[Bug 240475] [PATCH]: Add support for CPUID 0x16 in tsc_freq_cpuid() (TSC clock)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240475

            Bug ID: 240475
           Summary: [PATCH]: Add support for CPUID 0x16 in
                    tsc_freq_cpuid() (TSC clock)
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: neel at neelc.org

Created attachment 207346
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=207346&action=edit
Patch (Revision 1)

Keep in mind that I am new to the kernel.

Description:

If frequency for CPUID 0x15 is zero, fall back to CPUID 0x16 for the TSC clock
if available. This is needed on some Intel SOCs like Skylake and Kaby Lake
whcih report 0 for the crystal clock..

Inspired by Linux commit 604dc9170f2435d27da5039a3efd757dceadc684.

Tested on the HP Spectre x360 13-ap0043dx running 13-CURRENT with an accurate
clock using this patch (without the patch, the ticks are slower).

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