_UCON_RXIRQMODE;
 	wr_regl(port, S3C2410_UCON, ucon);
 
 	ourport->rx_enabled = 0;
-	uart_port_unlock_irqrestore(port, flags);
 }
 
 static void s3c24xx_serial_stop_tx(struct uart_port *port)
-- 
2.53.0