linux-kernel - [PATCH] tty: serial: fsl_lpuart: make sure to turn off break before enabling CTS

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Date:   Tue, 16 May 2023 18:11:38 +0800
From:   Sherry Sun <sherry.sun@....com>
To:     gregkh@...uxfoundation.org, jirislaby@...nel.org
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-imx@....com
Subject: [PATCH] tty: serial: fsl_lpuart: make sure to turn off break before enabling CTS

Due to one LPUART IP bug which treat the CTS as higher priority than the
break signal, that cause the break signal sending through UARTCTRL_SBK
may impacted by the CTS input if the HW flow control is enabled.

So commit c4c81db5cf8b ("tty: serial: fsl_lpuart: disable the CTS when
send break signal") try to workaround this IP issue, it disables CTS
before asserting SBK to avoid any interference from CTS, and re-enable
it when break off.

Here we enable CTS before turning off the break, there is still a risk
of the break signal been impacted by CTS input. The correct sequence is
to disable CTS before asserting SBK, and re-enable CTS after break off,
which ensures the break signal won't be impacted by CTS, so fix it.

Fixes: c4c81db5cf8b ("tty: serial: fsl_lpuart: disable the CTS when send break signal")
Signed-off-by: Sherry Sun <sherry.sun@....com>
---
 drivers/tty/serial/fsl_lpuart.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index 0e56fa64b4ce..d9e46f7b80e5 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -1550,20 +1550,19 @@ static void lpuart32_break_ctl(struct uart_port *port, int break_state)
 	modem = lpuart32_read(port, UARTMODIR);

 	if (break_state != 0) {
-		temp |= UARTCTRL_SBK;
 		/*
 		 * LPUART CTS has higher priority than SBK, need to disable CTS before
 		 * asserting SBK to avoid any interference if flow control is enabled.
 		 */
 		if (cflag & CRTSCTS && modem & UARTMODIR_TXCTSE)
 			lpuart32_write(port, modem & ~UARTMODIR_TXCTSE, UARTMODIR);
+		lpuart32_write(port, temp | UARTCTRL_SBK, UARTCTRL);
 	} else {
-		/* Re-enable the CTS when break off. */
+		lpuart32_write(port, temp, UARTCTRL);
+		/* Re-enable the CTS after break off. */
 		if (cflag & CRTSCTS && !(modem & UARTMODIR_TXCTSE))
 			lpuart32_write(port, modem | UARTMODIR_TXCTSE, UARTMODIR);
 	}
-
-	lpuart32_write(port, temp, UARTCTRL);
 }

 static void lpuart_setup_watermark(struct lpuart_port *sport)
-- 
2.17.1