Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 _DSD Device Properties Usage Rules 2 ---------------------------------- 3 4 Properties, Property Sets and Property Subsets 5 ---------------------------------------------- 6 7 The _DSD (Device Specific Data) configuration object, introduced in ACPI 5.1, 8 allows any type of device configuration data to be provided via the ACPI 9 namespace. In principle, the format of the data may be arbitrary, but it has to 10 be 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 12 the ACPI subsystem in the Linux kernel which automatically processes the data 13 packages associated with them and makes those data available to device drivers 14 as "device properties". 15 16 A device property is a data item consisting of a string key and a value (of a 17 specific type) associated with it. 18 19 In the ACPI _DSD context it is an element of the sub-package following the 20 generic Device Properties UUID in the _DSD return package as specified in the 21 Device Properties UUID definition document [1]. 22 23 It also may be regarded as the definition of a key and the associated data type 24 that can be returned by _DSD in the Device Properties UUID sub-package for a 25 given device. 26 27 A property set is a collection of properties applicable to a hardware entity 28 like a device. In the ACPI _DSD context it is the set of all properties that 29 can be returned in the Device Properties UUID sub-package for the device in 30 question. 31 32 Property subsets are nested collections of properties. Each of them is 33 associated with an additional key (name) allowing the subset to be referred 34 to as a whole (and to be treated as a separate entity). The canonical 35 representation of property subsets is via the mechanism specified in the 36 Hierarchical Properties Extension UUID definition document [2]. 37 38 Property sets may be hierarchical. That is, a property set may contain 39 multiple property subsets that each may contain property subsets of its 40 own and so on. 41 42 General Validity Rule for Property Sets 43 --------------------------------------- 44 45 Valid property sets must follow the guidance given by the Device Properties UUID 46 definition document [1]. 47 48 _DSD properties are intended to be used in addition to, and not instead of, the 49 existing mechanisms defined by the ACPI specification. Therefore, as a rule, 50 they should only be used if the ACPI specification does not make direct 51 provisions for handling the underlying use case. It generally is invalid to 52 return property sets which do not follow that rule from _DSD in data packages 53 associated with the Device Properties UUID. 54 55 Additional Considerations 56 ------------------------- 57 58 There are cases in which, even if the general rule given above is followed in 59 principle, the property set may still not be regarded as a valid one. 60 61 For 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 63 possibly leading to a conflict with AML methods in the ACPI namespace. In 64 particular, that may happen if the kernel code uses device properties to 65 manipulate hardware normally controlled by ACPI methods related to power 66 management, like _PSx and _DSW (for device objects) or _ON and _OFF (for power 67 resource objects), or by ACPI device disabling/enabling methods, like _DIS and 68 _SRS. 69 70 In all cases in which kernel code may do something that will confuse AML as a 71 result of using device properties, the device properties in question are not 72 suitable for the ACPI environment and consequently they cannot belong to a valid 73 property set. 74 75 Property Sets and Device Tree Bindings 76 -------------------------------------- 77 78 It often is useful to make _DSD return property sets that follow Device Tree 79 bindings. 80 81 In those cases, however, the above validity considerations must be taken into 82 account in the first place and returning invalid property sets from _DSD must be 83 avoided. For this reason, it may not be possible to make _DSD return a property 84 set following the given DT binding literally and completely. Still, for the 85 sake of code re-use, it may make sense to provide as much of the configuration 86 data as possible in the form of device properties and complement that with an 87 ACPI-specific mechanism suitable for the use case at hand. 88 89 In any case, property sets following DT bindings literally should not be 90 expected to automatically work in the ACPI environment regardless of their 91 contents. 92 93 References 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