Re: [PATCH] serial: 8250_omap: Fix gpio check for auto RTS and CTS

From: Adam Ford <aford173@gmail.com>
To: Yegor Yefremov <yegorslists@googlemail.com>
Cc: linux-serial@vger.kernel.org, "Adam Ford" <adam.ford@logicpd.com>,
	Linux-OMAP <linux-omap@vger.kernel.org>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Jiri Slaby" <jslaby@suse.com>, "Vignesh R" <vigneshr@ti.com>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Tony Lindgren" <tony@atomide.com>,
	"Douglas Anderson" <dianders@chromium.org>,
	"kernel list" <linux-kernel@vger.kernel.org>,
	"Nuno Gonçalves" <nunojpg@gmail.com>,
	"Giulio Benetti" <giulio.benetti@benettiengineering.com>,
	"Stefan Roese" <sr@denx.de>
Subject: Re: [PATCH] serial: 8250_omap: Fix gpio check for auto RTS and CTS
Date: Sun, 6 Oct 2019 06:38:34 -0500	[thread overview]
Message-ID: <CAHCN7xLgTiXxLrW4njTT8Zaiib6+F4xo07wk0-sQp8h+j=E3yw@mail.gmail.com> (raw)
In-Reply-To: <CAGm1_ksBaEN0OWR+dTwT9SgoybtnchcCKasjXf5ybP2fZfRF7A@mail.gmail.com>

On Sun, Oct 6, 2019 at 6:21 AM Yegor Yefremov
<yegorslists@googlemail.com> wrote:
>
> Hi Adam,
>
> On Sun, Oct 6, 2019 at 4:33 AM Adam Ford <aford173@gmail.com> wrote:
> >
> > There are two checks to see if the manual gpio is configured, but
> > these the check is seeing if the structure is NULL instead it
> > should check to see if there are CTS and/or RTS pins defined.
> >
> > This patch uses checks for those individual pins instead of
> > checking for the structure itself.
> >
> > Signed-off-by: Adam Ford <aford173@gmail.com>
> >
> > diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
> > index c68e2b3a1634..836e736ae188 100644
> > --- a/drivers/tty/serial/8250/8250_omap.c
> > +++ b/drivers/tty/serial/8250/8250_omap.c
> > @@ -141,7 +141,7 @@ static void omap8250_set_mctrl(struct uart_port *port, unsigned int mctrl)
> >
> >         serial8250_do_set_mctrl(port, mctrl);
> >
> > -       if (!up->gpios) {
> > +       if (!mctrl_gpio_to_gpiod(up->gpios, UART_GPIO_RTS)) {
> >                 /*
> >                  * Turn off autoRTS if RTS is lowered and restore autoRTS
> >                  * setting if RTS is raised
> > @@ -456,7 +456,8 @@ static void omap_8250_set_termios(struct uart_port *port,
> >         up->port.status &= ~(UPSTAT_AUTOCTS | UPSTAT_AUTORTS | UPSTAT_AUTOXOFF);
> >
> >         if (termios->c_cflag & CRTSCTS && up->port.flags & UPF_HARD_FLOW &&
> > -           !up->gpios) {
> > +           !mctrl_gpio_to_gpiod(up->gpios, UART_GPIO_RTS) &&
> > +           !mctrl_gpio_to_gpiod(up->gpios, UART_GPIO_CTS)) {
> >                 /* Enable AUTOCTS (autoRTS is enabled when RTS is raised) */
> >                 up->port.status |= UPSTAT_AUTOCTS | UPSTAT_AUTORTS;
> >                 priv->efr |= UART_EFR_CTS;
>
> Looks good to me but !up->gpios must remain as otherwise, we will get
> NULL pointer dereference. What do you think?

I was not seeing up->gpios ever NULL so the contents inside the check
never was executed.  When I removed the check, the performance came
back.  I looked at examples on how other devices checked for RTS and
CTS, and I noticed that the Atmel serial driver did something like the
above.

>
> Also adding some more people who can be interested in testing this approach.

I am open for ideas.  If something is better, but something is either
incorrectly setting up->gpios to non-NULL or the check for non-NULL is
wrong.

adam
>
> Cheers,
> Yegor