Hi,
On Thursday 01 November 2007, Ralf Baechle wrote:
> kernel/time/ntp.c contains the following piece of code:
>
> #define CLOCK_TICK_OVERFLOW (LATCH * HZ - CLOCK_TICK_RATE)
> #define CLOCK_TICK_ADJUST (((s64)CLOCK_TICK_OVERFLOW * NSEC_PER_SEC)
> / \ (s64)CLOCK_TICK_RATE)
>
> static void ntp_update_frequency(void)
> {
> u64 second_length = (u64)(tick_usec * NSEC_PER_USEC * USER_HZ)
> << TICK_LENGTH_SHIFT;
> second_length += (s64)CLOCK_TICK_ADJUST << TICK_LENGTH_SHIFT;
> second_length += (s64)time_freq << (TICK_LENGTH_SHIFT -
> SHIFT_NSEC);
>
> tick_length_base = second_length;
>
> do_div(second_length, HZ);
> tick_nsec = second_length >> TICK_LENGTH_SHIFT;
>
> do_div(tick_length_base, NTP_INTERVAL_FREQ);
> }
>
> So it uses CLOCK_TICK_RATE which on many systems but not all is defined to
> the i8253 input clock. But timekeeping on anything remotely modern makes
> little use of the i8253 so I wonder the intent was here.
The basic idea is to provide a base frequency adjustment, when I wrote this I
already wasn't entirely happy that it was hardcoded like this, but in the end
I simply reimplemented what the old code did.
It's not strictly needed, so if someone wants to add something like:
#ifndef CLOCK_TICK_RATE
#define CLOCK_TICK_ADJUST 0
#else
...
it would be fine with me.
bye, Roman
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