Why using ACPI drivers is not a good idea

Copyright:

© 2026, Intel Corporation

Author:

Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Even though binding drivers directly to struct acpi_device objects, also referred to as “ACPI device nodes”, allows basic functionality to be provided at least in some cases, there are problems with it, related to general consistency, sysfs layout, power management operation ordering, and code cleanliness.

First of all, ACPI device nodes represent firmware entities rather than hardware and in many cases they provide auxiliary information on devices enumerated independently (like PCI devices or CPUs). It is therefore generally questionable to assign resources to them because the entities represented by them do not decode addresses in the memory or I/O address spaces and do not generate interrupts or similar (all of that is done by hardware).

Second, as a general rule, a struct acpi_device can only be a parent of another struct acpi_device. If that is not the case, the location of the child device in the device hierarchy is at least confusing and it may not be straightforward to identify the piece of hardware providing functionality represented by it. However, binding a driver directly to an ACPI device node may cause that to happen if the given driver registers input devices or wakeup sources under it, for example.

Next, using system suspend and resume callbacks directly on ACPI device nodes is also questionable because it may cause ordering problems to appear. Namely, ACPI device nodes are registered before enumerating hardware corresponding to them and they land on the PM list in front of the majority of other device objects. Consequently, the execution ordering of their PM callbacks may be different from what is generally expected. Also, in general, dependencies returned by _DEP objects do not affect ACPI device nodes themselves, but the “physical” devices associated with them, which potentially is one more source of inconsistency related to treating ACPI device nodes as “real” device representation.

All of the above means that binding drivers to ACPI device nodes should generally be avoided and so struct acpi_driver objects should not be used.

Moreover, a device ID is necessary to bind a driver directly to an ACPI device node, but device IDs are not generally associated with all of them. Some of them contain alternative information allowing the corresponding pieces of hardware to be identified, for example represeted by an _ADR object return value, and device IDs are not used in those cases. In consequence, confusingly enough, binding an ACPI driver to an ACPI device node may even be impossible.

When that happens, the piece of hardware corresponding to the given ACPI device node is represented by another device object, like a struct pci_dev, and the ACPI device node is the “ACPI companion” of that device, accessible through its fwnode pointer used by the ACPI_COMPANION() macro. The ACPI companion holds additional information on the device configuration and possibly some “recipes” on device manipulation in the form of AML (ACPI Machine Language) bytecode provided by the platform firmware. Thus the role of the ACPI device node is similar to the role of a struct device_node on a system where Device Tree is used for platform description.

For consistency, this approach has been extended to the cases in which ACPI device IDs are used. Namely, in those cases, an additional device object is created to represent the piece of hardware corresponding to a given ACPI device node. By default, it is a platform device, but it may also be a PNP device, a CPU device, or another type of device, depending on what the given piece of hardware actually is. There are even cases in which multiple devices are “backed” or “accompanied” by one ACPI device node (e.g. ACPI device nodes corresponding to GPUs that may provide firmware interfaces for backlight brightness control in addition to GPU configuration information).

This means that it really should never be necessary to bind a driver directly to an ACPI device node because there is a “proper” device object representing the corresponding piece of hardware that can be bound to by a “proper” driver using the given ACPI device node as the device’s ACPI companion. Thus, in principle, there is no reason to use ACPI drivers and if they all were replaced with other driver types (for example, platform drivers), some code could be dropped and some complexity would go away.