[coreboot - Bug #623] On T480/S, missing P-state data from ACPI tables prevents Xen from performing frequency scaling

Issue #623 has been updated by Patrick Rudolph.


Set register "eist_enable" = "true" in devicetree.cb?

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Bug #623: On T480/S, missing P-state data from ACPI tables prevents Xen from performing frequency scaling
https://ticket.coreboot.org/issues/623#change-2212

* Author: herme herme
* Status: New
* Priority: Normal
* Target version: none
* Start date: 2026-01-16
* Affected versions: main
* Affected hardware: T480/S
* Affected OS: QubesOS
----------------------------------------
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](https://codeberg.org/libreboot/lbmk/issues/394) 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.





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