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

[Thomas Perrot (Schneider Electric)]

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@xxxxxxxxxxx>
Signed-off-by: Jérémie Dautheribes (Schneider Electric) <jeremie.dautheribes@xxxxxxxxxxx>
Signed-off-by: Thomas Perrot (Schneider Electric) <thomas.perrot@xxxxxxxxxxx>
---
 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@xxxxxxxxxxx>
 R:	Jérémie Dautheribes <jeremie.dautheribes@xxxxxxxxxxx>
 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@xxxxxxxxxxx>
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@xxxxxxxxxxx>
+ * Author: Thomas Perrot <thomas.perrot@xxxxxxxxxxx>
+ */
+
+#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@xxxxxxxxxxx>");
+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@xxxxxxxxxxx>
+ * Author: Thomas Perrot <thomas.perrot@xxxxxxxxxxx>
+ */
+
+#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