Lukas Wunner submitted a patch to the Linux-(kernel,ACPI) lists with an ACPI patch to support Apple _DSM properties. Comments included below:
While the rest of the world has standardized on _DSD as the way to store device properties in AML (introduced with ACPI 5.1 in 2014), Apple has been using a custom _DSM to achieve the same for much longer (ever since they switched from DeviceTree-based PowerPC to Intel in 2005, verified with MacOS X 10.4.11).
The theory of operation on macOS is as follows: AppleACPIPlatform.kext invokes mergeEFIproperties() and mergeDSMproperties() for each device to merge properties conveyed by EFI drivers as well as properties stored in AML into the I/O Kit registry from which they can be retrieved by drivers. We’ve been supporting EFI properties since commit 58c5475aba67 (“x86/efi: Retrieve and assign Apple device properties”). The present commit adds support for _DSM properties, thereby completing our support for Apple device properties. The _DSM properties are made available under the primary fwnode, the EFI properties under the secondary fwnode. So for devices which possess both property types, they can all be elegantly accessed with the uniform API in <linux/property.h>.
Until recently we had no need to support _DSM properties, they contained only uninteresting garbage. The situation has changed with MacBooks and MacBook Pros introduced since 2015: Their keyboard is attached with SPI instead of USB and the _CRS data which is necessary to initialize the spi driver only contains valid information if OSPM responds “false” to _OSI(“Darwin”). If OSPM responds “true”, _CRS is empty and the spi driver fails to initialize. The rationale is very simple, Apple only cares about macOS and Windows: On Windows, _CRS contains valid data, whereas on macOS it is empty. Instead, macOS gleans the necessary data from the _DSM properties.
Since Linux deliberately defaults to responding “true” to _OSI(“Darwin”), we need to emulate macOS’ behaviour by initializing the spi driver with data returned by the _DSM.
An out-of-tree driver for the SPI keyboard exists which currently binds to the ACPI device, invokes the _DSM, parses the returned package and instantiates an SPI device with the data gleaned from the _DSM:
By adding support for Apple’s _DSM properties in generic ACPI code, the out-of-tree driver will be able to register as a regular SPI device, significantly reducing its amount of code and improving its chances to be mainlined.
The SPI keyboard will not be the only user of this commit: E.g. on the MacBook8,1, the UART-attached Bluetooth device likewise returns empty _CRS data if OSPM returns “true” to _OSI(“Darwin”).
The _DSM returns a Package whose format unfortunately deviates slightly from the _DSD spec: The properties are marshalled up in a single Package as alternating key/value elements, unlike _DSD which stores them as a Package of 2-element Packages. The present commit therefore converts the Package to _DSD format and the ACPI core can then treat the data as if Apple would follow the standard.
Well, except for one small annoyance: The properties returned by the _DSM only ever have one of two types, Integer or Buffer. The former is retrievable as usual with device_property_read_u64(), but the latter is not part of the _DSD spec and it is not possible to retrieve Buffer properties with the device_property_read_*() functions due to the type checking performed in drivers/acpi/property.c. It is however possible to retrieve them with acpi_dev_get_property(). Apple is using the Buffer type somewhat sloppily to store null-terminated strings but also integers. The real data type is not distinguishable by the ACPI core and the onus is on the caller to use the contents of the Buffer in an appropriate way.
In case Apple moves to _DSD in the future, this commit first checks for _DSD and falls back to _DSM only if _DSD is not found.