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The Windows WMI-ACPI driver likely uses wmilib [1] to interact with
the WMI service in userspace. Said library uses plain byte buffers
for exchanging data, so the WMI-ACPI driver has to convert between
those byte buffers and ACPI objects returned by the ACPI firmware.
The format of the byte buffer is publicly documented [2], and after
some reverse eingineering of the WMI-ACPI driver using a set of custom
ACPI tables, the following conversion rules have been discovered:
- ACPI integers are always converted into a uint32
- ACPI strings are converted into special WMI strings
- ACPI buffers are copied as-is
- ACPI packages are unpacked
Extend the ACPI-WMI driver to also perform this kind of marshalling
for WMI data blocks, methods and events. Doing so gives us a number
of benefits:
- WMI drivers are not restricted to a fixed set of supported ACPI data
types anymore, see dell-wmi-aio (integer vs buffer) and
hp-wmi-sensors (string vs buffer)
- correct marshalling of WMI strings when data blocks are marked
as requiring ACPI strings instead of ACPI buffers
- development of WMI drivers without having to understand ACPI
This eventually should result in better compatibility with some
ACPI firmware implementations and in simpler WMI drivers. There are
however some differences between the original Windows driver and
the ACPI-WMI driver when it comes to ACPI object conversions:
- the Windows driver copies internal _ACPI_METHOD_ARGUMENT_V1 data
structures into the output buffer when encountering nested ACPI
packages. This is very likely an error inside the driver itself, so
we do not support nested ACPI packages.
- when converting WMI strings (UTF-16LE) into ACPI strings (ASCII),
the Windows driver replaces non-ascii characters (ä -> a, & -> ?)
instead of returning an error. This behavior is not documented
anywhere and might lead to severe errors in some cases (like
setting BIOS passwords over WMI), so we simply return an error.
As the current bus-based WMI API is based on ACPI buffers, a new
API is necessary. The legacy GUID-based WMI API is not extended to
support marshalling, as WMI drivers using said API are expected to
move to the bus-based WMI API in the future.
[1] https://learn.microsoft.com/de-de/windows-hardware/drivers/ddi/wmilib/
[2] https://learn.microsoft.com/en-us/windows-hardware/drivers/kernel/
driver-defined-wmi-data-items
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://patch.msgid.link/20260116204116.4030-2-W_Armin@gmx.de
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
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