1.. SPDX-License-Identifier: GPL-2.0
  2
  3==================================
  4_DSD Device Properties Usage Rules
  5==================================
  6
  7Properties, Property Sets and Property Subsets
  8==============================================
  9
 10The _DSD (Device Specific Data) configuration object, introduced in ACPI 5.1,
 11allows any type of device configuration data to be provided via the ACPI
 12namespace.  In principle, the format of the data may be arbitrary, but it has to
 13be identified by a UUID which must be recognized by the driver processing the
 14_DSD output.  However, there are generic UUIDs defined for _DSD recognized by
 15the ACPI subsystem in the Linux kernel which automatically processes the data
 16packages associated with them and makes those data available to device drivers
 17as "device properties".
 18
 19A device property is a data item consisting of a string key and a value (of a
 20specific type) associated with it.
 21
 22In the ACPI _DSD context it is an element of the sub-package following the
 23generic Device Properties UUID in the _DSD return package as specified in the
 24section titled "Well-Known _DSD UUIDs and Data Structure Formats" sub-section
 25"Device Properties UUID" in _DSD (Device Specific Data) Implementation Guide
 26document [1]_.
 27
 28It also may be regarded as the definition of a key and the associated data type
 29that can be returned by _DSD in the Device Properties UUID sub-package for a
 30given device.
 31
 32A property set is a collection of properties applicable to a hardware entity
 33like a device.  In the ACPI _DSD context it is the set of all properties that
 34can be returned in the Device Properties UUID sub-package for the device in
 35question.
 36
 37Property subsets are nested collections of properties.  Each of them is
 38associated with an additional key (name) allowing the subset to be referred
 39to as a whole (and to be treated as a separate entity).  The canonical
 40representation of property subsets is via the mechanism specified in the
 41section titled "Well-Known _DSD UUIDs and Data Structure Formats" sub-section
 42"Hierarchical Data Extension UUID" in _DSD (Device Specific Data)
 43Implementation Guide document [1]_.
 44
 45Property sets may be hierarchical.  That is, a property set may contain
 46multiple property subsets that each may contain property subsets of its
 47own and so on.
 48
 49General Validity Rule for Property Sets
 50=======================================
 51
 52Valid property sets must follow the guidance given by the Device Properties UUID
 53definition document [1].
 54
 55_DSD properties are intended to be used in addition to, and not instead of, the
 56existing mechanisms defined by the ACPI specification.  Therefore, as a rule,
 57they should only be used if the ACPI specification does not make direct
 58provisions for handling the underlying use case.  It generally is invalid to
 59return property sets which do not follow that rule from _DSD in data packages
 60associated with the Device Properties UUID.
 61
 62Additional Considerations
 63-------------------------
 64
 65There are cases in which, even if the general rule given above is followed in
 66principle, the property set may still not be regarded as a valid one.
 67
 68For example, that applies to device properties which may cause kernel code
 69(either a device driver or a library/subsystem) to access hardware in a way
 70possibly leading to a conflict with AML methods in the ACPI namespace.  In
 71particular, that may happen if the kernel code uses device properties to
 72manipulate hardware normally controlled by ACPI methods related to power
 73management, like _PSx and _DSW (for device objects) or _ON and _OFF (for power
 74resource objects), or by ACPI device disabling/enabling methods, like _DIS and
 75_SRS.
 76
 77In all cases in which kernel code may do something that will confuse AML as a
 78result of using device properties, the device properties in question are not
 79suitable for the ACPI environment and consequently they cannot belong to a valid
 80property set.
 81
 82Property Sets and Device Tree Bindings
 83======================================
 84
 85It often is useful to make _DSD return property sets that follow Device Tree
 86bindings.
 87
 88In those cases, however, the above validity considerations must be taken into
 89account in the first place and returning invalid property sets from _DSD must be
 90avoided.  For this reason, it may not be possible to make _DSD return a property
 91set following the given DT binding literally and completely.  Still, for the
 92sake of code re-use, it may make sense to provide as much of the configuration
 93data as possible in the form of device properties and complement that with an
 94ACPI-specific mechanism suitable for the use case at hand.
 95
 96In any case, property sets following DT bindings literally should not be
 97expected to automatically work in the ACPI environment regardless of their
 98contents.
 99
100References
101==========
102
103.. [1] https://github.com/UEFI/DSD-Guide