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The "Intel 64 and IA-32 Architectures Software Developer’s Manual Volume 4:
Model-Specific Registers" has the following table for the values from
freq_desc_byt:
000B: 083.3 MHz
001B: 100.0 MHz
010B: 133.3 MHz
011B: 116.7 MHz
100B: 080.0 MHz
Notice how for e.g the 83.3 MHz value there are 3 significant digits, which
translates to an accuracy of a 1000 ppm, where as a typical crystal
oscillator is 20 - 100 ppm, so the accuracy of the frequency format used in
the Software Developer’s Manual is not really helpful.
As far as we know Bay Trail SoCs use a 25 MHz crystal and Cherry Trail
uses a 19.2 MHz crystal, the crystal is the source clock for a root PLL
which outputs 1600 and 100 MHz. It is unclear if the root PLL outputs are
used directly by the CPU clock PLL or if there is another PLL in between.
This does not matter though, we can model the chain of PLLs as a single PLL
with a quotient equal to the quotients of all PLLs in the chain multiplied.
So we can create a simplified model of the CPU clock setup using a
reference clock of 100 MHz plus a quotient which gets us as close to the
frequency from the SDM as possible.
For the 83.3 MHz example from above this would give 100 MHz * 5 / 6 = 83
and 1/3 MHz, which matches exactly what has been measured on actual
hardware.
Use a simplified PLL model with a reference clock of 100 MHz for all Bay
and Cherry Trail models.
This has been tested on the following models:
CPU freq before: CPU freq after:
Intel N2840 2165.800 MHz 2166.667 MHz
Intel Z3736 1332.800 MHz 1333.333 MHz
Intel Z3775 1466.300 MHz 1466.667 MHz
Intel Z8350 1440.000 MHz 1440.000 MHz
Intel Z8750 1600.000 MHz 1600.000 MHz
This fixes the time drifting by about 1 second per hour (20 - 30 seconds
per day) on (some) devices which rely on the tsc_msr.c code to determine
the TSC frequency.
Reported-by: Vipul Kumar <vipulk0511@gmail.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20200223140610.59612-3-hdegoede@redhat.com
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