[PATCH v4 6/7] serial: sh-sci: Toggle TE when restoring SCSCR in console write on SCI

[Diogo Ivo <diogo.ivo.sx@renesas.com>]

From: Au Dang <au.dang.jx@renesas.com>

When SCI0 (PORT_SCI) is the system console on RZ/G2UL, the board can
hang at the serial console output stage during boot.

ftrace shows the following sequence:

1. sci_transmit_chars() is draining the TX kfifo through TXI
   interrupts with SCSCR=0xf0 (TE=1, TIE=1, RE=1, RIE=1).

2. serial_console_write() runs in the middle of that drain. It saves
   SCSCR=0xf0, switches to polling by writing SCSCR=0x30 (TE=1, TIE=0)
   and outputs the characters with sci_poll_put_char().

3. It then restores SCSCR=0xf0 in a single write. TE is 1 both before
   and after, so there is no TE transition.

4. sci_start_tx() is then called repeatedly with SCSCR unchanged, but
   no TXI is ever raised again, so the drain never resumes and the
   console hangs.

The RZ/G2UL User's Manual (section 23.3.6, "SCI Initialization") notes
that changing SCR.TE from 1 to 0 or from 0 to 1 while SCR.TIE is 1
generates a TXI interrupt request. Without a TE transition, the TXI
that would restart the drain is never generated.

For PORT_SCI, restore SCSCR in two steps: first with TE cleared, then
with the full saved value. This forces the TE 1->0->1 transition while
TIE is being re-enabled, so TXI fires and the interrupted drain
resumes.

Signed-off-by: Au Dang <au.dang.jx@renesas.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Diogo Ivo <diogo.ivo.sx@renesas.com>
---
 drivers/tty/serial/sh-sci.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index fc77536644ed..2d667086ec3a 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -3456,10 +3456,14 @@ static void serial_console_write(struct console *co, const char *s,
 		cpu_relax();
 
 	/* restore the SCSCR */
-	if (sci_port->ops->finish_console_write)
+	if (sci_port->ops->finish_console_write) {
 		sci_port->ops->finish_console_write(port, ctrl);
-	else
+	} else {
+		if (sci_port->type == PORT_SCI)
+			sci_port->ops->write_reg(port, regs->control, ctrl & ~SCSCR_TE);
+
 		sci_port->ops->write_reg(port, regs->control, ctrl);
+	}
 
 	if (locked)
 		uart_port_unlock_irqrestore(port, flags);

-- 
2.56.0