linux-kernel - Re: [PATCH REPOST] tty: serial: 8250: omap: restore registers on shutdown

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Message-ID: <56A3EFBD.9020209@hurleysoftware.com>
Date:	Sat, 23 Jan 2016 13:25:17 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	John Ogness <john.ogness@...utronix.de>, gregkh@...uxfoundation.org
Cc:	bigeasy@...utronix.de, tony@...mide.com, nsekhar@...com,
	peter.ujfalusi@...com, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH REPOST] tty: serial: 8250: omap: restore registers on
 shutdown

On 01/22/2016 08:28 AM, John Ogness wrote:
> If DMA is active during a shutdown, a delayed restore of the
> registers may be pending. The restore must be performed after
> the DMA is stopped, otherwise the delayed restore remains
> pending and will fire upon the first DMA TX complete of a
> totally different serial session.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>  This patch was originally posted and discussed here:
>  http://lkml.kernel.org/r/87egjp2r4a.fsf@linutronix.de
> 
>  The patch was not accepted because Peter expressed dissatisfaction with
>  the entire deferred implementation. (This patch fixes a bug in that
>  implementation.) However, no alternative solution was determined and the
>  bug continues to exist.

Yeah, I made that comment a while ago and still no one's fixed that.
What gives?

Regards,
Peter Hurley


>  Although we can certainly continue to debate about an alternate solution
>  to the deferred implementation, I think it makes sense to at least fix
>  the existing bug until an alternative exists.
> 
>  The bug can be easily produced with a `kill -9` on a process transmitting
>  on the serial port.
> 
>  patch against next-20160122
> 
>  drivers/tty/serial/8250/8250_omap.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
> index a2c0734..0ffec67 100644
> --- a/drivers/tty/serial/8250/8250_omap.c
> +++ b/drivers/tty/serial/8250/8250_omap.c
> @@ -664,9 +664,15 @@ static void omap_8250_shutdown(struct uart_port *port)
>  	up->ier = 0;
>  	serial_out(up, UART_IER, 0);
>  
> -	if (up->dma)
> +	if (up->dma) {
>  		serial8250_release_dma(up);
>  
> +		if (priv->delayed_restore) {
> +			priv->delayed_restore = 0;
> +			omap8250_restore_regs(up);
> +		}
> +	}
> +
>  	/*
>  	 * Disable break condition and FIFOs
>  	 */
>