coreboot

Hello

Currently, P-state data is missing from ACPI tables on T480/S. On my T480S running libreboot, this was confirmed by using the iasl command to decompile the tables at /sys/firmware/acpi/tables/; grepping for _PSS, _PCT and _PPC didn't return any results. Another user has confirmed that this is also the case for the similar T480 device, with latest release build of libreboot (26.01 RC1).

Meanwhile, in the vendor T480S BIOS, this data is present (contributed by @henk on #coreboot IRC channel):

# grep -r -e _PCT -e _PPC -e _PSS *.dsl
DSDT.dsl:                        If (CondRefOf (\_PR.PR00._PPC))
DSDT.dsl:                        If ((CondRefOf (\_PR.PR00._PSS) && CondRefOf (\_PR.PR00._PPC)))
SSDT1.dsl:    External (_PR_.PR00._PSS, MethodObj)    // 0 Arguments
SSDT1.dsl:        Name (_PPC, Zero)  // _PPC: Performance Present Capabilities
SSDT1.dsl:            If (CondRefOf (\_PR.PR00._PSS))
SSDT1.dsl:        Method (_PSS, 0, NotSerialized)  // _PSS: Performance Supported States
SSDT1.dsl:            If (CondRefOf (\_PR.PR00._PSS))
SSDT1.dsl:                Return (\_PR.PR00._PSS ())
SSDT1.dsl:            If (CondRefOf (\_PR.PR00._PSS))
SSDT8.dsl:            If (CondRefOf (\_PR.PR00._PSS))

The Linux kernel doesn't rely on this data to perform frequency scaling, because native support is provided by the intel_pstate driver. However, the Xen kernel lacks this native support. When the data is missing, as in my case, the Xen-based QubesOS runs underclocked and is only barely usable.