Executive brief
The Linux kernel's Coresight tracer (ETE component) was not properly saving hardware state when CPUs powered down, causing trace data loss and warnings during system suspend/resume. This affects systems using ARM Coresight tracing with ACPI firmware, particularly on development boards that support CPU power management.
Technical details
This is a logic error in the coresight ETE (Embedded Trace Extension) driver where the pm_save_enable parameter was not being applied to system-register-based ETMs and ETE devices, which always lose context on CPU power down. The fix forces save_state to true for sysreg ETMs and ACPI boots by introducing the etm4x_always_pm_save() function and using a local variable during device initialization. The root cause was that ETE's device-tree binding never documented the "arm,coresight-loses-context-with-cpu" property, and ACPI has no equivalent binding, leaving no way for users to enable the necessary state preservation. The patch ensures ETM/ETE trace state is always saved before power down, preventing loss of tracing data and eliminating spurious "External agent took claim tag" warnings.
Affected products
- Linux Linux kernel Multiple stable series (linux-5.x through linux-6.x and later)
Timeline
- 2026-08-28: disclosed
- 2026-05-05: patched: Upstream fix committed; backported to stable kernels