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

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Date: Fri,  1 Mar 2024 09:12:04 +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 v3] 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.
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