[RFC PATCH 2/4] mfd: Add driver for Realtek RTS490x I3C hub

From: zain_zhou

Date: Sun Sep 20 2026 - 06:09:46 EST

From: Yin Zhou <zain_zhou@xxxxxxxxxxxxxx>

Add the MFD core driver for the Realtek RTS490x family of I3C hubs.
The driver initializes the shared register map, identifies the hub
variant, and registers the regulator and I3C hub child devices.

Signed-off-by: Yin Zhou <zain_zhou@xxxxxxxxxxxxxx>
---
MAINTAINERS | 2 +
drivers/mfd/Kconfig | 11 +++
drivers/mfd/Makefile | 1 +
drivers/mfd/rts490x-core.c | 131 ++++++++++++++++++++++++++++++++++++
include/linux/mfd/rts490x.h | 53 +++++++++++++++
5 files changed, 198 insertions(+)
create mode 100644 drivers/mfd/rts490x-core.c
create mode 100644 include/linux/mfd/rts490x.h

diff --git a/MAINTAINERS b/MAINTAINERS
index a2c171a28cd2..659f553420eb 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -22998,6 +22998,8 @@ L: linux-i3c@xxxxxxxxxxxxxxxxxxx
L: linux-kernel@xxxxxxxxxxxxxxx
S: Maintained
F: Documentation/devicetree/bindings/i3c/realtek,rts490x.yaml
+F: drivers/mfd/rts490x-core.c
+F: include/linux/mfd/rts490x.h

REALTEK OTTO WATCHDOG
M: Sander Vanheule <sander@xxxxxxxxxxxxx>
diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index ee809b0c1b8f..6da8967de1a2 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -630,6 +630,17 @@ config MFD_P3H2X4X
This driver provides support for I3C hub and regulator, each subdriver
can be enabled independently depending on the required functionality.

+config MFD_RTS490X
+ tristate "Realtek RTS490x I3C hub"
+ depends on I3C
+ select MFD_CORE
+ select REGMAP_I3C
+ help
+ Enable support for the Realtek RTS490x family of I3C hub devices.
+ This driver provides the shared register map and parent state used by
+ the I3C hub and regulator child drivers. The hub is connected to the
+ host through I3C.
+
config MFD_PF1550
tristate "NXP PF1550 PMIC Support"
depends on I2C=y && OF
diff --git a/drivers/mfd/Makefile b/drivers/mfd/Makefile
index ec4e093f3540..97988277bee9 100644
--- a/drivers/mfd/Makefile
+++ b/drivers/mfd/Makefile
@@ -124,6 +124,7 @@ obj-$(CONFIG_MFD_MC13XXX_I2C) += mc13xxx-i2c.o

obj-$(CONFIG_MFD_P3H2X4X) += p3h2840.o
obj-$(CONFIG_MFD_PF1550) += pf1550.o
+obj-$(CONFIG_MFD_RTS490X) += rts490x-core.o

obj-$(CONFIG_MFD_NCT6694) += nct6694.o

diff --git a/drivers/mfd/rts490x-core.c b/drivers/mfd/rts490x-core.c
new file mode 100644
index 000000000000..c10718397167
--- /dev/null
+++ b/drivers/mfd/rts490x-core.c
@@ -0,0 +1,131 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025-2026 Realtek Semiconductor Corp. */
+
+#include <linux/bitfield.h>
+#include <linux/i3c/device.h>
+#include <linux/mfd/core.h>
+#include <linux/mfd/rts490x.h>
+#include <linux/regmap.h>
+
+struct rts490x_device_info {
+ const char *model;
+ u16 part_id;
+ u8 num_target_ports;
+};
+
+static const struct rts490x_device_info rts490x_devices[] = {
+ { "RTS4900", 0x4000, 4 },
+ { "RTS4901", 0x4100, 4 },
+ { "RTS4902", 0x8000, 8 },
+ { "RTS4903", 0x8100, 8 },
+ { "RTS4904", 0x4001, 4 },
+ { "RTS4906", 0x8001, 8 },
+};
+
+static const struct mfd_cell rts490x_cells[] = {
+ MFD_CELL_NAME("rts490x-regulator"),
+ MFD_CELL_NAME("rts490x-i3c-hub"),
+};
+
+static const struct regmap_config rts490x_regmap_config = {
+ .reg_bits = 8,
+ .val_bits = 8,
+ .max_register = 0xff,
+};
+
+static int rts490x_read_device_info(struct device *dev, struct rts490x *rts490x)
+{
+ unsigned int value;
+ u16 part_id;
+ int ret;
+ int i;
+
+ ret = regmap_read(rts490x->regmap, RTS490X_DEV_INFO_0, &value);
+ if (ret)
+ return dev_err_probe(dev, ret, "failed to read device information\n");
+
+ part_id = value << 8;
+
+ ret = regmap_read(rts490x->regmap, RTS490X_DEV_REV, &value);
+ if (ret)
+ return dev_err_probe(dev, ret, "failed to read device revision\n");
+
+ part_id |= FIELD_GET(RTS490X_DEV_REV_LDO_MASK, value);
+
+ for (i = 0; i < ARRAY_SIZE(rts490x_devices); i++) {
+ if (rts490x_devices[i].part_id != part_id)
+ continue;
+
+ rts490x->model = rts490x_devices[i].model;
+ rts490x->part_id = part_id;
+ rts490x->num_target_ports = rts490x_devices[i].num_target_ports;
+ return 0;
+ }
+
+ return dev_err_probe(dev, -ENODEV,
+ "unsupported RTS490x device ID 0x%04x\n", part_id);
+}
+
+static int rts490x_probe(struct i3c_device *i3cdev)
+{
+ struct device *dev = i3cdev_to_dev(i3cdev);
+ struct i3c_device_info info;
+ struct rts490x *rts490x;
+ int ret;
+
+ i3c_device_get_info(i3cdev, &info);
+ if (I3C_PID_MANUF_ID(info.pid) != RTS490X_MANUFACTURER_ID)
+ return -ENODEV;
+
+ rts490x = devm_kzalloc(dev, sizeof(*rts490x), GFP_KERNEL);
+ if (!rts490x)
+ return -ENOMEM;
+
+ ret = devm_mutex_init(dev, &rts490x->protected_reg_lock);
+ if (ret)
+ return ret;
+
+ rts490x->regmap = devm_regmap_init_i3c(i3cdev, &rts490x_regmap_config);
+ if (IS_ERR(rts490x->regmap))
+ return dev_err_probe(dev, PTR_ERR(rts490x->regmap),
+ "failed to initialize regmap\n");
+
+ rts490x->i3cdev = i3cdev;
+ i3cdev_set_drvdata(i3cdev, rts490x);
+
+ ret = rts490x_read_device_info(dev, rts490x);
+ if (ret)
+ return ret;
+
+ ret = devm_mfd_add_devices(dev, PLATFORM_DEVID_AUTO,
+ rts490x_cells, ARRAY_SIZE(rts490x_cells),
+ NULL, 0, NULL);
+ if (ret)
+ return dev_err_probe(dev, ret, "failed to add child devices\n");
+
+ dev_info(dev, "%s I3C hub with %u target ports\n",
+ rts490x->model, rts490x->num_target_ports);
+
+ return 0;
+}
+
+static const struct i3c_device_id rts490x_i3c_ids[] = {
+ {
+ .match_flags = I3C_MATCH_DCR | I3C_MATCH_MANUF,
+ .dcr = I3C_DCR_HUB,
+ .manuf_id = RTS490X_MANUFACTURER_ID,
+ },
+ { }
+};
+MODULE_DEVICE_TABLE(i3c, rts490x_i3c_ids);
+
+static struct i3c_driver rts490x_driver = {
+ .driver.name = "rts490x",
+ .probe = rts490x_probe,
+ .id_table = rts490x_i3c_ids,
+};
+module_i3c_driver(rts490x_driver);
+
+MODULE_AUTHOR("Yin Zhou <zain_zhou@xxxxxxxxxxxxxx>");
+MODULE_DESCRIPTION("Realtek RTS490x I3C hub MFD driver");
+MODULE_LICENSE("GPL");
diff --git a/include/linux/mfd/rts490x.h b/include/linux/mfd/rts490x.h
new file mode 100644
index 000000000000..2af77d22dab9
--- /dev/null
+++ b/include/linux/mfd/rts490x.h
@@ -0,0 +1,53 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2025-2026 Realtek Semiconductor Corp. */
+#ifndef _LINUX_MFD_RTS490X_H
+#define _LINUX_MFD_RTS490X_H
+
+#include <linux/bits.h>
+#include <linux/mutex.h>
+#include <linux/types.h>
+
+#define RTS490X_MAX_TARGET_PORTS 8
+
+/* Device information registers shared by the MFD children. */
+#define RTS490X_DEV_INFO_0 0x00
+#define RTS490X_DEV_REV 0x0b
+#define RTS490X_DEV_REV_LDO_MASK GENMASK(7, 6)
+
+/* Regulator registers shared with the regulator child. */
+#define RTS490X_LDO_CONF 0x16
+#define RTS490X_LDO_AND_PULLUP_CONF 0x19
+
+/* Protected-register access shared by the MFD children. */
+#define RTS490X_PROTECTION_CODE 0x10
+#define RTS490X_REGISTERS_LOCK_CODE 0x00
+#define RTS490X_REGISTERS_UNLOCK_CODE 0x69
+
+#define RTS490X_MANUFACTURER_ID 0x025d
+
+struct i3c_device;
+struct regmap;
+struct rts490x_hub;
+
+/**
+ * struct rts490x - shared state for RTS490x MFD children
+ * @i3cdev: upstream I3C device
+ * @regmap: register map shared by all child drivers
+ * @protected_reg_lock: serializes protected-register transactions
+ * @model: detected chip model
+ * @part_id: hardware part identifier
+ * @num_target_ports: number of implemented target ports
+ * @hub: hub child state used by the hub-device IBI handler
+ */
+struct rts490x {
+ struct i3c_device *i3cdev;
+ struct regmap *regmap;
+ /* Serializes protected-register transactions across child drivers. */
+ struct mutex protected_reg_lock;
+ const char *model;
+ u16 part_id;
+ u8 num_target_ports;
+ struct rts490x_hub *hub;
+};
+
+#endif /* _LINUX_MFD_RTS490X_H */
--
2.34.1