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

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Date:   Wed, 9 May 2018 13:28:50 +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 Wed, May 09, 2018 at 12:42:56PM +0200, Uwe Kleine-König wrote:
> On Wed, May 09, 2018 at 12:20:58PM +0200, Sebastian Reichel wrote:
> > Hi,
> > 
> > On Tue, May 08, 2018 at 08:46:12PM +0200, Uwe Kleine-König wrote:
> > > On Tue, May 08, 2018 at 03:40:47PM +0200, Sebastian Reichel wrote:
> > > > 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.
> > > 
> > > In which context (kernel version, operating mode (e.g. rs485)) did these
> > > happen? What does "crash" mean? The kernel did just hang or produced an
> > > oops? If the latter, can you show it/them?
> > 
> > I pasted the oops, that triggered writing the patches (Linux 4.8, no
> > rs485, 4MHz baudrate). I think, that the actual issue has already been
> > fixed upstream between 4.13 and current master.
> > 
> > -- Sebastian
> > 
> > ...
> > [  302.516696] Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0938000
> 
> This is usually a missing clk. Alternatively RX is disabled and the
> RXDATA register is read. Scrolling through
> 
> 	git log v4.13..linus/master -- drivers/tty/serial/imx.c
> 
> I didn't find a candidate for fixing that.

It happens while the port is being torn apart. I think the following
patches from you are very good candidates. Especially since the
remaining diff fixing the issue in the customer's old kernel has a
smilar change:

76821e222c18 - serial: imx: ensure that RX irqs are off if RX is off (9 weeks ago) <Uwe Kleine-König>
dedc64e02f5d - serial: imx: Stop to receive in .stop_rx() (9 weeks ago) <Uwe Kleine-König>

-- Sebastian

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