[SRU][F][J][PATCH 0/1] ACPI: CPPC: Disable FIE if registers in PCC regions

[Shiyou Huang]

Buglink: https://bugs.launchpad.net/ubuntu-realtime/+bug/1993326

SRU Justification:

[Impact]
With cppc_fie enabled, users can see 2% - 11% CPU overhead even when the system is idle. On real time kernel, cppc_fie leads to higher latencies up to 120 microseconds.
The following patch:
https://github.com/torvalds/linux/commit/ae2df912d1a557a3548be83da20851ac55f42ab3
fixes this issue by allowing users to disable cppc_fie at boot time.

[Test Case]
Running 'cyclictest' on an 80-core arm system with the following command:
 `sudo cyclictest -m -Sp90 -i200 -l100000 -h 400`
Without the patch, the max latency reported is 121 us while with the patch the max latency is 20 us.

[Regression Potential]
This patch only affects the aarch64 architecture and is limited to the CPPC_CPUFREQ driver. Any regression is limited to this driver. From our testing, we find that there are no regressions by this patch.

[Other Info]
None

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