1_DSD Device Properties Usage Rules 2---------------------------------- 3 4Properties, Property Sets and Property Subsets 5---------------------------------------------- 6 7The _DSD (Device Specific Data) configuration object, introduced in ACPI 5.1, 8allows any type of device configuration data to be provided via the ACPI 9namespace. In principle, the format of the data may be arbitrary, but it has to 10be identified by a UUID which must be recognized by the driver processing the 11_DSD output. However, there are generic UUIDs defined for _DSD recognized by 12the ACPI subsystem in the Linux kernel which automatically processes the data 13packages associated with them and makes those data available to device drivers 14as "device properties". 15 16A device property is a data item consisting of a string key and a value (of a 17specific type) associated with it. 18 19In the ACPI _DSD context it is an element of the sub-package following the 20generic Device Properties UUID in the _DSD return package as specified in the 21Device Properties UUID definition document [1]. 22 23It also may be regarded as the definition of a key and the associated data type 24that can be returned by _DSD in the Device Properties UUID sub-package for a 25given device. 26 27A property set is a collection of properties applicable to a hardware entity 28like a device. In the ACPI _DSD context it is the set of all properties that 29can be returned in the Device Properties UUID sub-package for the device in 30question. 31 32Property subsets are nested collections of properties. Each of them is 33associated with an additional key (name) allowing the subset to be referred 34to as a whole (and to be treated as a separate entity). The canonical 35representation of property subsets is via the mechanism specified in the 36Hierarchical Properties Extension UUID definition document [2]. 37 38Property sets may be hierarchical. That is, a property set may contain 39multiple property subsets that each may contain property subsets of its 40own and so on. 41 42General Validity Rule for Property Sets 43--------------------------------------- 44 45Valid property sets must follow the guidance given by the Device Properties UUID 46definition document [1]. 47 48_DSD properties are intended to be used in addition to, and not instead of, the 49existing mechanisms defined by the ACPI specification. Therefore, as a rule, 50they should only be used if the ACPI specification does not make direct 51provisions for handling the underlying use case. It generally is invalid to 52return property sets which do not follow that rule from _DSD in data packages 53associated with the Device Properties UUID. 54 55Additional Considerations 56------------------------- 57 58There are cases in which, even if the general rule given above is followed in 59principle, the property set may still not be regarded as a valid one. 60 61For example, that applies to device properties which may cause kernel code 62(either a device driver or a library/subsystem) to access hardware in a way 63possibly leading to a conflict with AML methods in the ACPI namespace. In 64particular, that may happen if the kernel code uses device properties to 65manipulate hardware normally controlled by ACPI methods related to power 66management, like _PSx and _DSW (for device objects) or _ON and _OFF (for power 67resource objects), or by ACPI device disabling/enabling methods, like _DIS and 68_SRS. 69 70In all cases in which kernel code may do something that will confuse AML as a 71result of using device properties, the device properties in question are not 72suitable for the ACPI environment and consequently they cannot belong to a valid 73property set. 74 75Property Sets and Device Tree Bindings 76-------------------------------------- 77 78It often is useful to make _DSD return property sets that follow Device Tree 79bindings. 80 81In those cases, however, the above validity considerations must be taken into 82account in the first place and returning invalid property sets from _DSD must be 83avoided. For this reason, it may not be possible to make _DSD return a property 84set following the given DT binding literally and completely. Still, for the 85sake of code re-use, it may make sense to provide as much of the configuration 86data as possible in the form of device properties and complement that with an 87ACPI-specific mechanism suitable for the use case at hand. 88 89In any case, property sets following DT bindings literally should not be 90expected to automatically work in the ACPI environment regardless of their 91contents. 92 93References 94---------- 95 96[1] http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf 97[2] http://www.uefi.org/sites/default/files/resources/_DSD-hierarchical-data-extension-UUID-v1.1.pdf 98