linux-kernel - Re: [PATCH] gpio: ts4900: Do not set DAT and OE together

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Date:   Tue, 8 Mar 2022 09:38:50 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Kris Bahnsen <kris@...eddedts.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Featherston <mark@...eddedts.com>
Subject: Re: [PATCH] gpio: ts4900: Do not set DAT and OE together

On Mon, Mar 7, 2022 at 8:40 PM Kris Bahnsen <kris@...eddedts.com> wrote:
>
> On Mon, 2022-03-07 at 10:13 +0100, Bartosz Golaszewski wrote:
> > On Fri, Mar 4, 2022 at 11:15 PM Kris Bahnsen <kris@...eddedts.com> wrote:
> > >
> > > From: Mark Featherston <mark@...eddedTS.com>
> > >
> > > This works around an issue with the hardware where both OE and
> > > DAT are exposed in the same register. If both are updated
> > > simultaneously, the harware makes no guarantees that OE or DAT
> > > will actually change in any given order and may result in a
> > > glitch of a few ns on a GPIO pin when changing direction and value
> > > in a single write.
> > >
> > > Setting direction to input now only affects OE bit. Setting
> > > direction to output updates DAT first, then OE.
> > >
> > > Signed-off-by: Mark Featherston <mark@...eddedTS.com>
> > > Signed-off-by: Kris Bahnsen <kris@...eddedTS.com>
> > > ---
> > >  drivers/gpio/gpio-ts4900.c | 25 ++++++++++++++++++++-----
> > >  1 file changed, 20 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/gpio/gpio-ts4900.c b/drivers/gpio/gpio-ts4900.c
> > > index d885032cf814..fbabfca030c0 100644
> > > --- a/drivers/gpio/gpio-ts4900.c
> > > +++ b/drivers/gpio/gpio-ts4900.c
> > > @@ -1,7 +1,8 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > >  /*
> > >   * Digital I/O driver for Technologic Systems I2C FPGA Core
> > >   *
> > > - * Copyright (C) 2015 Technologic Systems
> > > + * Copyright (C) 2015-2018 Technologic Systems
> > >   * Copyright (C) 2016 Savoir-Faire Linux
> > >   *
> > >   * This program is free software; you can redistribute it and/or
> > > @@ -55,19 +56,33 @@ static int ts4900_gpio_direction_input(struct gpio_chip *chip,
> > >  {
> > >         struct ts4900_gpio_priv *priv = gpiochip_get_data(chip);
> > >
> > > -       /*
> > > -        * This will clear the output enable bit, the other bits are
> > > -        * dontcare when this is cleared
> > > +       /* Only clear the OE bit here, requires a RMW. Prevents potential issue
> > > +        * with OE and data getting to the physical pin at different times.
> > >          */
> > > -       return regmap_write(priv->regmap, offset, 0);
> > > +       return regmap_update_bits(priv->regmap, offset, TS4900_GPIO_OE, 0);
> > >  }
> > >
> > >  static int ts4900_gpio_direction_output(struct gpio_chip *chip,
> > >                                         unsigned int offset, int value)
> > >  {
> > >         struct ts4900_gpio_priv *priv = gpiochip_get_data(chip);
> > > +       unsigned int reg;
> > >         int ret;
> > >
> > > +       /* If changing from an input to an output, we need to first set the
> > > +        * proper data bit to what is requested and then set OE bit. This
> > > +        * prevents a glitch that can occur on the IO line
> > > +        */
> > > +       regmap_read(priv->regmap, offset, &reg);
> > > +       if (!(reg & TS4900_GPIO_OE)) {
> > > +               if (value)
> > > +                       reg = TS4900_GPIO_OUT;
> > > +               else
> > > +                       reg &= ~TS4900_GPIO_OUT;
> > > +
> > > +               regmap_write(priv->regmap, offset, reg);
> > > +       }
> > > +
> > >         if (value)
> > >                 ret = regmap_write(priv->regmap, offset, TS4900_GPIO_OE |
> > >                                                          TS4900_GPIO_OUT);
> > > --
> > > 2.11.0
> > >
> >
> > This looks like a fix, can you add a Fixes tag?
> >
> > Bart
> >
>
> Please excuse my ignorance (and email client issues) I am still learning the
> submission process. I'm not sure what kind of Fixes tag to add in this scenario.
>
> This GPIO issue has existed since the driver's inception. It is a quirk of
> hardware that has always existed on this platform. The driver was originally
> implemented by Savoir-faire Linux. We discovered the issue and have had it
> patched in our trees for years and wanted to push it upstream.
>
> There is no public discussion on it, it was found and patched. And, aside from
> the first commit of this driver, there is no commit that introduced any issue.
>
> Can you please advise what kind of Fixes tag is appropriate in this situation?
>
> Additionally, if I do add a Fixes tag, would that warrant a v2 patch? Or would
> it just need to be an email response that includes it?
>
> Kris

Yeah, just send a v2 with "Fixes: <short commit hash> ("<commit
title>")" referring to the original patch that added the driver.

We'll send it for stable.

Bart