linux-kernel - [PATCH] tty: serial: fsl_lpuart: skip waiting for transmission complete when UARTCTRL

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Date:   Thu, 23 Feb 2023 17:39:41 +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: skip waiting for transmission complete when UARTCTRL_SBK is asserted

According to LPUART RM, Transmission Complete Flag becomes 0 if queuing
a break character by writing 1 to CTRL[SBK], so here need to skip
waiting for transmission complete when UARTCTRL_SBK is asserted,
otherwise the kernel may stuck here.
And actually set_termios() adds transmission completion waiting to avoid
data loss or data breakage when changing the baud rate, but we don't
need to worry about this when queuing break characters.

Signed-off-by: Sherry Sun <sherry.sun@....com>
---
 drivers/tty/serial/fsl_lpuart.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index e945f41b93d4..f9e164abf920 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -2240,9 +2240,15 @@ lpuart32_set_termios(struct uart_port *port, struct ktermios *termios,
 	/* update the per-port timeout */
 	uart_update_timeout(port, termios->c_cflag, baud);
 
-	/* wait transmit engin complete */
-	lpuart32_write(&sport->port, 0, UARTMODIR);
-	lpuart32_wait_bit_set(&sport->port, UARTSTAT, UARTSTAT_TC);
+	/*
+	 * LPUART Transmission Complete Flag may never be set while queuing a break
+	 * character, so skip waiting for transmission complete when UARTCTRL_SBK is
+	 * asserted.
+	 */
+	if (!(old_ctrl & UARTCTRL_SBK)) {
+		lpuart32_write(&sport->port, 0, UARTMODIR);
+		lpuart32_wait_bit_set(&sport->port, UARTSTAT, UARTSTAT_TC);
+	}
 
 	/* disable transmit and receive */
 	lpuart32_write(&sport->port, old_ctrl & ~(UARTCTRL_TE | UARTCTRL_RE),
-- 
2.17.1