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

Lee Jones
2026-03-31T13:08:55Z

On Tue, 24 Mar 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.
>
> 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>
> + */

Consider updating the Copyright date - we're pretty deep into 2026 at this point.

> +#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",
> + },
> +};

MFD_CELL_BASIC()

> +/*
> + * 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 = {

Assuming 'rsp' means response, let's just write that out in full.

Readability wins over brevity every time.

> + .addr = client->addr,
> + .flags = I2C_M_RD,
> + .buf = &rsp,
> + .len = 1,
> + };
> + int ret;

Since some I2C host controllers might use DMA, should we ensure that the
'rsp' buffer is allocated in DMA-safe memory rather than on the stack to
prevent potential cache-line corruption?

Also allocation of structs during in declaration statements is rough!

And adding that u8 in the middle is just rubbing it in.

> + 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,

Are you sure? Why 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);

dev_err_probe()

> +
> + return devm_mfd_add_devices(&client->dev, PLATFORM_DEVID_NONE,
> + aaeon_mcu_devs, ARRAY_SIZE(aaeon_mcu_devs),
> + NULL, 0, NULL);

Why PLATFORM_DEVID_NONE over AUTO here?

> +}
> +
> +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>
> + */

As above.

> +
> +#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))

Where else is this used?

> +#endif /* __LINUX_MFD_AAEON_MCU_H */
>
> --
> 2.53.0
>

--
Lee Jones [李琼斯]