linux-kernel - [PATCH] tty: serial: imx: fix rs485 rx after tx

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Date:   Fri, 16 Jun 2023 12:47:23 +0200
From:   Martin Fuzzey <martin.fuzzey@...wbird.group>
To:     linux-serial@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Fabio Estevam <festevam@...il.com>,
        Marek Vasut <marex@...x.de>, linux-kernel@...r.kernel.org
Subject: [PATCH] tty: serial: imx: fix rs485 rx after tx

Since commit 79d0224f6bf2 ("tty: serial: imx: Handle RS485 DE signal
active high") RS485 reception no longer works after a transmission.

The following scenario shows the problem:
	1) Open a port in RS485 mode
	2) Receive data from remote (OK)
	3) Transmit data to remote (OK)
	4) Receive data from remote (Nothing received)

In RS485 mode, imx_uart_start_tx() calls imx_uart_stop_rx() and, when the
transmission is complete, imx_uart_stop_tx() calls imx_uart_start_rx().

Since the above commit imx_uart_stop_rx() now sets the loopback bit but
imx_uart_start_rx() does not clear it causing the hardware to remain in
loopback mode and not receive external data.

Fix this by moving the existing loopback disable code to a helper function
and calling it from imx_uart_start_rx() too.

Fixes: 79d0224f6bf2 ("tty: serial: imx: Handle RS485 DE signal active high")
Cc: stable@...r.kernel.org
Signed-off-by: Martin Fuzzey <martin.fuzzey@...wbird.group>
---
 drivers/tty/serial/imx.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index c5e17569c3ad..3fe8ff241522 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -369,6 +369,16 @@ static void imx_uart_soft_reset(struct imx_port *sport)
 	sport->idle_counter = 0;
 }
 
+static void imx_uart_disable_loopback_rs485(struct imx_port *sport)
+{
+	unsigned int uts;
+
+	/* See SER_RS485_ENABLED/UTS_LOOP comment in imx_uart_probe() */
+	uts = imx_uart_readl(sport, imx_uart_uts_reg(sport));
+	uts &= ~UTS_LOOP;
+	imx_uart_writel(sport, uts, imx_uart_uts_reg(sport));
+}
+
 /* called with port.lock taken and irqs off */
 static void imx_uart_start_rx(struct uart_port *port)
 {
@@ -390,6 +400,7 @@ static void imx_uart_start_rx(struct uart_port *port)
 	/* Write UCR2 first as it includes RXEN */
 	imx_uart_writel(sport, ucr2, UCR2);
 	imx_uart_writel(sport, ucr1, UCR1);
+	imx_uart_disable_loopback_rs485(sport);
 }
 
 /* called with port.lock taken and irqs off */
@@ -1422,7 +1433,7 @@ static int imx_uart_startup(struct uart_port *port)
 	int retval;
 	unsigned long flags;
 	int dma_is_inited = 0;
-	u32 ucr1, ucr2, ucr3, ucr4, uts;
+	u32 ucr1, ucr2, ucr3, ucr4;
 
 	retval = clk_prepare_enable(sport->clk_per);
 	if (retval)
@@ -1521,10 +1532,7 @@ static int imx_uart_startup(struct uart_port *port)
 		imx_uart_writel(sport, ucr2, UCR2);
 	}
 
-	/* See SER_RS485_ENABLED/UTS_LOOP comment in imx_uart_probe() */
-	uts = imx_uart_readl(sport, imx_uart_uts_reg(sport));
-	uts &= ~UTS_LOOP;
-	imx_uart_writel(sport, uts, imx_uart_uts_reg(sport));
+	imx_uart_disable_loopback_rs485(sport);
 
 	spin_unlock_irqrestore(&sport->port.lock, flags);
 
-- 
2.25.1