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

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Message-ID: <CAMRc=MeHT4pX1ZRbOz0owDDec5rv+FE84rp464ugffbH5PuS5w@mail.gmail.com>
Date:   Mon, 7 Mar 2022 10:13:11 +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 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