Patch "serial: 8250_early: Only set divisor if valid clk & baud" has been added to the 4.14-stable tree

[<gregkh@linuxfoundation.org>]

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 the 4.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     serial-8250_early-only-set-divisor-if-valid-clk-baud.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>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

From: Matt Redfearn <matt.redfearn@mips.com>

commit 0ff3ab701963a845d52337ded7eebf2d1a14fe00 upstream.

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")
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(-)

--- a/drivers/tty/serial/8250/8250_early.c
+++ b/drivers/tty/serial/8250/8250_early.c
@@ -125,12 +125,14 @@ static void __init init_port(struct earl
 	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,


Patches currently in stable-queue which might be from matt.redfearn@mips.com are

queue-4.14/mips-add-custom-serial.h-with-base_baud-override-for-generic-kernel.patch
queue-4.14/serial-8250_early-only-set-divisor-if-valid-clk-baud.patch