Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [RFC] x86, tsc: Add kcmdline args for skipping tsc calibration
 sequences
From:   patrickg <patrickg@supermicro.com>
To:     "Brown, Len" <len.brown@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     "mingo@kernel.org" <mingo@kernel.org>,
        "Du, Alek" <alek.du@intel.com>,
        "arjan@linux.intel.com" <arjan@linux.intel.com>,
        "Tang, Feng" <feng.tang@intel.com>
References: <fdf96605-a4a0-049b-51c9-1e68cc2a9b93@supermicro.com>
 <1A7043D5F58CCB44A599DFD55ED4C94849A1AACF@FMSMSX126.amr.corp.intel.com>
 <c7582a8c-8cc1-eff3-2452-a8d42d23fea1@supermicro.com>
Message-ID: <cb798dcd-8cd6-8a92-9f6c-50bec0afe204@supermicro.com>
Date:   Tue, 24 Jul 2018 12:45:22 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.1
MIME-Version: 1.0
In-Reply-To: <c7582a8c-8cc1-eff3-2452-a8d42d23fea1@supermicro.com>
Content-Type: text/plain; charset="utf-8"
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

K, did significant poking.

native_calibrate_cpu is getting precidence no matter what because on SKL server, native_calibrate_tsc is always returning zero (Note that there is a caveat 2 lines down).

In native_calibrate_tsc, I'm seeing it always return zero after the `switch (boot_cpu_data.x86_model)`.  crystal_khz is zero so it rolls through that, never assigns it.

Now I'm apparently not testing on production CPU's.  I've requested that they drop em' in so I can ensure `CPUID 15H TSC/Crystal ratio` doesn't differ between the ES and Prod silicon, since that will effect the math 

Anyways, Then after calibrate is found to be zero, it's utilizing the CPUID calculation from native_calibrate_cpu for tsc_khz since that's returning a 'proper' value when it does the reassignment:

---
    if (tsc_khz == 0) /* Hits here and reassigns tsc_khz to the cpuid calculation. */
        tsc_khz = cpu_khz;
    else if (abs(cpu_khz - tsc_khz) * 10 > tsc_khz)
        cpu_khz = tsc_khz;
---

I'll post another update when I've checked with Prod CPU's, /probably/ tomorrow.