net: dsa: qca8k: introduce reset via gpio feature

The QCA8337(N) has a RESETn signal on Pin B42 that
triggers a chip reset if the line is pulled low.
The datasheet says that: "The active low duration
must be greater than 10 ms".

This can hopefully fix some of the issues related
to pin strapping in OpenWrt for the EA8500 which
suffers from detection issues after a SoC reset.

Please note that the qca8k_probe() function does
currently require to read the chip's revision
register for identification purposes.

Signed-off-by: Christian Lamparter <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/dsa/qca8k.c

···
 #include <linux/of_platform.h>
 #include <linux/if_bridge.h>
 #include <linux/mdio.h>
+#include <linux/gpio.h>
 #include <linux/etherdevice.h>
 
 #include "qca8k.h"
···
 
 	priv->bus = mdiodev->bus;
 	priv->dev = &mdiodev->dev;
+
+	priv->reset_gpio = devm_gpiod_get_optional(priv->dev, "reset",
+						   GPIOD_ASIS);
+	if (IS_ERR(priv->reset_gpio))
+		return PTR_ERR(priv->reset_gpio);
+
+	if (priv->reset_gpio) {
+		gpiod_set_value_cansleep(priv->reset_gpio, 1);
+		/* The active low duration must be greater than 10 ms
+		 * and checkpatch.pl wants 20 ms.
+		 */
+		msleep(20);
+		gpiod_set_value_cansleep(priv->reset_gpio, 0);
+	}
 
 	/* read the switches ID register */
 	id = qca8k_read(priv, QCA8K_REG_MASK_CTRL);

drivers/net/dsa/qca8k.h

···
 
 #include <linux/delay.h>
 #include <linux/regmap.h>
+#include <linux/gpio.h>
 
 #define QCA8K_NUM_PORTS					7
 
···
 	struct mutex reg_mutex;
 	struct device *dev;
 	struct dsa_switch_ops ops;
+	struct gpio_desc *reset_gpio;
 };
 
 struct qca8k_mib_desc {