Messages in this thread | | | Date | Wed, 21 Dec 2022 23:35:36 +0100 | From | "Christian A. Ehrhardt" <> | Subject | Re: [PATCH v2] Add hardcoded crystal clock for KabyLake |
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Hi,
On Mon, Nov 14, 2022 at 11:58:54PM +0100, Thomas Gleixner wrote: > On Thu, Oct 20 2022 at 10:18, Dave Hansen wrote: > > On 10/20/22 10:13, Peter Zijlstra wrote: > >> And why, pray *WHY* can't Intel simply write the correct information in > >> CPUID leaf 15h. I mean, they defined the leaf, might as well use it, no? > > > > Is the data that's in the leaf just wrong? Doesn't that mean that the > > CPUID leaf on these models is violating the architecture contract? That > > sounds like something that deserves an erratum. > > > > Is there a documented erratum? > > I don't know. The code has this comment: > > /* > * Some Intel SoCs like Skylake and Kabylake don't report the crystal > * clock, but we can easily calculate it to a high degree of accuracy > * by considering the crystal ratio and the CPU speed. > */
Latest (April 2022) version of the SDM clearly states that the above comment is wrong. CPUID.16h has the following note: | Data is returned from this interface in accordance with the processor's | specification and does not reflect actual values. Suitable use of this | data includes the display of processor information in like manner to the | processor brand string and for determining the appropriate range to use | when displaying processor information e.g. frequency history graphs. The | returned information should not be used for any other purpose as the | returned information does not accurately correlate to information / | counters returned by other processor interfaces.
Thus using CPUID.16h to determine the crystal clock frequency is wrong. This difference is significant. I have one Kaby Lake latop where the CPUID.16h reported frequency is 1900MHz but the real frequency is only 1896MHz. This amounts to a time drift of about 8s/hour if the wrong TSC frequency is used for time keeping.
Basically, I think this commit: 604dc9170 (x86/tsc: Use CPUID.0x16 to calculate ...) needs to be reverted.
> so those SoCs fail to expose clock in leaf 15h and then the information > in leaf 16h is so inaccurate that the calculation is off. > > Sigh. It's 2022 and we are still relying on crystalball mechanisms to > figure out the damned crystal frequency. > > The specification of leaf 15h is: > > 15H Time Stamp Counter and Nominal Core Crystal Clock Information Leaf > NOTES: > If EBX[31:0] is 0, the TSC/”core crystal clock” ratio is not enumerated. > If ECX is 0, the nominal core crystal clock frequency is not enumerated. > > IOW, this CPUID leaf is defined to be useless and leaves it up to the > SoC integration to provide this information or not. It needs even two > fields to chose from to make it useless...
The SDM (now?) has some hints on how to do this. This is hidden here:
Vol.3 Chapter 19.7.3: Determining the Processor Base Frequency
This chapter contains a table that lists the correct crystal clock frequencies for CPU models that do not enumerate it via CPUID.15h.
regards Christian
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