On Mon, Jan 5, 2026 at 8:40 PM Randy Dunlap [off-list ref] wrote:

On 1/5/26 3:25 AM, Rafael J. Wysocki wrote:

quoted

From: Rafael J. Wysocki <redacted>

[snip]

quoted

Link: https://uefi.org/specs/ACPI/6.6/02_Definition_of_Terms.html#term-ACPI-Namespace [1]
Signed-off-by: Rafael J. Wysocki <redacted>
---

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/ (local)

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
 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:: <isonum.txt>
+
+=========================================
+Why using ACPI drivers is not a good idea
+=========================================
+
+:Copyright: |copy| 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 questionable either because it may cause ordering problems to appear.

Missing the "or ..." following "either because ...".
Or did you mean something like "also" instead of "either"?

Yes, I'll change it into "is also questionable".

quoted

+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 represented 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) byte code
+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 (eg. ACPI device nodes

                                                      e.g.

OK