How does the Kernel determine processor speed at boot?

How does the Kernel determine the processor speed displayed in the boost.msg/dmesg log?

<6>Extended CMOS year: 2000
<6>TSC: PIT calibration confirmed by PMTIMER.
<6>TSC: using PIT calibration value
<4>Detected 2593.093 MHz processor.
<7>spurious 8259A interrupt: IRQ7.
<4>Console: colour dummy device 80x25
<6>console [tty0] enabled
<6>Checking aperture...

I would like to get this value and use it in conjunction with TSC for a high resolution / high performance timer. I’m currently using value ready from /proc/cpuinfo, but some recent experiments show that value is not correct; at least the /proc/cpuinfo value is not correct relative to gettimeofday().

Experiments show that the value displayed in /var/log/boot.msg (2.6.27 kernel) and /var/log/dmsg (2.6.32 kernel) is correct relative to gettimeofday().

I’m trying to find an alternative to opening a file in /var/log and reading the value, maybe a system call?

Please don’t flame me on the use of TSC for keeping track of time. I’m well aware of all the related issues. For my use case, TSC is ideal (if I can just get the correct frequency.)

Regards,
Matt

You can look into
/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq

Originally Posted by bsat You can look into /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq

Value found in /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq is from the cpufreq module. Those values seem to always be nice round numbers, i.e. 2600000, when the actual frequency of processor detected at boot is 2593093. That is enough error to throw off my calculations of time. Also, I actually need the frequency of the TSC, not the CPU frequency, which may be stepped up or down relative to TSC.

The Processor frequency detected at boot does seem to match the TSC frequency.

-Matt

You can read code in time.c and find out how to get exact CPU clock.