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 104