gpio: fxl6408: add I2C GPIO expander driver

Add minimal driver for Fairchild FXL6408 8-bit I2C-controlled GPIO expander
using the generic regmap based GPIO driver (GPIO_REGMAP).

The driver implements setting the GPIO direction, reading inputs
and writing outputs.

In addition to that the FXL6408 has the following functionalities:
- allows to monitor input ports for data transitions with an interrupt pin
- all inputs can be configured with pull-up or pull-down resistors

Datasheet: https://www.onsemi.com/download/data-sheet/pdf/fxl6408-d.pdf
Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Co-developed-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[Bartosz: order includes alphabetically]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/Kconfig

@@ -1007,6 +1007,16 @@ config GPIO_ADNP
 	  enough to represent all pins, but the driver will assume a
 	  register layout for 64 pins (8 registers).
 
+config GPIO_FXL6408
+	tristate "FXL6408 I2C GPIO expander"
+	select GPIO_REGMAP
+	select REGMAP_I2C
+	help
+	  GPIO driver for Fairchild Semiconductor FXL6408 GPIO expander.
+
+	  To compile this driver as a module, choose M here: the module will
+	  be called gpio-fxl6408.
+
 config GPIO_GW_PLD
 	tristate "Gateworks PLD GPIO Expander"
 	depends on OF_GPIO

drivers/gpio/Makefile

@@ -60,6 +60,7 @@ obj-$(CONFIG_GPIO_EP93XX)		+= gpio-ep93xx.o
 obj-$(CONFIG_GPIO_EXAR)			+= gpio-exar.o
 obj-$(CONFIG_GPIO_F7188X)		+= gpio-f7188x.o
 obj-$(CONFIG_GPIO_FTGPIO010)		+= gpio-ftgpio010.o
+obj-$(CONFIG_GPIO_FXL6408)		+= gpio-fxl6408.o
 obj-$(CONFIG_GPIO_GE_FPGA)		+= gpio-ge.o
 obj-$(CONFIG_GPIO_GPIO_MM)		+= gpio-gpio-mm.o
 obj-$(CONFIG_GPIO_GRGPIO)		+= gpio-grgpio.o

drivers/gpio/gpio-fxl6408.c

@@ -0,0 +1,158 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * FXL6408 GPIO driver
+ *
+ * Copyright 2023 Toradex
+ *
+ * Author: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
+ */
+
+#include <linux/err.h>
+#include <linux/gpio/regmap.h>
+#include <linux/i2c.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/regmap.h>
+
+#define FXL6408_REG_DEVICE_ID		0x01
+#define FXL6408_MF_FAIRCHILD		0b101
+#define FXL6408_MF_SHIFT		5
+
+/* Bits set here indicate that the GPIO is an output. */
+#define FXL6408_REG_IO_DIR		0x03
+
+/*
+ * Bits set here, when the corresponding bit of IO_DIR is set, drive
+ * the output high instead of low.
+ */
+#define FXL6408_REG_OUTPUT		0x05
+
+/* Bits here make the output High-Z, instead of the OUTPUT value. */
+#define FXL6408_REG_OUTPUT_HIGH_Z	0x07
+
+/* Returns the current status (1 = HIGH) of the input pins. */
+#define FXL6408_REG_INPUT_STATUS	0x0f
+
+/*
+ * Return the current interrupt status
+ * This bit is HIGH if input GPIO != default state (register 09h).
+ * The flag is cleared after being read (bit returns to 0).
+ * The input must go back to default state and change again before this flag is raised again.
+ */
+#define FXL6408_REG_INT_STS		0x13
+
+#define FXL6408_NGPIO			8
+
+static const struct regmap_range rd_range[] = {
+	{ FXL6408_REG_DEVICE_ID, FXL6408_REG_DEVICE_ID },
+	{ FXL6408_REG_IO_DIR, FXL6408_REG_OUTPUT },
+	{ FXL6408_REG_INPUT_STATUS, FXL6408_REG_INPUT_STATUS },
+};
+
+static const struct regmap_range wr_range[] = {
+	{ FXL6408_REG_DEVICE_ID, FXL6408_REG_DEVICE_ID },
+	{ FXL6408_REG_IO_DIR, FXL6408_REG_OUTPUT },
+	{ FXL6408_REG_OUTPUT_HIGH_Z, FXL6408_REG_OUTPUT_HIGH_Z },
+};
+
+static const struct regmap_range volatile_range[] = {
+	{ FXL6408_REG_DEVICE_ID, FXL6408_REG_DEVICE_ID },
+	{ FXL6408_REG_INPUT_STATUS, FXL6408_REG_INPUT_STATUS },
+};
+
+static const struct regmap_access_table rd_table = {
+	.yes_ranges = rd_range,
+	.n_yes_ranges = ARRAY_SIZE(rd_range),
+};
+
+static const struct regmap_access_table wr_table = {
+	.yes_ranges = wr_range,
+	.n_yes_ranges = ARRAY_SIZE(wr_range),
+};
+
+static const struct regmap_access_table volatile_table = {
+	.yes_ranges = volatile_range,
+	.n_yes_ranges = ARRAY_SIZE(volatile_range),
+};
+
+static const struct regmap_config regmap = {
+	.reg_bits = 8,
+	.val_bits = 8,
+
+	.max_register = FXL6408_REG_INT_STS,
+	.wr_table = &wr_table,
+	.rd_table = &rd_table,
+	.volatile_table = &volatile_table,
+
+	.cache_type = REGCACHE_RBTREE,
+	.num_reg_defaults_raw = FXL6408_REG_INT_STS + 1,
+};
+
+static int fxl6408_identify(struct device *dev, struct regmap *regmap)
+{
+	int val, ret;
+
+	ret = regmap_read(regmap, FXL6408_REG_DEVICE_ID, &val);
+	if (ret)
+		return dev_err_probe(dev, ret, "error reading DEVICE_ID\n");
+	if (val >> FXL6408_MF_SHIFT != FXL6408_MF_FAIRCHILD)
+		return dev_err_probe(dev, -ENODEV, "invalid device id 0x%02x\n", val);
+
+	return 0;
+}
+
+static int fxl6408_probe(struct i2c_client *client)
+{
+	struct device *dev = &client->dev;
+	int ret;
+	struct gpio_regmap_config gpio_config = {
+		.parent = dev,
+		.ngpio = FXL6408_NGPIO,
+		.reg_dat_base = GPIO_REGMAP_ADDR(FXL6408_REG_INPUT_STATUS),
+		.reg_set_base = GPIO_REGMAP_ADDR(FXL6408_REG_OUTPUT),
+		.reg_dir_out_base = GPIO_REGMAP_ADDR(FXL6408_REG_IO_DIR),
+		.ngpio_per_reg = FXL6408_NGPIO,
+	};
+
+	gpio_config.regmap = devm_regmap_init_i2c(client, &regmap);
+	if (IS_ERR(gpio_config.regmap))
+		return dev_err_probe(dev, PTR_ERR(gpio_config.regmap),
+				     "failed to allocate register map\n");
+
+	ret = fxl6408_identify(dev, gpio_config.regmap);
+	if (ret)
+		return ret;
+
+	/* Disable High-Z of outputs, so that our OUTPUT updates actually take effect. */
+	ret = regmap_write(gpio_config.regmap, FXL6408_REG_OUTPUT_HIGH_Z, 0);
+	if (ret)
+		return dev_err_probe(dev, ret, "failed to write 'output high Z' register\n");
+
+	return PTR_ERR_OR_ZERO(devm_gpio_regmap_register(dev, &gpio_config));
+}
+
+static const __maybe_unused struct of_device_id fxl6408_dt_ids[] = {
+	{ .compatible = "fcs,fxl6408" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, fxl6408_dt_ids);
+
+static const struct i2c_device_id fxl6408_id[] = {
+	{ "fxl6408", 0 },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, fxl6408_id);
+
+static struct i2c_driver fxl6408_driver = {
+	.driver = {
+		.name	= "fxl6408",
+		.of_match_table = fxl6408_dt_ids,
+	},
+	.probe_new	= fxl6408_probe,
+	.id_table	= fxl6408_id,
+};
+module_i2c_driver(fxl6408_driver);
+
+MODULE_AUTHOR("Emanuele Ghidoli <emanuele.ghidoli@toradex.com>");
+MODULE_DESCRIPTION("FXL6408 GPIO driver");
+MODULE_LICENSE("GPL");