serial: imx: only set receiver level if it is zero

With commit a81dbd04 ("serial: imx: set receiver level before
starting uart") we set the receiver level to its default value. This
caused a regression when using SDMA, where the receiver level is 9
instead of 8 (default). This change will first check if the receiver
level is zero and only then set it to the default. This still avoids the
interrupt storm when the receiver level is zero.

Fixes: a81dbd04 ("serial: imx: set receiver level before starting uart")
Cc: stable <stable@kernel.org>
Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Link: https://lore.kernel.org/r/20240703112543.148304-1-eichest@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -120,6 +120,7 @@
 #define UCR4_OREN	(1<<1)	/* Receiver overrun interrupt enable */
 #define UCR4_DREN	(1<<0)	/* Recv data ready interrupt enable */
 #define UFCR_RXTL_SHF	0	/* Receiver trigger level shift */
+#define UFCR_RXTL_MASK	0x3F	/* Receiver trigger 6 bits wide */
 #define UFCR_DCEDTE	(1<<6)	/* DCE/DTE mode select */
 #define UFCR_RFDIV	(7<<7)	/* Reference freq divider mask */
 #define UFCR_RFDIV_REG(x)	(((x) < 7 ? 6 - (x) : 6) << 7)
@@ -1933,7 +1934,7 @@ static int imx_uart_rs485_config(struct uart_port *port, struct ktermios *termio
 				 struct serial_rs485 *rs485conf)
 {
 	struct imx_port *sport = (struct imx_port *)port;
-	u32 ucr2;
+	u32 ucr2, ufcr;
 
 	if (rs485conf->flags & SER_RS485_ENABLED) {
 		/* Enable receiver if low-active RTS signal is requested */
@@ -1953,7 +1954,10 @@ static int imx_uart_rs485_config(struct uart_port *port, struct ktermios *termio
 	/* Make sure Rx is enabled in case Tx is active with Rx disabled */
 	if (!(rs485conf->flags & SER_RS485_ENABLED) ||
 	    rs485conf->flags & SER_RS485_RX_DURING_TX) {
-		imx_uart_setup_ufcr(sport, TXTL_DEFAULT, RXTL_DEFAULT);
+		/* If the receiver trigger is 0, set it to a default value */
+		ufcr = imx_uart_readl(sport, UFCR);
+		if ((ufcr & UFCR_RXTL_MASK) == 0)
+			imx_uart_setup_ufcr(sport, TXTL_DEFAULT, RXTL_DEFAULT);
 		imx_uart_start_rx(port);
 	}