Re: [PATCH v5 3/5] mfd: aaeon: Add SRG-IMX8P MCU driver

Lee Jones
2026-04-30T13:11:03Z

On Wed, 08 Apr 2026, Thomas Perrot (Schneider Electric) wrote:

> 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.
>
> I2C transfers use heap-allocated DMA-safe buffers rather than
> stack-allocated ones, as required by I2C controllers that perform DMA.
>
> Regmap caching is enabled (REGCACHE_MAPLE) with a volatile_reg
> callback that marks GPIO input read registers (opcode 0x72) and the
> watchdog status register (opcode 0x63, arg 0x02) as volatile. All
> other registers written by the driver (GPIO direction,
> GPO state, watchdog control) are stable and can be safely cached.
>
> 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 | 204 ++++++++++++++++++++++++++++++++++++++++++
> include/linux/mfd/aaeon-mcu.h | 40 +++++++++
> 5 files changed, 257 insertions(+)
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index ea9d55f76f35..f91b6a1826d0 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 aace5766b38a..82ec1d8e7224 100644
> --- a/drivers/mfd/Kconfig
> +++ b/drivers/mfd/Kconfig
> @@ -1561,6 +1561,16 @@ config ABX500_CORE
> remain unchanged when IC changes. Binding of the functions to
> actual register access is done by the IC core driver.
>
> +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 AB8500_CORE
> bool "ST-Ericsson AB8500 Mixed Signal Power Management chip"
> depends on ABX500_CORE && MFD_DB8500_PRCMU
> diff --git a/drivers/mfd/Makefile b/drivers/mfd/Makefile
> index e75e8045c28a..34db5b033584 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 000000000000..3b4e2d891534
> --- /dev/null
> +++ b/drivers/mfd/aaeon-mcu.c
> @@ -0,0 +1,204 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Aaeon MCU driver
> + *
> + * Copyright (C) 2026 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/aaeon-mcu.h>
> +#include <linux/mfd/core.h>
> +#include <linux/platform_device.h>
> +#include <linux/regmap.h>
> +#include <linux/slab.h>
> +
> +struct aaeon_mcu {
> + struct i2c_client *client;
> + u8 *cmd; /* DMA-safe 3-byte write buffer [opcode, arg, value] */
> + u8 *response; /* DMA-safe 1-byte read buffer for MCU acknowledgment */
> +};
> +
> +static const struct mfd_cell aaeon_mcu_devs[] = {
> + MFD_CELL_BASIC("aaeon-mcu-wdt", NULL, NULL, 0, 0),
> + MFD_CELL_BASIC("aaeon-mcu-gpio", NULL, NULL, 0, 0),
> +};
> +
> +/* Number of bytes in a MCU command: [opcode, arg, value] */
> +#define AAEON_MCU_CMD_LEN 3
> +
> +/*
> + * 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 aaeon_mcu *mcu = context;
> + struct i2c_client *client = mcu->client;
> + struct i2c_msg write_msg;
> + /* The MCU always sends a response byte after each command; discard it. */
> + struct i2c_msg response_msg;
> + int ret;
> +
> + memcpy(mcu->cmd, data, count);
> +
> + write_msg.addr = client->addr;
> + write_msg.flags = 0;
> + write_msg.buf = mcu->cmd;
> + write_msg.len = count;
> +
> + response_msg.addr = client->addr;
> + response_msg.flags = I2C_M_RD;
> + response_msg.buf = mcu->response;
> + response_msg.len = 1;
> +
> + ret = i2c_transfer(client->adapter, &write_msg, 1);
> + if (ret < 0)
> + return ret;
> + if (ret != 1)
> + return -EIO;
> +
> + ret = i2c_transfer(client->adapter, &response_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 aaeon_mcu *mcu = context;
> + struct i2c_client *client = mcu->client;
> + struct i2c_msg write_msg;
> + struct i2c_msg read_msg;
> + int ret;
> +
> + /*
> + * reg_buf holds the 2-byte big-endian register address [opcode, arg].
> + * Append a trailing 0x00 to form the full 3-byte MCU command.
> + */
> + mcu->cmd[0] = ((u8 *)reg_buf)[0];
> + mcu->cmd[1] = ((u8 *)reg_buf)[1];
> + mcu->cmd[2] = 0x00;
> +
> + write_msg.addr = client->addr;
> + write_msg.flags = 0;
> + write_msg.buf = mcu->cmd;
> + write_msg.len = AAEON_MCU_CMD_LEN;
> +
> + read_msg.addr = client->addr;
> + read_msg.flags = I2C_M_RD;
> + read_msg.buf = val_buf;
> + read_msg.len = val_size;
> +
> + 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 bool aaeon_mcu_volatile_reg(struct device *dev, unsigned int reg)
> +{
> + /*
> + * GPIO input registers are driven by external signals and can change
> + * at any time without CPU involvement, always read from hardware.
> + *
> + * The watchdog status register reflects hardware state and can change
> + * autonomously.
> + *
> + * All other registers are written by the driver and their values are
> + * stable, so they can be safely cached.
> + */
> + if ((reg >> 8) == AAEON_MCU_READ_GPIO_OPCODE)
> + return true;
> + if (reg == AAEON_MCU_REG(AAEON_MCU_CONTROL_WDT_OPCODE, 0x02))
> + return true;
> + return false;
> +}
> +
> +static const struct regmap_config aaeon_mcu_regmap_config = {
> + .reg_bits = 16,
> + .val_bits = 8,
> + .reg_format_endian = REGMAP_ENDIAN_BIG,
> + .max_register = AAEON_MCU_MAX_REGISTER,
> + .volatile_reg = aaeon_mcu_volatile_reg,
> + .cache_type = REGCACHE_MAPLE,
> +};
> +
> +static int aaeon_mcu_probe(struct i2c_client *client)
> +{
> + struct aaeon_mcu *mcu;

Nit: We usually call this ddata, but it's not a blocker.

> + struct regmap *regmap;
> +
> + mcu = devm_kzalloc(&client->dev, sizeof(*mcu), GFP_KERNEL);
> + if (!mcu)
> + return -ENOMEM;
> +
> + mcu->client = client;
> +
> + mcu->cmd = devm_kzalloc(&client->dev, AAEON_MCU_CMD_LEN * sizeof(*mcu->cmd), GFP_KERNEL);
> + if (!mcu->cmd)
> + return -ENOMEM;
> +
> + mcu->response = devm_kzalloc(&client->dev, sizeof(*mcu->response), GFP_KERNEL);
> + if (!mcu->response)
> + return -ENOMEM;
> +
> + regmap = devm_regmap_init(&client->dev, &aaeon_mcu_regmap_bus,
> + mcu, &aaeon_mcu_regmap_config);
> + if (IS_ERR(regmap))
> + return dev_err_probe(&client->dev, PTR_ERR(regmap),
> + "failed to initialize regmap\n");
> +
> + return devm_mfd_add_devices(&client->dev, PLATFORM_DEVID_AUTO,
> + 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",

Nit: This should be a '-'.

> + .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 000000000000..3a1aeec85d60
> --- /dev/null
> +++ b/include/linux/mfd/aaeon-mcu.h
> @@ -0,0 +1,40 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +/*
> + * Aaeon MCU driver definitions
> + *
> + * Copyright (C) 2026 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))
> +
> +/*
> + * Opcode for GPIO input reads. These registers are volatile, their values
> + * are driven by external signals and can change without CPU involvement.
> + * Used by the MFD driver's volatile_reg callback to bypass the regmap cache.
> + */
> +#define AAEON_MCU_READ_GPIO_OPCODE 0x72
> +
> +/*
> + * Opcode for watchdog control and status commands.
> + * The status register (arg=0x02) reflects hardware state and is volatile.
> + */
> +#define AAEON_MCU_CONTROL_WDT_OPCODE 0x63
> +
> +/*
> + * Highest register address in the MCU register map.
> + * The WRITE_GPIO opcode (0x77) with the highest GPIO argument (0x0B = 11,
> + * i.e. MAX_GPIOS - 1) produces the largest encoded address.
> + */
> +#define AAEON_MCU_MAX_REGISTER AAEON_MCU_REG(0x77, 0x0B)
> +
> +#endif /* __LINUX_MFD_AAEON_MCU_H */
>
> --
> 2.53.0
>

--
Lee Jones