linux-kernel - [PATCH v2] gpio: 74x164: Enable output pins after registers are reset

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Date: Thu, 29 Feb 2024 10:45:56 +0200
From: Arturas Moskvinas <arturas.moskvinas@...il.com>
To: linus.walleij@...aro.org,
	brgl@...ev.pl,
	u.kleine-koenig@...gutronix.de,
	andriy.shevchenko@...ux.intel.com
Cc: linux-gpio@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Arturas Moskvinas <arturas.moskvinas@...il.com>
Subject: [PATCH v2] gpio: 74x164: Enable output pins after registers are reset

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 in chip.
Change fixes 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.

[1] - https://elixir.bootlin.com/linux/v6.7.4/source/drivers/gpio/gpio-74x164.c#L130
[2] - https://elixir.bootlin.com/linux/v6.7.4/source/drivers/gpio/gpio-74x164.c#L150

Signed-off-by: Arturas Moskvinas <arturas.moskvinas@...il.com>

---
v1 -> v2
* Updated commit message to contain more information why a 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