serial: sh-sci: prevent lockup on full TTY buffers

This change “serial: sh-sci: prevent lockup on full TTY buffers” in Linux kernel is authored by Ulrich Hecht <ulrich.hecht+renesas [at]> on Thu Feb 15 13:02:27 2018 +0100.

serial: sh-sci: prevent lockup on full TTY buffers

When the TTY buffers fill up to the configured maximum, a system lockup

[  598.820128] INFO: rcu_preempt detected stalls on CPUs/tasks:
[  598.825796]  0-...!: (1 GPs behind) idle=5a6/2/0 softirq=1974/1974 fqs=1
[  598.832577]  (detected by 3, t=62517 jiffies, g=296, c=295, q=126)
[  598.838755] Task dump for CPU 0:
[  598.841977] swapper/0       R  running task        0     0      0 0x00000022
[  598.849023] Call trace:
[  598.851476]  __switch_to+0x98/0xb0
[  598.854870]            (null)

This can be prevented by doing a dummy read of the RX data register.

This issue affects both HSCIF and SCIF ports. Reported for R-Car H3 ES2.0;
reproduced and fixed on H3 ES1.1. Probably affects other R-Car platforms
as well.

Reported-by: Yoshihiro Shimoda <>
Signed-off-by: Ulrich Hecht <>
Reviewed-by: Geert Uytterhoeven <>
Cc: stable <>
Tested-by: Nguyen Viet Dung <>
Signed-off-by: Greg Kroah-Hartman <>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 7842055.

There are 2 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/tty/serial/sh-sci.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 7257c07..44adf9d 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -885,6 +885,8 @@ static void sci_receive_chars(struct uart_port *port)
 		/* Tell the rest of the system the news. New characters! */
 	} else {
+		/* TTY buffers full; read from RX reg to prevent lockup */
+		serial_port_in(port, SCxRDR);
 		serial_port_in(port, SCxSR); /* dummy read */
 		sci_clear_SCxSR(port, SCxSR_RDxF_CLEAR(port));

The commit for this change in Linux stable tree is 7842055 (patch).

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