patch "serial: 8250_early: Only set divisor if valid clk & baud" added to tty-linus

patch "serial: 8250_early: Only set divisor if valid clk & baud" added to tty-linus

[gregkh]

This is a note to let you know that I've just added the patch titled

    serial: 8250_early: Only set divisor if valid clk & baud

to my tty git tree which can be found at
    git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-linus branch.

The patch will show up in the next release of the linux-next tree
(usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the
next -rc kernel release.

If you have any questions about this process, please let me know.


>From 0ff3ab701963a845d52337ded7eebf2d1a14fe00 Mon Sep 17 00:00:00 2001
From: Matt Redfearn <matt.redfearn@mips.com>
Date: Wed, 22 Nov 2017 09:57:28 +0000
Subject: serial: 8250_early: Only set divisor if valid clk & baud

If either uartclk or baud are 0, avoid calculating and setting a divisor
based on them since the output will almost certainly be garbage.

This also allows platforms such as the MIPS generic kernel, which has no
way to know a valid BASE_BASE for the board it is actually booted on at
compile time, to set BASE_BAUD to 0 and avoid early_8250 setting a bad
divisor.

This fixes a regression caused by commit 31cb9a8575ca ("earlycon:
initialise baud field of earlycon device structure"), which changed the
behavior of of_setup_earlycon such that it sets a baud rate in the
earlycon structure where previously it was left as 0. All boards
supported by the MIPS generic kernel started outputting garbage from the
boot console due to an incorrect divisor being set.

Fixes: 31cb9a8575ca ("earlycon: initialise baud field of earlycon device structure")
Cc: stable <stable@vger.kernel.org> # 4.14
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/serial/8250/8250_early.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_early.c b/drivers/tty/serial/8250/8250_early.c
index 362c25ff188a..ae6a256524d8 100644
--- a/drivers/tty/serial/8250/8250_early.c
+++ b/drivers/tty/serial/8250/8250_early.c
@@ -122,12 +122,14 @@ static void __init init_port(struct earlycon_device *device)
 	serial8250_early_out(port, UART_FCR, 0);	/* no fifo */
 	serial8250_early_out(port, UART_MCR, 0x3);	/* DTR + RTS */
 
-	divisor = DIV_ROUND_CLOSEST(port->uartclk, 16 * device->baud);
-	c = serial8250_early_in(port, UART_LCR);
-	serial8250_early_out(port, UART_LCR, c | UART_LCR_DLAB);
-	serial8250_early_out(port, UART_DLL, divisor & 0xff);
-	serial8250_early_out(port, UART_DLM, (divisor >> 8) & 0xff);
-	serial8250_early_out(port, UART_LCR, c & ~UART_LCR_DLAB);
+	if (port->uartclk && device->baud) {
+		divisor = DIV_ROUND_CLOSEST(port->uartclk, 16 * device->baud);
+		c = serial8250_early_in(port, UART_LCR);
+		serial8250_early_out(port, UART_LCR, c | UART_LCR_DLAB);
+		serial8250_early_out(port, UART_DLL, divisor & 0xff);
+		serial8250_early_out(port, UART_DLM, (divisor >> 8) & 0xff);
+		serial8250_early_out(port, UART_LCR, c & ~UART_LCR_DLAB);
+	}
 }
 
 int __init early_serial8250_setup(struct earlycon_device *device,
-- 
2.15.0