linux-kernel - [PATCH v3] serial: sh-sci: disable DMA for uart

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Date:   Tue, 14 May 2019 23:29:34 -0400
From:   "George G. Davis" <george_davis@...tor.com>
To:     Eugeniu Rosca <erosca@...adit-jv.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Simon Horman <horms+renesas@...ge.net.au>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
CC:     Chris Brandt <chris.brandt@...esas.com>,
        Ulrich Hecht <ulrich.hecht+renesas@...il.com>,
        Andy Lowe <andy_lowe@...tor.com>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS 
        <devicetree@...r.kernel.org>, Magnus Damm <magnus.damm@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "George G. Davis" <george_davis@...tor.com>,
        <stable@...r.kernel.org>
Subject: [PATCH v3] serial: sh-sci: disable DMA for uart_console

As noted in commit 84b40e3b57ee ("serial: 8250: omap: Disable DMA for
console UART"), UART console lines use low-level PIO only access functions
which will conflict with use of the line when DMA is enabled, e.g. when
the console line is also used for systemd messages. So disable DMA
support for UART console lines.

Reported-by: Michael Rodin <mrodin@...adit-jv.com>
Link: https://patchwork.kernel.org/patch/10929511/
Tested-by: Eugeniu Rosca <erosca@...adit-jv.com>
Reviewed-by: Simon Horman <horms+renesas@...ge.net.au>
Reviewed-by: Wolfram Sang <wsa+renesas@...g-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>
Cc: stable@...r.kernel.org
Signed-off-by: George G. Davis <george_davis@...tor.com>
---
v2: Clarify comment regarding DMA support on kernel console,
    add {Tested,Reviewed}-by:, and Cc: linux-stable lines.
v3: Change Fixes: reference to Link: reference and add another Reviewed-by.
---
 drivers/tty/serial/sh-sci.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 3cd139752d3f..abc705716aa0 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1557,6 +1557,13 @@ static void sci_request_dma(struct uart_port *port)
 
 	dev_dbg(port->dev, "%s: port %d\n", __func__, port->line);
 
+	/*
+	 * DMA on console may interfere with Kernel log messages which use
+	 * plain putchar(). So, simply don't use it with a console.
+	 */
+	if (uart_console(port))
+		return;
+
 	if (!port->dev->of_node)
 		return;
 
-- 
2.7.4