Re: [PATCH 1/2] power: reset: Add QEMU virt-ctrl driver

[Sebastian Reichel <sebastian.reichel@collabora.com>]

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Hi,

On Mon, Jan 12, 2026 at 06:22:56PM +0000, Kuan-Wei Chiu wrote:
> Add a new driver for the 'virt-ctrl' device found on QEMU virt machines
> (e.g. m68k). This device provides a simple interface for system reset
> and power off [1].
> 
> This driver registers a restart handler for system reboot and sets the
> global pm_power_off callback for system shutdown. It is designed to be
> generic and can be reused by other architectures utilizing this QEMU
> device.
> 
> Link: https://gitlab.com/qemu-project/qemu/-/blob/v10.2.0/hw/misc/virt_ctrl.c [1]
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
> ---
>  MAINTAINERS                          |  6 ++
>  drivers/power/reset/Kconfig          | 10 +++
>  drivers/power/reset/Makefile         |  1 +
>  drivers/power/reset/qemu-virt-ctrl.c | 98 ++++++++++++++++++++++++++++
>  4 files changed, 115 insertions(+)
>  create mode 100644 drivers/power/reset/qemu-virt-ctrl.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 0d044a58cbfe..2586e4162304 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -21179,6 +21179,12 @@ S:	Maintained
>  F:	drivers/firmware/qemu_fw_cfg.c
>  F:	include/uapi/linux/qemu_fw_cfg.h
>  
> +QEMU VIRT MACHINE SYSTEM CONTROLLER DRIVER
> +M:	Kuan-Wei Chiu <visitorckw@gmail.com>
> +L:	linux-pm@vger.kernel.org
> +S:	Maintained
> +F:	drivers/power/reset/qemu-virt-ctrl.c
> +
>  QLOGIC QL41xxx FCOE DRIVER
>  M:	Saurav Kashyap <skashyap@marvell.com>
>  M:	Javed Hasan <jhasan@marvell.com>
> diff --git a/drivers/power/reset/Kconfig b/drivers/power/reset/Kconfig
> index f6c1bcbb57de..1ca1c2913cbc 100644
> --- a/drivers/power/reset/Kconfig
> +++ b/drivers/power/reset/Kconfig
> @@ -354,4 +354,14 @@ config POWER_MLXBF
>  	help
>  	  This driver supports reset or low power mode handling for Mellanox BlueField.
>  
> +config POWER_RESET_QEMU_VIRT_CTRL
> +	bool "QEMU Virt Machine System Controller"
> +	depends on HAS_IOMEM
> +	help
> +	  This driver supports the system reset and power off functionality
> +	  provided by the QEMU 'virt-ctrl' device.
> +
> +	  Say Y here if you are running Linux on a QEMU virtual machine that
> +	  provides this controller, such as the m68k virt machine.
> +
>  endif
> diff --git a/drivers/power/reset/Makefile b/drivers/power/reset/Makefile
> index 0e4ae6f6b5c5..d7ae97241a83 100644
> --- a/drivers/power/reset/Makefile
> +++ b/drivers/power/reset/Makefile
> @@ -41,3 +41,4 @@ obj-$(CONFIG_SYSCON_REBOOT_MODE) += syscon-reboot-mode.o
>  obj-$(CONFIG_POWER_RESET_SC27XX) += sc27xx-poweroff.o
>  obj-$(CONFIG_NVMEM_REBOOT_MODE) += nvmem-reboot-mode.o
>  obj-$(CONFIG_POWER_MLXBF) += pwr-mlxbf.o
> +obj-$(CONFIG_POWER_RESET_QEMU_VIRT_CTRL) += qemu-virt-ctrl.o
> diff --git a/drivers/power/reset/qemu-virt-ctrl.c b/drivers/power/reset/qemu-virt-ctrl.c
> new file mode 100644
> index 000000000000..f1acd22172ce
> --- /dev/null
> +++ b/drivers/power/reset/qemu-virt-ctrl.c
> @@ -0,0 +1,98 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * QEMU Virt Machine System Controller Driver
> + *
> + * Copyright (C) 2026 Kuan-Wei Chiu <visitorckw@gmail.com>
> + */
> +
> +#include <linux/io.h>
> +#include <linux/module.h>
> +#include <linux/platform_device.h>
> +#include <linux/reboot.h>
> +
> +/* Registers */
> +#define VIRT_CTRL_REG_FEATURES	0x00
> +#define VIRT_CTRL_REG_CMD	0x04
> +
> +/* Commands */
> +#define CMD_NOOP	0
> +#define CMD_RESET	1
> +#define CMD_HALT	2
> +#define CMD_PANIC	3
> +
> +struct qemu_virt_ctrl {
> +	void __iomem *base;
> +	struct notifier_block restart_nb;
> +};
> +
> +static void __iomem *qemu_virt_ctrl_base;
> +
> +static void qemu_virt_ctrl_power_off(void)
> +{
> +	if (qemu_virt_ctrl_base)
> +		iowrite32be(CMD_HALT, qemu_virt_ctrl_base + VIRT_CTRL_REG_CMD);
> +}
> +
> +static int qemu_virt_ctrl_restart(struct notifier_block *nb, unsigned long action,
> +				  void *data)
> +{
> +	struct qemu_virt_ctrl *vc = container_of(nb, struct qemu_virt_ctrl, restart_nb);
> +
> +	iowrite32be(CMD_RESET, vc->base + VIRT_CTRL_REG_CMD);
> +
> +	return NOTIFY_DONE;
> +}
> +
> +static int qemu_virt_ctrl_probe(struct platform_device *pdev)
> +{
> +	struct qemu_virt_ctrl *vc;
> +	int ret;
> +
> +	vc = devm_kzalloc(&pdev->dev, sizeof(*vc), GFP_KERNEL);
> +	if (!vc)
> +		return -ENOMEM;
> +
> +	vc->base = devm_platform_ioremap_resource(pdev, 0);
> +	if (IS_ERR(vc->base))
> +		return PTR_ERR(vc->base);
> +
> +	qemu_virt_ctrl_base = vc->base;
> +
> +	vc->restart_nb.notifier_call = qemu_virt_ctrl_restart;
> +	vc->restart_nb.priority = 128;
> +
> +	ret = register_restart_handler(&vc->restart_nb);
> +	if (ret)
> +		return dev_err_probe(&pdev->dev, ret,
> +				     "cannot register restart handler\n");

Please use devm_register_sys_off_handler().

> +	if (!pm_power_off)
> +		pm_power_off = qemu_virt_ctrl_power_off;

same.

> +	platform_set_drvdata(pdev, vc);
> +
> +	return 0;
> +}
> +
> +static void qemu_virt_ctrl_remove(struct platform_device *pdev)
> +{
> +	struct qemu_virt_ctrl *vc = platform_get_drvdata(pdev);
> +
> +	unregister_restart_handler(&vc->restart_nb);
> +
> +	if (pm_power_off == qemu_virt_ctrl_power_off)
> +		pm_power_off = NULL;
> +}
> +
> +static struct platform_driver qemu_virt_ctrl_driver = {
> +	.probe = qemu_virt_ctrl_probe,
> +	.remove = qemu_virt_ctrl_remove,
> +	.driver = {
> +		.name = "qemu-virt-ctrl",
> +	},
> +};
> +module_platform_driver(qemu_virt_ctrl_driver);
> +
> +MODULE_AUTHOR("Kuan-Wei Chiu <visitorckw@gmail.com>");
> +MODULE_DESCRIPTION("QEMU Virt Machine System Controller Driver");
> +MODULE_LICENSE("GPL");

You mark this as module_platform_driver() and add MODULE_*, but the
Kconfig option is bool. To be usable as a module it is missing setup
of .id_table in the platform_driver struct for auto-loading.

Greetings,

-- Sebastian

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