feat(config): enable PL031 RTC + HCTOSYS on arm64

[AprilNEA]

Companion to arcboxlabs/arcbox#373 (ABX-416).

The ArcBox HV backend now exposes a PL031 RTC backed by the host wall clock. This enables:

• CONFIG_RTC_CLASS=y + CONFIG_RTC_DRV_PL031=y — bind the device via the AMBA bus
• CONFIG_RTC_HCTOSYS=y (+ _DEVICE="rtc0") — set CLOCK_REALTIME from rtc0 at boot, so the guest has correct wall time before the agent's clock-sync ping (TLS validation during early boot)

Inert under VZ, which exposes no PL031. Local build verification against the new arcbox PL031 device in progress.

[greptile-apps]

Greptile Summary

This PR enables early wall-clock initialization for the arm64 ArcBox kernel. The main changes are:

• Adds built-in RTC class support.
• Enables CONFIG_RTC_DRV_PL031 for the HV-backed PL031 RTC.
• Enables CONFIG_RTC_HCTOSYS from rtc0 at boot.
• Keeps CONFIG_RTC_SYSTOHC disabled so the guest does not write time back to the host-backed RTC.

Confidence Score: 5/5

Safe to merge with minimal risk.

The change is limited to arm64 kernel config and matches the described one-way RTC-to-system clock flow.

No files require special attention.

T-Rex Logs

What T-Rex did

• Verified that the after RTC config shows all RTC/HCTOSYS assertions passing, as captured in the after-log evidence.
• Compared against the prior HEAD^ config, which showed a failed assertion for missing explicit SYSTOHC disablement in the before-log evidence.
• Documented the prerequisites that were missing for a local arm64 olddefconfig preservation check, including Docker, the arm64 cross-compiler, QEMU arm64, and dtc, as recorded in the prerequisites-after log.
• Described the olddefconfig blocker evidence that these missing prerequisites prevented a preserved make ARCH=arm64 olddefconfig step, as shown in the olddefconfig-after log.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
configs/arcbox-arm64.config	Adds built-in PL031 RTC and HCTOSYS configuration for arm64 guests; no blocking issues identified.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
participant HV as ArcBox HV backend
participant AMBA as AMBA bus
participant RTC as PL031 RTC driver
participant Kernel as Linux timekeeping
participant Agent as ArcBox agent

HV->>AMBA: Expose PL031 RTC device
AMBA->>RTC: Bind built-in `CONFIG_RTC_DRV_PL031`
RTC->>Kernel: Register `rtc0`
Kernel->>RTC: `CONFIG_RTC_HCTOSYS` reads `rtc0` at boot
RTC-->>Kernel: Host-backed wall clock
Kernel->>Kernel: Set `CLOCK_REALTIME`
Agent->>Kernel: Later clock-sync ping runs after early TLS validation

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
participant HV as ArcBox HV backend
participant AMBA as AMBA bus
participant RTC as PL031 RTC driver
participant Kernel as Linux timekeeping
participant Agent as ArcBox agent

HV->>AMBA: Expose PL031 RTC device
AMBA->>RTC: Bind built-in `CONFIG_RTC_DRV_PL031`
RTC->>Kernel: Register `rtc0`
Kernel->>RTC: `CONFIG_RTC_HCTOSYS` reads `rtc0` at boot
RTC-->>Kernel: Host-backed wall clock
Kernel->>Kernel: Set `CLOCK_REALTIME`
Agent->>Kernel: Later clock-sync ping runs after early TLS validation

Loading

_{Reviews (2): Last reviewed commit: "fix(config): explicitly disable RTC_SYST..." | Re-trigger Greptile}

[chatgpt-codex-connector]

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