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

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Message-ID: <20190513135114.GA20443@kunai>
Date:   Mon, 13 May 2019 15:51:14 +0200
From:   Wolfram Sang <wsa@...-dreams.de>
To:     "George G. Davis" <ggdavisiv@...il.com>
Cc:     Eugeniu Rosca <erosca@...adit-jv.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        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>,
        Simon Horman <horms+renesas@...ge.net.au>,
        Chris Brandt <chris.brandt@...esas.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.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>
Subject: Re: [PATCH] serial: sh-sci: disable DMA for uart_console

On Thu, May 09, 2019 at 10:43:30AM -0400, George G. Davis wrote:
> 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.
> 
> Fixes: https://patchwork.kernel.org/patch/10929511/
> Reported-by: Michael Rodin <mrodin@...adit-jv.com>
> Cc: Eugeniu Rosca <erosca@...adit-jv.com>
> Signed-off-by: George G. Davis <george_davis@...tor.com>
> ---
>  drivers/tty/serial/sh-sci.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
> index 3cd139752d3f..885b56b1d4e4 100644
> --- a/drivers/tty/serial/sh-sci.c
> +++ b/drivers/tty/serial/sh-sci.c
> @@ -1557,6 +1557,9 @@ static void sci_request_dma(struct uart_port *port)
>  
>  	dev_dbg(port->dev, "%s: port %d\n", __func__, port->line);
>  
> +	if (uart_console(port))
> +		return; /* Cannot use DMA on console */

Minor nit: maybe the comment can be made more specific?

/*
 * DMA on console may interfere with Kernel log messages which use
 * plain putchar(). So, simply don't use it with a console.
 */

Other than that:

Reviewed-by: Wolfram Sang <wsa+renesas@...g-engineering.com>

Much better than dropping the properties, as Geert noted.


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