linux-kernel - Re: [PATCH v3] gpio: 74x164: Enable output pins after registers are reset

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Date: Fri, 1 Mar 2024 08:23:27 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Arturas Moskvinas <arturas.moskvinas@...il.com>
Cc: linus.walleij@...aro.org, u.kleine-koenig@...gutronix.de, 
	andriy.shevchenko@...ux.intel.com, linux-gpio@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] gpio: 74x164: Enable output pins after registers are reset

On Fri, Mar 1, 2024 at 8:12 AM Arturas Moskvinas
<arturas.moskvinas@...il.com> wrote:
>
> Chip outputs are enabled[1] before actual reset is performed[2] which might
> cause pin output value to flip flop if previous pin value was set to 1.
> Fix that behavior by making sure chip is fully reset before all outputs are
> enabled.
>
> Flip-flop can be noticed when module is removed and inserted again and one of
> the pins was changed to 1 before removal. 100 microsecond flipping is
> noticeable on oscilloscope (100khz SPI bus).
>
> For a properly reset chip - output is enabled around 100 microseconds (on 100khz
> SPI bus) later during probing process hence should be irrelevant behavioral
> change.
>
> Fixes: 7ebc194d0fd4 (gpio: 74x164: Introduce 'enable-gpios' property)
> Link: https://elixir.bootlin.com/linux/v6.7.4/source/drivers/gpio/gpio-74x164.c#L130 [1]
> Link: https://elixir.bootlin.com/linux/v6.7.4/source/drivers/gpio/gpio-74x164.c#L150 [2]
> Signed-off-by: Arturas Moskvinas <arturas.moskvinas@...il.com>
>
> ---
> v2 -> v3
> * Updated commit message to be imperatively moody
> * Converted links to proper tags
> * Added Fixes tag
>
> v1 -> v2
> * Updated commit message to contain more information why change is made.
> ---
>  drivers/gpio/gpio-74x164.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpio/gpio-74x164.c b/drivers/gpio/gpio-74x164.c
> index e00c33310517..753e7be039e4 100644
> --- a/drivers/gpio/gpio-74x164.c
> +++ b/drivers/gpio/gpio-74x164.c
> @@ -127,8 +127,6 @@ static int gen_74x164_probe(struct spi_device *spi)
>         if (IS_ERR(chip->gpiod_oe))
>                 return PTR_ERR(chip->gpiod_oe);
>
> -       gpiod_set_value_cansleep(chip->gpiod_oe, 1);
> -
>         spi_set_drvdata(spi, chip);
>
>         chip->gpio_chip.label = spi->modalias;
> @@ -153,6 +151,8 @@ static int gen_74x164_probe(struct spi_device *spi)
>                 goto exit_destroy;
>         }
>
> +       gpiod_set_value_cansleep(chip->gpiod_oe, 1);
> +
>         ret = gpiochip_add_data(&chip->gpio_chip, chip);
>         if (!ret)
>                 return 0;
>
> base-commit: d206a76d7d2726f3b096037f2079ce0bd3ba329b
> --
> 2.44.0
>

Queued for fixes.

Bart