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

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Date:   Sun, 6 Oct 2019 06:38:34 -0500
From:   Adam Ford <aford173@...il.com>
To:     Yegor Yefremov <yegorslists@...glemail.com>
Cc:     linux-serial@...r.kernel.org, Adam Ford <adam.ford@...icpd.com>,
        Linux-OMAP <linux-omap@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Vignesh R <vigneshr@...com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Tony Lindgren <tony@...mide.com>,
        Douglas Anderson <dianders@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Nuno Gonçalves <nunojpg@...il.com>,
        Giulio Benetti <giulio.benetti@...ettiengineering.com>,
        Stefan Roese <sr@...x.de>
Subject: Re: [PATCH] serial: 8250_omap: Fix gpio check for auto RTS and CTS

On Sun, Oct 6, 2019 at 6:21 AM Yegor Yefremov
<yegorslists@...glemail.com> wrote:
>
> Hi Adam,
>
> On Sun, Oct 6, 2019 at 4:33 AM Adam Ford <aford173@...il.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@...il.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