RE: [PATCH v3] driver core/ACPI: Introduce companion_bus_register()

[Michael Kelley <mhklinux@outlook.com>]

From: Rafael J. Wysocki <rafael@kernel.org> Sent: Friday, September 25, 2026 2:24 AM
> 
> The ACPI bus type does not allow drivers to be matched to devices, so
> the sysfs attributes related to drivers created for it are useless and
> their existence is confusing.
> 
> Moreover, it is better to prevent drivers from being registered and
> looked up for a bus like that.
> 
> To allow skipping the creation of those sysfs attributes and preventing
> driver registration and lookup for the ACPI bus type, introduce a
> "companion" bus type concept and add a special registration function
> for registering "companion" bus types, companion_bus_register().
> 
> The "drivers" directory under the ACPI bus type is still needed because
> there are versions of systemd that depend on it [1].
> 
> Link: https://lore.kernel.org/linux-acpi/SN6PR02MB41575266A4580339E186E5D9D4812@SN6PR02MB4157.namprd02.prod.outlook.com/ [1]
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Tested this against 7.3.0-rc2, running on my Ubuntu 20.04 instance
with the older systemd, and as a Hyper-V guest. The VM booted
as expected, whereas the boot failed with v2 of this patch.

Tested-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>

> ---
> 
> v2 -> v3:
>    * Retain /sys/bus/acpi/drivers/ because the lack of it confuses some
>      versions of systemd (Michael Kelley)
>    * Add flag in struct subsys_private to indicate a bus with no drivers
>    * Add ACK from Greg (tentatively, but the patch is generally along the
>      same lines as before)
>    * Update the changelog
> 
> 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/base.h        |    2 +
>  drivers/base/bus.c         |   63 +++++++++++++++++++++++++++++++++++----------
>  include/linux/device/bus.h |    1
>  4 files changed, 54 insertions(+), 20 deletions(-)
> 
> --- a/drivers/acpi/bus.c
> +++ b/drivers/acpi/bus.c
> @@ -1110,11 +1110,6 @@ 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);
> @@ -1122,7 +1117,6 @@ static int acpi_device_uevent(const stru
> 
>  const struct bus_type acpi_bus_type = {
>  	.name		= "acpi",
> -	.match		= acpi_bus_match,
>  	.uevent		= acpi_device_uevent,
>  };
> 
> @@ -1441,7 +1435,7 @@ 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/base.h
> +++ b/drivers/base/base.h
> @@ -27,6 +27,7 @@
>   * @drivers_autoprobe: gate whether new devices are automatically attached to
>   *		       registered drivers, or new drivers automatically attach
>   *		       to existing devices.
> + * @no_drivers: gate whether drivers can be registered.
>   * @bus: pointer back to the struct bus_type that this structure is associated
>   *	 with.
>   * @dev_root: Default device to use as the parent.
> @@ -51,6 +52,7 @@ struct subsys_private {
>  	struct klist klist_drivers;
>  	struct blocking_notifier_head bus_notifier;
>  	unsigned int drivers_autoprobe:1;
> +	unsigned int no_drivers:1;
>  	const struct bus_type *bus;
>  	struct device *dev_root;
> 
> --- 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->no_drivers) {
> +		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 +957,8 @@ 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;
> +	priv->no_drivers = !use_drivers;
> 
>  	retval = kset_register(&priv->subsys);
>  	if (retval)
> @@ -989,9 +987,11 @@ 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 +1016,41 @@ 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 +1447,11 @@ struct device_driver *driver_find(const
>  	if (!sp)
>  		return NULL;
> 
> +	if (sp->no_drivers) {
> +		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);
> 
> 
>