From nobody Sat Feb 7 18:00:50 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1C8C71624C5; Tue, 6 Jan 2026 12:27:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767702440; cv=none; b=gKVbV52hFxZdhMaKcGgnZ+ncJ+9bUGk3jEGHmDz82IzxFcH+iozThvIB/SrOmPmgMriTXl5BksL1DJWWYh5ZtTQRdtBrh0cl72hVPrW8w4O9UUSAg3G1AJ0FqnOm96H1Jmxvoj/mK8Dm0epD+rPl/vBoHYMgH3Ts+dL++dYA6nM= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767702440; c=relaxed/simple; bh=0g0q3+yeUhWNl+UuUZgUuMzE5sKCflZGQ0dod3qEcuM=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type; b=emX5rO3aa/So9qBi2ZHo/QroufO1laaE8ijN8V3eikzduDmNxOacJMH9ZtOGCGbVqxV8+9nGwVNj6Zx0GBLzKmFkuWB9po6sPoGVZIqmAApqRctUvcPGyScmQhpEEnKSXiv9aGfvBH+MJ97pvHg/YYPm2Oh/pwymb8JVbDiLdC0= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=FiohRHXn; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="FiohRHXn" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5E345C116C6; Tue, 6 Jan 2026 12:27:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1767702439; bh=0g0q3+yeUhWNl+UuUZgUuMzE5sKCflZGQ0dod3qEcuM=; h=From:To:Cc:Subject:Date:From; b=FiohRHXn3sEIjh0qmA8patOEkRjPK+inXS2P0IhbnxkA9nJcHEOmdpSHsSkhG9AUY wvyIZZjYXjJWjp5LsSXNUG+udXd92lxSbaW2lkXkpycQ6Sd77QaZBSzB6NMk+YbScP 8REm3iea/Ug/7tTn4USDnE0ryqihmpquFMmssNlBg+wyi4tQXIDsNTQjSNxgl95ucE s9/9S3DM3dX3gaRstRb94UDl2eHLh42Q/3PdP5GizRivp2TZ0H/mz7Y0WZiqIR9NIk EvMMuvK1QHZW0LoOBz5O10PoBgu6vLDE9x40euDRwHimLz7T2XGmTtt8wTpd/9ygdK hheXKXjyAgUPw== From: "Rafael J. Wysocki" To: Linux ACPI Cc: LKML , Andy Shevchenko , Bjorn Helgaas , Greg Kroah-Hartman , Hans de Goede , Linux Documentation , Mika Westerberg , Zhang Rui , Armin Wolf , Danilo Krummrich , Ilpo Jarvinen , Mario Limonciello , Randy Dunlap Subject: [PATCH v2] ACPI: Documentation: driver-api: Disapprove of using ACPI drivers Date: Tue, 06 Jan 2026 13:27:14 +0100 Message-ID: <2396510.ElGaqSPkdT@rafael.j.wysocki> Organization: Linux Kernel Development Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Rafael J. Wysocki Sadly, there is quite a bit of technical debt related to the kernel's ACPI support subsystem and one of the most significant pieces of it is the existence and use of ACPI drivers represented by struct acpi_driver objects. Those drivers are bound directly to struct acpi_device objects, also referred to as "ACPI device nodes", representing device objects in the ACPI namespace defined as: A hierarchical tree structure in OS-controlled memory that contains named objects. These objects may be data objects, control method objects, bus/device package objects, and so on. according to the ACPI specification [1]. The above definition implies, although rather indirectly, that the objects in question don't really represent hardware. They are just "device package objects" containing some information on the devices present in the given platform that is known to the platform firmware. Although the platform firmware can be the only source of information on some devices, the information provided by it alone may be insufficient for device enumeration in general. If that is the case, binding a driver directly to a given ACPI device node clearly doesn't make sense. If the device in question is enumerated through a hardware interface, it will be represented by a device object matching that interface, like a struct pci_dev, and the ACPI device node corresponding to it will be treated as its "ACPI companions" whose role is to amend the "native" enumeratiom mechanism. For the sake of consistency and confusion avoidance, it is better to treat ACPI device nodes in general as ACPI companions of other device objects representing hardware. In some cases though it appeared easier to take a shortcut and use an ACPI driver binding directly to an ACPI device node. Moreover, there were corner cases in which that was the only choice, but they all have been addressed now. In all cases in which an ACPI driver might be used, the ACPI device node it might bind to is an ACPI companion of another device object representing a piece of hardware. It is thus better to use a driver binding to the latter than to use an ACPI driver and leave the other device object alone, not just because doing so is more consistent and less confusing, but also because using ACPI drivers may lead to potential functional deficiencies, like possible ordering issues related to power management. Unfortunately, there are quite a few ACPI drivers in use and, as a rule, they bind to ACPI device nodes that are ACPI companions of platform devices, so in fact they play the role of platform drivers although in a kind of convoluted way. An effort has been under way to replace them with platform drivers, which is relatively straightforward in the vast majority of cases, but it has not been pursued very aggressively so far, mostly due to the existence of the corner cases mentioned above. However, since those corner cases are gone now, it makes sense to spend more time on driver conversions with the ultimate goal to get rid of struct acpi_driver and the related code from the kernel. To that end, add a document explaining why using ACPI drivers is not a good idea, so it need not be explained from scratch on every attempt to convert an ACPI driver to a platform one. Link: https://uefi.org/specs/ACPI/6.6/02_Definition_of_Terms.html#term-ACPI= -Namespace [1] Signed-off-by: Rafael J. Wysocki Reviewed-by: Andy Shevchenko Reviewed-by: Armin Wolf Reviewed-by: Danilo Krummrich Reviewed-by: Mario Limonciello (AMD) Reviewed-by: Randy Dunlap --- v1 -> v2: * Fixed a typo in the changelog (Andy). * Addressed two review comments from Randy ("is questionable either" -> "is also questionable" and "eg." -> "e.g."). * Added R-bys from Andy and Armin. --- Although this patch can be applied independently, it actually depends on some ACPI changes in linux-next and on https://lore.kernel.org/linux-acpi/12824456.O9o76ZdvQC@rafael.j.wysocki/ so it is better to handle it along with that material. --- Documentation/driver-api/acpi/acpi-drivers.rst | 80 ++++++++++++++++++++= +++++ Documentation/driver-api/acpi/index.rst | 1=20 2 files changed, 81 insertions(+) --- /dev/null +++ b/Documentation/driver-api/acpi/acpi-drivers.rst @@ -0,0 +1,80 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. include:: + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Why using ACPI drivers is not a good idea +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +:Copyright: |copy| 2026, Intel Corporation + +:Author: Rafael J. Wysocki + +Even though binding drivers directly to struct acpi_device objects, also +referred to as "ACPI device nodes", allows basic functionality to be provi= ded +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 gene= rally +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 an= other +struct acpi_device. If that is not the case, the location of the child de= vice +in the device hierarchy is at least confusing and it may not be straightfo= rward +to identify the piece of hardware providing functionality represented by i= t. +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 no= des +is also questionable because it may cause ordering problems to appear. Na= mely, +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 t= he +"physical" devices associated with them, which potentially is one more sou= rce +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 de= vice +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, confus= ingly +enough, binding an ACPI driver to an ACPI device node may even be impossib= le. + +When that happens, the piece of hardware corresponding to the given ACPI d= evice +node is represented by another device object, like a struct pci_dev, and t= he +ACPI device node is the "ACPI companion" of that device, accessible throug= h its +fwnode pointer used by the ACPI_COMPANION() macro. The ACPI companion hol= ds +additional information on the device configuration and possibly some "reci= pes" +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 d= evice +node. By default, it is a platform device, but it may also be a PNP devic= e, 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 direc= tly 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 princ= iple, +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 a= nd +some complexity would go away. --- a/Documentation/driver-api/acpi/index.rst +++ b/Documentation/driver-api/acpi/index.rst @@ -7,3 +7,4 @@ ACPI Support =20 linuxized-acpica scan_handlers + acpi-drivers