Patch "serial: 8250: omap: Disable DMA for console UART" has been added to the 4.9-stable tree

Patch "serial: 8250: omap: Disable DMA for console UART" has been added to the 4.9-stable tree

[gregkh]

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

    serial: 8250: omap: Disable DMA for console UART

to the 4.9-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-omap-disable-dma-for-console-uart.patch
and it can be found in the queue-4.9 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 foo@baz Mon Apr  9 17:09:24 CEST 2018
From: Vignesh R <vigneshr@ti.com>
Date: Sat, 22 Apr 2017 18:37:19 +0530
Subject: serial: 8250: omap: Disable DMA for console UART

From: Vignesh R <vigneshr@ti.com>


[ Upstream commit 84b40e3b57eef1417479c00490dd4c9f6e5ffdbc ]

Kernel always writes log messages to console via
serial8250_console_write()->serial8250_console_putchar() which directly
accesses UART_TX register _without_ using DMA.

But, if other processes like systemd using same UART port, then these
writes are handled by a different code flow using 8250_omap driver where
there is provision to use DMA.

It seems that it is possible that both DMA and CPU might simultaneously
put data to UART FIFO and lead to potential loss of data due to FIFO
overflow and weird data corruption. This happens when both kernel
console and userspace tries to write simultaneously to the same UART
port. Therefore, disable DMA on kernel console port to avoid potential
race between CPU and DMA.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/serial/8250/8250_omap.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -613,6 +613,10 @@ static int omap_8250_startup(struct uart
 	up->lsr_saved_flags = 0;
 	up->msr_saved_flags = 0;
 
+	/* Disable DMA for console UART */
+	if (uart_console(port))
+		up->dma = NULL;
+
 	if (up->dma) {
 		ret = serial8250_request_dma(up);
 		if (ret) {


Patches currently in stable-queue which might be from vigneshr@ti.com are

queue-4.9/serial-8250-omap-disable-dma-for-console-uart.patch