Re: [PATCH v3 1/2] gpio: ts4900: Do not set DAT and OE together

Bartosz Golaszewski
2022-03-10T09:07:21Z

On Thu, Mar 10, 2022 at 2:16 AM Kris Bahnsen <kris@embeddedts.com> wrote:
>
> From: Mark Featherston <mark@embeddedTS.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.
>
> Fixes: 9c6686322d74 ("gpio: add Technologic I2C-FPGA gpio support")
>
> Signed-off-by: Mark Featherston <mark@embeddedTS.com>
> Signed-off-by: Kris Bahnsen <kris@embeddedTS.com>
> ---
> drivers/gpio/gpio-ts4900.c | 24 +++++++++++++++++++-----
> 1 file changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpio/gpio-ts4900.c b/drivers/gpio/gpio-ts4900.c
> index d885032cf814..d918d2df4de2 100644
> --- a/drivers/gpio/gpio-ts4900.c
> +++ b/drivers/gpio/gpio-ts4900.c
> @@ -1,7 +1,7 @@
> /*
> * 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 +55,33 @@ 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
>

Applied for fixes, thanks!

Bart