linux-kernel - [PATCH] serial: mvebu-uart: fix tx lost characters at power off

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Date:   Wed, 23 Dec 2020 20:19:31 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Gregory CLEMENT <gregory.clement@...tlin.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Marek Behún <kabel@...nel.org>
Subject: [PATCH] serial: mvebu-uart: fix tx lost characters at power off

Commit c685af1108d7 ("serial: mvebu-uart: fix tx lost characters") fixed tx
lost characters at low baud rates but started causing tx lost characters
when kernel is going to power off or reboot.

TX_EMP tells us when transmit queue is empty therefore all characters were
transmitted. TX_RDY tells us when CPU can send a new character.

Therefore we need to use different check prior transmitting new character
and different check after all characters were sent.

This patch splits polling code into two functions: wait_for_xmitr() which
waits for TX_RDY and wait_for_xmite() which waits for TX_EMP.

When rebooting A3720 platform without this patch on UART is print only:
[   42.699�

And with this patch on UART is full output:
[   39.530216] reboot: Restarting system

Fixes: c685af1108d7 ("serial: mvebu-uart: fix tx lost characters")
Signed-off-by: Pali Rohár <pali@...nel.org>
---
 drivers/tty/serial/mvebu-uart.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-uart.c
index 118b29912289..e0c00a1b0763 100644
--- a/drivers/tty/serial/mvebu-uart.c
+++ b/drivers/tty/serial/mvebu-uart.c
@@ -648,6 +648,14 @@ static void wait_for_xmitr(struct uart_port *port)
 				  (val & STAT_TX_RDY(port)), 1, 10000);
 }
 
+static void wait_for_xmite(struct uart_port *port)
+{
+	u32 val;
+
+	readl_poll_timeout_atomic(port->membase + UART_STAT, val,
+				  (val & STAT_TX_EMP), 1, 10000);
+}
+
 static void mvebu_uart_console_putchar(struct uart_port *port, int ch)
 {
 	wait_for_xmitr(port);
@@ -675,7 +683,7 @@ static void mvebu_uart_console_write(struct console *co, const char *s,
 
 	uart_console_write(port, s, count, mvebu_uart_console_putchar);
 
-	wait_for_xmitr(port);
+	wait_for_xmite(port);
 
 	if (ier)
 		writel(ier, port->membase + UART_CTRL(port));
-- 
2.20.1