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.. SPDX-License-Identifier: GPL-2.0
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==================================
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_DSD Device Properties Usage Rules
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==================================
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Properties, Property Sets and Property Subsets
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==============================================
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The _DSD (Device Specific Data) configuration object, introduced in ACPI 5.1,
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allows any type of device configuration data to be provided via the ACPI
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namespace. In principle, the format of the data may be arbitrary, but it has to
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be identified by a UUID which must be recognized by the driver processing the
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_DSD output. However, there are generic UUIDs defined for _DSD recognized by
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the ACPI subsystem in the Linux kernel which automatically processes the data
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packages associated with them and makes those data available to device drivers
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as "device properties".
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A device property is a data item consisting of a string key and a value (of a
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specific type) associated with it.
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In the ACPI _DSD context it is an element of the sub-package following the
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generic Device Properties UUID in the _DSD return package as specified in the
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Device Properties UUID definition document [1]_.
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It also may be regarded as the definition of a key and the associated data type
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that can be returned by _DSD in the Device Properties UUID sub-package for a
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given device.
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A property set is a collection of properties applicable to a hardware entity
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like a device. In the ACPI _DSD context it is the set of all properties that
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can be returned in the Device Properties UUID sub-package for the device in
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question.
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Property subsets are nested collections of properties. Each of them is
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associated with an additional key (name) allowing the subset to be referred
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to as a whole (and to be treated as a separate entity). The canonical
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representation of property subsets is via the mechanism specified in the
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Hierarchical Properties Extension UUID definition document [2]_.
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Property sets may be hierarchical. That is, a property set may contain
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multiple property subsets that each may contain property subsets of its
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own and so on.
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General Validity Rule for Property Sets
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=======================================
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Valid property sets must follow the guidance given by the Device Properties UUID
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definition document [1].
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_DSD properties are intended to be used in addition to, and not instead of, the
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existing mechanisms defined by the ACPI specification. Therefore, as a rule,
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they should only be used if the ACPI specification does not make direct
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provisions for handling the underlying use case. It generally is invalid to
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return property sets which do not follow that rule from _DSD in data packages
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associated with the Device Properties UUID.
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Additional Considerations
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-------------------------
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There are cases in which, even if the general rule given above is followed in
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principle, the property set may still not be regarded as a valid one.
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For example, that applies to device properties which may cause kernel code
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(either a device driver or a library/subsystem) to access hardware in a way
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possibly leading to a conflict with AML methods in the ACPI namespace. In
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particular, that may happen if the kernel code uses device properties to
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manipulate hardware normally controlled by ACPI methods related to power
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management, like _PSx and _DSW (for device objects) or _ON and _OFF (for power
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resource objects), or by ACPI device disabling/enabling methods, like _DIS and
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_SRS.
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In all cases in which kernel code may do something that will confuse AML as a
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result of using device properties, the device properties in question are not
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suitable for the ACPI environment and consequently they cannot belong to a valid
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property set.
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Property Sets and Device Tree Bindings
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======================================
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It often is useful to make _DSD return property sets that follow Device Tree
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bindings.
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In those cases, however, the above validity considerations must be taken into
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account in the first place and returning invalid property sets from _DSD must be
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avoided. For this reason, it may not be possible to make _DSD return a property
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set following the given DT binding literally and completely. Still, for the
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sake of code re-use, it may make sense to provide as much of the configuration
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data as possible in the form of device properties and complement that with an
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ACPI-specific mechanism suitable for the use case at hand.
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In any case, property sets following DT bindings literally should not be
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expected to automatically work in the ACPI environment regardless of their
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contents.
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References
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==========
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.. [1] http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf
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.. [2] http://www.uefi.org/sites/default/files/resources/_DSD-hierarchical-data-extension-UUID-v1.1.pdf
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