[PATCH v4 3/5] mfd: aaeon: Add SRG-IMX8P MCU driver

Thomas Perrot (Schneider Electric)
2026-03-24T19:24:55Z

Add Multi-Function Device (MFD) driver for the Aaeon SRG-IMX8P
embedded controller. This driver provides the core I2C communication
interface and registers child devices (GPIO and watchdog controllers).

The driver implements a custom regmap bus over I2C to match the MCU's
fixed 3-byte command format [opcode, arg, value]. Register addresses
are encoded as 16-bit values (opcode << 8 | arg) using the
AAEON_MCU_REG() macro defined in the shared header. The regmap
instance is shared with child drivers via dev_get_regmap(). Concurrent
I2C accesses from child drivers are serialized by regmap's built-in
locking.

Co-developed-by: Jérémie Dautheribes (Schneider Electric) <jeremie.dautheribes@bootlin.com>
Signed-off-by: Jérémie Dautheribes (Schneider Electric) <jeremie.dautheribes@bootlin.com>
Signed-off-by: Thomas Perrot (Schneider Electric) <thomas.perrot@bootlin.com>
---
MAINTAINERS | 2 +
drivers/mfd/Kconfig | 10 +++
drivers/mfd/Makefile | 1 +
drivers/mfd/aaeon-mcu.c | 155 ++++++++++++++++++++++++++++++++++++++++++
include/linux/mfd/aaeon-mcu.h | 20 ++++++
5 files changed, 188 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index ea9d55f76f3509c7f6ba6d1bc86ca2e2e71aa954..f91b6a1826d04bef8a0f88221f6c8e8a3652cd77 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -191,6 +191,8 @@ M: Thomas Perrot <thomas.perrot@bootlin.com>
R: Jérémie Dautheribes <jeremie.dautheribes@bootlin.com>
S: Maintained
F: Documentation/devicetree/bindings/mfd/aaeon,srg-imx8p-mcu.yaml
+F: drivers/mfd/aaeon-mcu.c
+F: include/linux/mfd/aaeon-mcu.h

AAEON UPBOARD FPGA MFD DRIVER
M: Thomas Richard <thomas.richard@bootlin.com>
diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index aace5766b38aa5e46e32a8a7b42eea238159fbcf..7a1ceedece899faad7a03a1fe7b1c91b72253c05 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -1574,6 +1574,16 @@ config AB8500_CORE
the irq_chip parts for handling the Mixed Signal chip events.
This chip embeds various other multimedia functionalities as well.

+config MFD_AAEON_MCU
+ tristate "Aaeon SRG-IMX8P MCU Driver"
+ depends on I2C || COMPILE_TEST
+ select MFD_CORE
+ help
+ Select this option to enable support for the Aaeon SRG-IMX8P
+ onboard microcontroller (MCU). This driver provides the core
+ functionality to communicate with the MCU over I2C. The MCU
+ provides GPIO and watchdog functionality.
+
config MFD_DB8500_PRCMU
bool "ST-Ericsson DB8500 Power Reset Control Management Unit"
depends on UX500_SOC_DB8500
diff --git a/drivers/mfd/Makefile b/drivers/mfd/Makefile
index e75e8045c28afae975ac61d282b3b85af5440119..34db5b033584368b7a269b1eef12528a74baf8f5 100644
--- a/drivers/mfd/Makefile
+++ b/drivers/mfd/Makefile
@@ -8,6 +8,7 @@ obj-$(CONFIG_MFD_88PM860X) += 88pm860x.o
obj-$(CONFIG_MFD_88PM800) += 88pm800.o 88pm80x.o
obj-$(CONFIG_MFD_88PM805) += 88pm805.o 88pm80x.o
obj-$(CONFIG_MFD_88PM886_PMIC) += 88pm886.o
+obj-$(CONFIG_MFD_AAEON_MCU) += aaeon-mcu.o
obj-$(CONFIG_MFD_ACT8945A) += act8945a.o
obj-$(CONFIG_MFD_SM501) += sm501.o
obj-$(CONFIG_ARCH_BCM2835) += bcm2835-pm.o
diff --git a/drivers/mfd/aaeon-mcu.c b/drivers/mfd/aaeon-mcu.c
new file mode 100644
index 0000000000000000000000000000000000000000..5a969890d201c027eb25c324b4d4d89b1f8c563e
--- /dev/null
+++ b/drivers/mfd/aaeon-mcu.c
@@ -0,0 +1,155 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Aaeon MCU driver
+ *
+ * Copyright (C) 2025 Bootlin
+ * Author: Jérémie Dautheribes <jeremie.dautheribes@bootlin.com>
+ * Author: Thomas Perrot <thomas.perrot@bootlin.com>
+ */
+
+#include <linux/err.h>
+#include <linux/i2c.h>
+#include <linux/mfd/core.h>
+#include <linux/platform_device.h>
+#include <linux/regmap.h>
+
+static const struct mfd_cell aaeon_mcu_devs[] = {
+ {
+ .name = "aaeon-mcu-wdt",
+ },
+ {
+ .name = "aaeon-mcu-gpio",
+ },
+};
+
+/*
+ * Custom regmap bus for the Aaeon MCU I2C protocol.
+ *
+ * The MCU uses a fixed 3-byte command format [opcode, arg, value] followed
+ * by a 1-byte response. It requires a STOP condition between the command
+ * write and the response read, so two separate i2c_transfer() calls are
+ * issued. The regmap lock serialises concurrent accesses from the GPIO
+ * and watchdog child drivers.
+ *
+ * Register addresses are encoded as a 16-bit big-endian value where the
+ * high byte is the opcode and the low byte is the argument, matching the
+ * wire layout produced by regmap for reg_bits=16.
+ */
+
+static int aaeon_mcu_regmap_write(void *context, const void *data, size_t count)
+{
+ struct i2c_client *client = context;
+ /* data = [opcode, arg, value] as formatted by regmap */
+ struct i2c_msg write_msg = {
+ .addr = client->addr,
+ .flags = 0,
+ .buf = (u8 *)data,
+ .len = count,
+ };
+ u8 rsp;
+ /* The MCU always sends a response byte after each command; discard it. */
+ struct i2c_msg rsp_msg = {
+ .addr = client->addr,
+ .flags = I2C_M_RD,
+ .buf = &rsp,
+ .len = 1,
+ };
+ int ret;
+
+ ret = i2c_transfer(client->adapter, &write_msg, 1);
+ if (ret < 0)
+ return ret;
+ if (ret != 1)
+ return -EIO;
+
+ ret = i2c_transfer(client->adapter, &rsp_msg, 1);
+ if (ret < 0)
+ return ret;
+ if (ret != 1)
+ return -EIO;
+
+ return 0;
+}
+
+static int aaeon_mcu_regmap_read(void *context, const void *reg_buf,
+ size_t reg_size, void *val_buf, size_t val_size)
+{
+ struct i2c_client *client = context;
+ /*
+ * reg_buf holds the 2-byte big-endian register address [opcode, arg].
+ * Append a trailing 0x00 to form the full 3-byte MCU command.
+ */
+ u8 cmd[3] = { ((u8 *)reg_buf)[0], ((u8 *)reg_buf)[1], 0x00 };
+ struct i2c_msg write_msg = {
+ .addr = client->addr,
+ .flags = 0,
+ .buf = cmd,
+ .len = sizeof(cmd),
+ };
+ struct i2c_msg read_msg = {
+ .addr = client->addr,
+ .flags = I2C_M_RD,
+ .buf = val_buf,
+ .len = val_size,
+ };
+ int ret;
+
+ ret = i2c_transfer(client->adapter, &write_msg, 1);
+ if (ret < 0)
+ return ret;
+ if (ret != 1)
+ return -EIO;
+
+ ret = i2c_transfer(client->adapter, &read_msg, 1);
+ if (ret < 0)
+ return ret;
+ if (ret != 1)
+ return -EIO;
+
+ return 0;
+}
+
+static const struct regmap_bus aaeon_mcu_regmap_bus = {
+ .write = aaeon_mcu_regmap_write,
+ .read = aaeon_mcu_regmap_read,
+};
+
+static const struct regmap_config aaeon_mcu_regmap_config = {
+ .reg_bits = 16,
+ .val_bits = 8,
+ .reg_format_endian = REGMAP_ENDIAN_BIG,
+ .cache_type = REGCACHE_NONE,
+};
+
+static int aaeon_mcu_probe(struct i2c_client *client)
+{
+ struct regmap *regmap;
+
+ regmap = devm_regmap_init(&client->dev, &aaeon_mcu_regmap_bus,
+ client, &aaeon_mcu_regmap_config);
+ if (IS_ERR(regmap))
+ return PTR_ERR(regmap);
+
+ return devm_mfd_add_devices(&client->dev, PLATFORM_DEVID_NONE,
+ aaeon_mcu_devs, ARRAY_SIZE(aaeon_mcu_devs),
+ NULL, 0, NULL);
+}
+
+static const struct of_device_id aaeon_mcu_of_match[] = {
+ { .compatible = "aaeon,srg-imx8p-mcu" },
+ {},
+};
+MODULE_DEVICE_TABLE(of, aaeon_mcu_of_match);
+
+static struct i2c_driver aaeon_mcu_driver = {
+ .driver = {
+ .name = "aaeon_mcu",
+ .of_match_table = aaeon_mcu_of_match,
+ },
+ .probe = aaeon_mcu_probe,
+};
+module_i2c_driver(aaeon_mcu_driver);
+
+MODULE_DESCRIPTION("Aaeon MCU Driver");
+MODULE_AUTHOR("Jérémie Dautheribes <jeremie.dautheribes@bootlin.com>");
+MODULE_LICENSE("GPL");
diff --git a/include/linux/mfd/aaeon-mcu.h b/include/linux/mfd/aaeon-mcu.h
new file mode 100644
index 0000000000000000000000000000000000000000..861003f6dfd20424c3785008bd2cf89aaa1715b9
--- /dev/null
+++ b/include/linux/mfd/aaeon-mcu.h
@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Aaeon MCU driver definitions
+ *
+ * Copyright (C) 2025 Bootlin
+ * Author: Jérémie Dautheribes <jeremie.dautheribes@bootlin.com>
+ * Author: Thomas Perrot <thomas.perrot@bootlin.com>
+ */
+
+#ifndef __LINUX_MFD_AAEON_MCU_H
+#define __LINUX_MFD_AAEON_MCU_H
+
+/*
+ * MCU register address: the high byte is the command opcode, the low
+ * byte is the argument. This matches the 3-byte wire format
+ * [opcode, arg, value] used by the MCU I2C protocol.
+ */
+#define AAEON_MCU_REG(op, arg) (((op) << 8) | (arg))
+
+#endif /* __LINUX_MFD_AAEON_MCU_H */

--
2.53.0