ACPI / property: Document usage rules for _DSD properties

Following some discussions during the Kernel Summit and LPC, document
what can be returned from ACPI _DSD as device properties and when it
is valid to use the special PRP0001 device ID.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviwed-by: Mark Brown <broonie@kernel.org>

Documentation/acpi/DSD-properties-rules.txt

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

Documentation/acpi/enumeration.txt

@@ -415,3 +415,12 @@ the "compatible" property in the _DSD or a _CID as long as one of their
 ancestors provides a _DSD with a valid "compatible" property.  Such device
 objects are then simply regarded as additional "blocks" providing hierarchical
 configuration information to the driver of the composite ancestor device.
+
+However, PRP0001 can only be returned from either _HID or _CID of a device
+object if all of the properties returned by the _DSD associated with it (either
+the _DSD of the device object itself or the _DSD of its ancestor in the
+"composite device" case described above) can be used in the ACPI environment.
+Otherwise, the _DSD itself is regarded as invalid and therefore the "compatible"
+property returned by it is meaningless.
+
+Refer to DSD-properties-rules.txt for more information.