linux-kernel - Re: [PATCH] serial: imx: Improve PIO prevention if TX DMA has been started

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Date:   Sun, 13 Aug 2017 00:07:56 +0200
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     linux-serial@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Fabio Estevam <fabio.estevam@....com>,
        linux-kernel@...r.kernel.org, Ian Jamison <ian.dev@...ver.com>
Subject: Re: [PATCH] serial: imx: Improve PIO prevention if TX DMA has been
 started

On Sat, Aug 12, 2017 at 09:54:51PM +0200, Uwe Kleine-König wrote:
> On Sat, Aug 12, 2017 at 05:12:10PM +0200, Clemens Gruber wrote:
> > The imx_transmit_buffer function should return if TX DMA has already
> > been started and not just skip over the buffer PIO write loop. (Which
> > did fix the initial problem, but could have unintentional side-effects)
> > 
> > Tested on an i.MX6Q board with half-duplex RS-485 and with RS-232.
> > 
> > Cc: Ian Jamison <ian.dev@...ver.com>
> > Cc: Uwe-Kleine König <u.kleine-koenig@...gutronix.de>
> > Fixes: 514ab34dbad6 ("serial: imx: Prevent TX buffer PIO write when a
> > DMA has been started")
> 
> AFAIK no newline in the Fixes: line.

Thanks. A checkpatch warning for this would be great.

> 
> > Signed-off-by: Clemens Gruber <clemens.gruber@...ruber.com>
> > ---
> >  drivers/tty/serial/imx.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> > index 80934e7bd67f..fce538eb8c77 100644
> > --- a/drivers/tty/serial/imx.c
> > +++ b/drivers/tty/serial/imx.c
> > @@ -452,13 +452,14 @@ static inline void imx_transmit_buffer(struct imx_port *sport)
> >  		if (sport->dma_is_txing) {
> >  			temp |= UCR1_TDMAEN;
> >  			writel(temp, sport->port.membase + UCR1);
> > +			return;
> >  		} else {
> >  			writel(temp, sport->port.membase + UCR1);
> >  			imx_dma_tx(sport);
> >  		}
> 
> Shouldn't the return go here?

Yes, it can also go here (and probably should). The problem of
xmit->tail jumping over xmit->head occurs only if we are already DMA
txing and then go into the PIO loop, but not the first time after
calling imx_dma_tx. That's why the v1 passed the tests too.
I'll have to conduct a few more tests and if they succeed I'll send a
v2 where we return in both cases (already txing and starting to).

> Did you understand the problem? Can you say why this only hurts in RS485
> half-duplex but not (as it seems) in regular rs232 mode?

I am not sure anyone understands (yet) why it a) only hurts RS-485 and
b) only occurs on SMP systems.
If you have more insight, please share it. :)

Cheers,
Clemens