linux-kernel - Re: [PATCH 2/2] serial: imx: dma_unmap

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Date:   Tue, 8 May 2018 15:40:47 +0200
From:   Sebastian Reichel <sebastian.reichel@...labora.co.uk>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
        Fabio Estevam <fabio.estevam@....com>,
        Shawn Guo <shawnguo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] serial: imx: dma_unmap_sg buffers on shutdown

Hi,

On Tue, May 08, 2018 at 08:43:51AM +0200, Uwe Kleine-König wrote:
> On Mon, May 07, 2018 at 11:36:10PM +0200, Sebastian Reichel wrote:
> > This properly unmaps DMA SG on device shutdown.
> > 
> > Reported-by: Nandor Han <nandor.han@...com>
> > Suggested-by: Nandor Han <nandor.han@...com>
> > Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.co.uk>
> > ---
> >  drivers/tty/serial/imx.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> > index 3ca767b1162a..6c53e74244ec 100644
> > --- a/drivers/tty/serial/imx.c
> > +++ b/drivers/tty/serial/imx.c
> > @@ -1425,10 +1425,18 @@ static void imx_uart_shutdown(struct uart_port *port)
> >  	u32 ucr1, ucr2;
> >  
> >  	if (sport->dma_is_enabled) {
> > -		sport->dma_is_rxing = 0;
> > -		sport->dma_is_txing = 0;
> >  		dmaengine_terminate_sync(sport->dma_chan_tx);
> > +		if (sport->dma_is_txing) {
> > +			dma_unmap_sg(sport->port.dev, &sport->tx_sgl[0],
> > +				     sport->dma_tx_nents, DMA_TO_DEVICE);
> > +			sport->dma_is_txing = 0;
> > +		}
> 
> did you find this because the kernel crashed or consumed more and more
> memory, or is this "only" a finding of reading the source code? If the
> former it would be great to point out in the commit log, if the latter,
> I wonder if this is a real problem that warrants a stable backport.

A bit of both. One of Collabora's customers had a (scarce) kernel crash
in imx-serial and modified multiple things in the driver. The crash is
gone, but it's not clear which change fixed it. I could not
reproduce the crash so far and I'm currently rebasing and splitting
their changes into upstreamable portions with proper patch
descriptions. From reading the source this looked like a real issue.

-- Sebastian

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