i2c: Add Nuvoton NCT6694 I2C support

This driver supports I2C adapter functionality for NCT6694 MFD
device based on USB interface.

Each I2C controller uses the default baudrate of 100kHz, which
can be overridden via module parameters.

Acked-by: Andi Shyti <[e-mail]>
Reviewed-by: Wolfram Sang <[e-mail]>
Signed-off-by: Ming Yu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

MAINTAINERS

···
 M:	Ming Yu <[e-mail]>
 S:	Supported
 F:	drivers/gpio/gpio-nct6694.c
+F:	drivers/i2c/busses/i2c-nct6694.c
 F:	drivers/mfd/nct6694.c
 F:	include/linux/mfd/nct6694.h
 

drivers/i2c/busses/Kconfig

···
 	  This driver can also be built as a module.  If so, the module
 	  will be called i2c-ljca.
 
+config I2C_NCT6694
+	tristate "Nuvoton NCT6694 I2C adapter support"
+	depends on MFD_NCT6694
+	help
+	  If you say yes to this option, support will be included for Nuvoton
+	  NCT6694, a USB to I2C interface.
+
+	  This driver can also be built as a module. If so, the module will
+	  be called i2c-nct6694.
+
 config I2C_CP2615
 	tristate "Silicon Labs CP2615 USB sound card and I2C adapter"
 	depends on USB

drivers/i2c/busses/Makefile

···
 obj-$(CONFIG_I2C_DIOLAN_U2C)	+= i2c-diolan-u2c.o
 obj-$(CONFIG_I2C_DLN2)		+= i2c-dln2.o
 obj-$(CONFIG_I2C_LJCA)		+= i2c-ljca.o
+obj-$(CONFIG_I2C_NCT6694)	+= i2c-nct6694.o
 obj-$(CONFIG_I2C_CP2615) += i2c-cp2615.o
 obj-$(CONFIG_I2C_PARPORT)	+= i2c-parport.o
 obj-$(CONFIG_I2C_PCI1XXXX)	+= i2c-mchp-pci1xxxx.o

drivers/i2c/busses/i2c-nct6694.c

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Nuvoton NCT6694 I2C adapter driver based on USB interface.
+ *
+ * Copyright (C) 2025 Nuvoton Technology Corp.
+ */
+
+#include <linux/i2c.h>
+#include <linux/idr.h>
+#include <linux/kernel.h>
+#include <linux/mfd/nct6694.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+
+/*
+ * USB command module type for NCT6694 I2C controller.
+ * This defines the module type used for communication with the NCT6694
+ * I2C controller over the USB interface.
+ */
+#define NCT6694_I2C_MOD			0x03
+
+/* Command 00h - I2C Deliver */
+#define NCT6694_I2C_DELIVER		0x00
+#define NCT6694_I2C_DELIVER_SEL		0x00
+
+#define NCT6694_I2C_MAX_XFER_SIZE	64
+#define NCT6694_I2C_MAX_DEVS		6
+
+static unsigned char br_reg[NCT6694_I2C_MAX_DEVS] = {[0 ... (NCT6694_I2C_MAX_DEVS - 1)] = 0xFF};
+
+module_param_array(br_reg, byte, NULL, 0644);
+MODULE_PARM_DESC(br_reg,
+		 "I2C Baudrate register per adapter: (0=25K, 1=50K, 2=100K, 3=200K, 4=400K, 5=800K, 6=1M), default=2");
+
+enum nct6694_i2c_baudrate {
+	NCT6694_I2C_BR_25K = 0,
+	NCT6694_I2C_BR_50K,
+	NCT6694_I2C_BR_100K,
+	NCT6694_I2C_BR_200K,
+	NCT6694_I2C_BR_400K,
+	NCT6694_I2C_BR_800K,
+	NCT6694_I2C_BR_1M
+};
+
+struct __packed nct6694_i2c_deliver {
+	u8 port;
+	u8 br;
+	u8 addr;
+	u8 w_cnt;
+	u8 r_cnt;
+	u8 rsv[11];
+	u8 write_data[NCT6694_I2C_MAX_XFER_SIZE];
+	u8 read_data[NCT6694_I2C_MAX_XFER_SIZE];
+};
+
+struct nct6694_i2c_data {
+	struct device *dev;
+	struct nct6694 *nct6694;
+	struct i2c_adapter adapter;
+	struct nct6694_i2c_deliver deliver;
+	unsigned char port;
+	unsigned char br;
+};
+
+static int nct6694_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
+{
+	struct nct6694_i2c_data *data = adap->algo_data;
+	struct nct6694_i2c_deliver *deliver = &data->deliver;
+	static const struct nct6694_cmd_header cmd_hd = {
+		.mod = NCT6694_I2C_MOD,
+		.cmd = NCT6694_I2C_DELIVER,
+		.sel = NCT6694_I2C_DELIVER_SEL,
+		.len = cpu_to_le16(sizeof(*deliver))
+	};
+	int ret, i;
+
+	for (i = 0; i < num; i++) {
+		struct i2c_msg *msg_temp = &msgs[i];
+
+		memset(deliver, 0, sizeof(*deliver));
+
+		deliver->port = data->port;
+		deliver->br = data->br;
+		deliver->addr = i2c_8bit_addr_from_msg(msg_temp);
+		if (msg_temp->flags & I2C_M_RD) {
+			deliver->r_cnt = msg_temp->len;
+			ret = nct6694_write_msg(data->nct6694, &cmd_hd, deliver);
+			if (ret < 0)
+				return ret;
+
+			memcpy(msg_temp->buf, deliver->read_data, msg_temp->len);
+		} else {
+			deliver->w_cnt = msg_temp->len;
+			memcpy(deliver->write_data, msg_temp->buf, msg_temp->len);
+			ret = nct6694_write_msg(data->nct6694, &cmd_hd, deliver);
+			if (ret < 0)
+				return ret;
+		}
+	}
+
+	return num;
+}
+
+static u32 nct6694_i2c_func(struct i2c_adapter *adapter)
+{
+	return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;
+}
+
+static const struct i2c_adapter_quirks nct6694_i2c_quirks = {
+	.max_read_len = NCT6694_I2C_MAX_XFER_SIZE,
+	.max_write_len = NCT6694_I2C_MAX_XFER_SIZE,
+};
+
+static const struct i2c_algorithm nct6694_i2c_algo = {
+	.xfer = nct6694_i2c_xfer,
+	.functionality = nct6694_i2c_func,
+};
+
+static int nct6694_i2c_set_baudrate(struct nct6694_i2c_data *data)
+{
+	if (data->port >= NCT6694_I2C_MAX_DEVS) {
+		dev_err(data->dev, "Invalid I2C port index %d\n", data->port);
+		return -EINVAL;
+	}
+
+	if (br_reg[data->port] > NCT6694_I2C_BR_1M) {
+		dev_warn(data->dev, "Invalid baudrate %d for I2C%d, using 100K\n",
+			 br_reg[data->port], data->port);
+		br_reg[data->port] = NCT6694_I2C_BR_100K;
+	}
+
+	data->br = br_reg[data->port];
+
+	return 0;
+}
+
+static void nct6694_i2c_ida_free(void *d)
+{
+	struct nct6694_i2c_data *data = d;
+	struct nct6694 *nct6694 = data->nct6694;
+
+	ida_free(&nct6694->i2c_ida, data->port);
+}
+
+static int nct6694_i2c_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct nct6694 *nct6694 = dev_get_drvdata(dev->parent);
+	struct nct6694_i2c_data *data;
+	int ret;
+
+	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	data->dev = dev;
+	data->nct6694 = nct6694;
+
+	ret = ida_alloc(&nct6694->i2c_ida, GFP_KERNEL);
+	if (ret < 0)
+		return ret;
+	data->port = ret;
+
+	ret = devm_add_action_or_reset(dev, nct6694_i2c_ida_free, data);
+	if (ret)
+		return ret;
+
+	ret = nct6694_i2c_set_baudrate(data);
+	if (ret)
+		return ret;
+
+	sprintf(data->adapter.name, "NCT6694 I2C Adapter %d", data->port);
+	data->adapter.owner = THIS_MODULE;
+	data->adapter.algo = &nct6694_i2c_algo;
+	data->adapter.quirks = &nct6694_i2c_quirks;
+	data->adapter.dev.parent = dev;
+	data->adapter.algo_data = data;
+
+	platform_set_drvdata(pdev, data);
+
+	return devm_i2c_add_adapter(dev, &data->adapter);
+}
+
+static struct platform_driver nct6694_i2c_driver = {
+	.driver = {
+		.name	= "nct6694-i2c",
+	},
+	.probe		= nct6694_i2c_probe,
+};
+
+module_platform_driver(nct6694_i2c_driver);
+
+MODULE_DESCRIPTION("USB-I2C adapter driver for NCT6694");
+MODULE_AUTHOR("Ming Yu <[e-mail]>");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:nct6694-i2c");