[PATCH 2/2] gpio: pcf857x: add support for reset-gpios on (most) PCA967x

[Quentin Schulz <foss+kernel@0leil.net>]

From: Quentin Schulz <quentin.schulz@cherry.de>

The PCA9670, PCA9671, PCA9672 and PCA9673 all have a RESETN input pin
that is used to reset the I2C GPIO expander.

One needs to hold this pin low for at least 4us and the reset should be
finished after about 100us according to the datasheet[1]. Once the reset
is done, the "registers and I2C-bus state machine will be held in their
default state until the RESET input is once again HIGH.".

Because the logic is reset, the latch values eventually provided in the
Device Tree via lines-initial-states property are inapplicable so they
are simply ignored if a reset GPIO is provided.

[1] https://www.nxp.com/docs/en/data-sheet/PCA9670.pdf 8.5 and fig 22.
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
---
 drivers/gpio/gpio-pcf857x.c | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/drivers/gpio/gpio-pcf857x.c b/drivers/gpio/gpio-pcf857x.c
index 7c57eaeb0afeba8953d998d8eec60a65b40efb6d..94077208e24ae99a1e8762e783f0eabc580fa520 100644
--- a/drivers/gpio/gpio-pcf857x.c
+++ b/drivers/gpio/gpio-pcf857x.c
@@ -5,6 +5,7 @@
  * Copyright (C) 2007 David Brownell
  */
 
+#include <linux/delay.h>
 #include <linux/gpio/driver.h>
 #include <linux/i2c.h>
 #include <linux/interrupt.h>
@@ -272,12 +273,11 @@ static const struct irq_chip pcf857x_irq_chip = {
 
 static int pcf857x_probe(struct i2c_client *client)
 {
+	struct gpio_desc *rstn_gpio;
 	struct pcf857x *gpio;
-	unsigned int n_latch = 0;
+	unsigned int n_latch;
 	int status;
 
-	device_property_read_u32(&client->dev, "lines-initial-states", &n_latch);
-
 	/* Allocate, initialize, and register this gpio_chip. */
 	gpio = devm_kzalloc(&client->dev, sizeof(*gpio), GFP_KERNEL);
 	if (!gpio)
@@ -297,6 +297,29 @@ static int pcf857x_probe(struct i2c_client *client)
 	gpio->chip.direction_output	= pcf857x_output;
 	gpio->chip.ngpio		= (uintptr_t)i2c_get_match_data(client);
 
+	rstn_gpio = devm_gpiod_get_optional(&client->dev, "reset", GPIOD_OUT_HIGH);
+	if (IS_ERR(rstn_gpio)) {
+		return dev_err_probe(&client->dev, PTR_ERR(rstn_gpio),
+				     "failed to get reset GPIO\n");
+	}
+
+	if (rstn_gpio) {
+		/* Reset already held with devm_gpiod_get_optional with GPIOD_OUT_HIGH */
+		usleep_range(4, 8); /* tw(rst) > 4us */
+		gpiod_set_value(rstn_gpio, 0);
+		usleep_range(100, 200); /* trst > 100uS */
+
+		/*
+		 * Reset "will initialize to their default states of all I/Os to
+		 * inputs with weak current source to VDD", which is the same as
+		 * writing 1 for all I/Os which is 0 in n_latch.
+		 */
+		n_latch = 0;
+	} else {
+		device_property_read_u32(&client->dev, "lines-initial-states",
+					 &n_latch);
+	}
+
 	/* NOTE:  the OnSemi jlc1562b is also largely compatible with
 	 * these parts, notably for output.  It has a low-resolution
 	 * DAC instead of pin change IRQs; and its inputs can be the

-- 
2.48.1