[Bug 234455] CPU frequency scaling fails for multiple cores

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234455

            Bug ID: 234455
           Summary: CPU frequency scaling fails for multiple cores
           Product: Base System
           Version: 12.0-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: dfrg at xsmail.com

An intel i5-8400 processor has 6 cores. On both FreeBSD 11.2 and 12.0-RELEASE
frequency scaling via both powerd or explicit sysctl is broken unless all but
one core is disabled via BIOS.

# sysctl dev.cpu.0.freq_levels
dev.cpu.0.freq_levels: 2801/65000 2800/65000 2700/61690 2500/55888 2400/52759
2200/47301 2100/44946 1900/39204 1800/37000 1700/34277 1500/29546 1400/27531
1200/22557 1100/20688 900/16549 800/14296

# sysctl dev.cpu.0.freq=2800
dev.cpu.0.freq: 800 -> 800

It appears cf_set_method() returns EPERM at sys/kern_cpu.c:287 and commenting
out that conditional restores frequency scaling when multiple cores are
enabled.

Whilst this ugly local patch suffices its obviously not correct and I'm not
sure how to proceed from here.

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