RE: [PATCH v2] driver core/ACPI: Introduce companion_bus_register()

[Michael Kelley <mhklinux@outlook.com>]

From: Rafael J. Wysocki <rafael@kernel.org> Sent: Wednesday, September 9, 2026 9:41 AM
> 
> The ACPI bus type does not allow drivers to be registered, so the sysfs
> attributes related to drivers created for it and its devices are
> useless, and its drivers/ directory is always empty.  All of that is
> confusing and wasteful.
> 
> To allow skipping the creation of those sysfs attributes, introduce a
> "companion" bus type concept and add a special registration function for
> registering "companion" bus types, companion_bus_register().
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

As a heads up, this patch breaks Linux guests booting on Hyper-V.
Something goes wrong in the VMBus driver getting loaded and
initialized (drivers/hv/vmbus_drv.c). It may be that the VMBus
driver is doing something wrong or making an invalid assumption.
I'll debug the problem today or tomorrow, but any insight you can
offer would be appreciated. I'm working with linux-next20260921.

Michael

> ---
> 
> v1 -> v2:
>    * Address Sashiko feedback regarding possible leaks of references in two
>      places:
> 
>      https://sashiko.dev/#/patchset/8753121.T7Z3S40VBb%40rafael.j.wysocki
> 
> ---
>  drivers/acpi/bus.c         |    8 -----
>  drivers/base/bus.c         |   72 ++++++++++++++++++++++++++++++++++-----------
>  include/linux/device/bus.h |    1
>  3 files changed, 57 insertions(+), 24 deletions(-)
> 
> --- a/drivers/acpi/bus.c
> +++ b/drivers/acpi/bus.c
> @@ -1120,11 +1120,6 @@ EXPORT_SYMBOL_GPL(acpi_driver_match_devi
>                                ACPI Bus operations
>     -------------------------------------------------------------------------- */
> 
> -static int acpi_bus_match(struct device *dev, const struct device_driver *drv)
> -{
> -	return 0;
> -}
> -
>  static int acpi_device_uevent(const struct device *dev, struct kobj_uevent_env *env)
>  {
>  	return __acpi_device_uevent_modalias(to_acpi_device(dev), env);
> @@ -1132,7 +1127,6 @@ static int acpi_device_uevent(const stru
> 
>  const struct bus_type acpi_bus_type = {
>  	.name		= "acpi",
> -	.match		= acpi_bus_match,
>  	.uevent		= acpi_device_uevent,
>  };
> 
> @@ -1451,7 +1445,7 @@ static int __init acpi_bus_init(void)
>  	 */
>  	acpi_root_dir = proc_mkdir(ACPI_BUS_FILE_ROOT, NULL);
> 
> -	result = bus_register(&acpi_bus_type);
> +	result = companion_bus_register(&acpi_bus_type);
>  	if (!result)
>  		return 0;
> 
> --- a/drivers/base/bus.c
> +++ b/drivers/base/bus.c
> @@ -738,6 +738,11 @@ int bus_add_driver(struct device_driver
>  	if (!sp)
>  		return -EINVAL;
> 
> +	if (!sp->drivers_kset) {
> +		error = -ENXIO;
> +		goto out_put_bus;
> +	}
> +
>  	/*
>  	 * Reference in sp is now incremented and will be dropped when
>  	 * the driver is removed from the bus
> @@ -930,15 +935,7 @@ static ssize_t bus_uevent_store(const st
>  static struct bus_attribute bus_attr_uevent = __ATTR(uevent, 0200, NULL,
>  						     bus_uevent_store);
> 
> -/**
> - * bus_register - register a driver-core subsystem
> - * @bus: bus to register
> - *
> - * Once we have that, we register the bus with the kobject
> - * infrastructure, then register the children subsystems it has:
> - * the devices and drivers that belong to the subsystem.
> - */
> -int bus_register(const struct bus_type *bus)
> +static int bus_register_internal(const struct bus_type *bus, bool use_drivers)
>  {
>  	int retval;
>  	struct subsys_private *priv;
> @@ -960,7 +957,7 @@ int bus_register(const struct bus_type *
> 
>  	bus_kobj->kset = bus_kset;
>  	bus_kobj->ktype = &bus_ktype;
> -	priv->drivers_autoprobe = 1;
> +	priv->drivers_autoprobe = use_drivers;
> 
>  	retval = kset_register(&priv->subsys);
>  	if (retval)
> @@ -976,10 +973,12 @@ int bus_register(const struct bus_type *
>  		goto bus_devices_fail;
>  	}
> 
> -	priv->drivers_kset = kset_create_and_add("drivers", NULL, bus_kobj);
> -	if (!priv->drivers_kset) {
> -		retval = -ENOMEM;
> -		goto bus_drivers_fail;
> +	if (use_drivers) {
> +		priv->drivers_kset = kset_create_and_add("drivers", NULL, bus_kobj);
> +		if (!priv->drivers_kset) {
> +			retval = -ENOMEM;
> +			goto bus_drivers_fail;
> +		}
>  	}
> 
>  	INIT_LIST_HEAD(&priv->interfaces);
> @@ -989,9 +988,11 @@ int bus_register(const struct bus_type *
>  	klist_init(&priv->klist_devices, klist_devices_get, klist_devices_put);
>  	klist_init(&priv->klist_drivers, NULL, NULL);
> 
> -	retval = add_probe_files(bus);
> -	if (retval)
> -		goto bus_probe_files_fail;
> +	if (use_drivers) {
> +		retval = add_probe_files(bus);
> +		if (retval)
> +			goto bus_probe_files_fail;
> +	}
> 
>  	retval = sysfs_create_groups(bus_kobj, bus->bus_groups);
>  	if (retval)
> @@ -1016,9 +1017,41 @@ out:
>  	kfree(priv);
>  	return retval;
>  }
> +
> +/**
> + * bus_register - register a driver-core subsystem
> + * @bus: bus to register
> + *
> + * Once we have that, we register the bus with the kobject
> + * infrastructure, then register the children subsystems it has:
> + * the devices and drivers that belong to the subsystem.
> + */
> +int bus_register(const struct bus_type *bus)
> +{
> +	return bus_register_internal(bus, true);
> +}
>  EXPORT_SYMBOL_GPL(bus_register);
> 
>  /**
> + * companion_bus_register - register a companion bus type
> + * @bus: companion bus to register
> + *
> + * A companion bus is a bus without drivers.  Devices that belong to it can be
> + * bound to other devices as their "companions" and represent interfaces that
> + * can be used by the drivers of those other devices.  They may also be used for
> + * the enumeration of those other devices.
> + *
> + * The ACPI bus is a specific example of a companion bus.
> + *
> + * Registering a companion bus is like registering a regular bus except that it
> + * skips the creation of sysfs interfaces related to drivers for @bus.
> + */
> +int companion_bus_register(const struct bus_type *bus)
> +{
> +	return bus_register_internal(bus, false);
> +}
> +
> +/**
>   * bus_unregister - remove a bus from the system
>   * @bus: bus.
>   *
> @@ -1415,6 +1448,11 @@ struct device_driver *driver_find(const
>  	if (!sp)
>  		return NULL;
> 
> +	if (!sp->drivers_kset) {
> +		subsys_put(sp);
> +		return NULL;
> +	}
> +
>  	k = kset_find_obj(sp->drivers_kset, name);
>  	subsys_put(sp);
>  	if (!k)
> --- a/include/linux/device/bus.h
> +++ b/include/linux/device/bus.h
> @@ -113,6 +113,7 @@ struct bus_type {
>  	bool need_parent_lock;
>  };
> 
> +int __must_check companion_bus_register(const struct bus_type *bus);
>  int __must_check bus_register(const struct bus_type *bus);
> 
>  void bus_unregister(const struct bus_type *bus);
> 
> 
>