On Fri, Mar 06, 2020 at 04:44:14PM +0100, Gerd Hoffmann wrote:
> Factor out TimerKHz and ShiftTSC calculation to tsctimer_configure().
>
> Signed-off-by: Gerd Hoffmann <kra...@redhat.com>
> ---
> src/hw/timer.c | 21 +++++++++++++--------
> 1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/src/hw/timer.c b/src/hw/timer.c
> index bdcb3bfca211..ce3d63cd75f6 100644
> --- a/src/hw/timer.c
> +++ b/src/hw/timer.c
> @@ -58,6 +58,18 @@ u8 ShiftTSC VARFSEG;
> * Internal timer setup
> ****************************************************************/
>
> +static void
> +tsctimer_configure(u64 t)
> +{
> + while (t >= (1<<24)) {
> + ShiftTSC++;
> + t = (t + 1) >> 1;
> + }
> + TimerKHz = DIV_ROUND_UP((u32)t, 1000 * PMTIMER_TO_PIT);
> + TimerPort = 0;
> + dprintf(1, "CPU Mhz=%u\n", (TimerKHz << ShiftTSC) / 1000);
> +}
> +
> #define CALIBRATE_COUNT 0x800 // Approx 1.7ms
>
> // Calibrate the CPU time-stamp-counter
> @@ -87,14 +99,7 @@ tsctimer_setup(void)
> dprintf(6, "tsc calibrate start=%u end=%u diff=%u\n"
> , (u32)start, (u32)end, (u32)diff);
> u64 t = DIV_ROUND_UP(diff * PMTIMER_HZ, CALIBRATE_COUNT);
> - while (t >= (1<<24)) {
> - ShiftTSC++;
> - t = (t + 1) >> 1;
> - }
> - TimerKHz = DIV_ROUND_UP((u32)t, 1000 * PMTIMER_TO_PIT);
> - TimerPort = 0;
> -
> - dprintf(1, "CPU Mhz=%u\n", (TimerKHz << ShiftTSC) / 1000);
> + tsctimer_configure(t);
> }
The PMTIMER_TO_PIT in DIV_ROUND_UP((u32)t, 1000 * PMTIMER_TO_PIT) is
directly tied to the measurement interval performed in
tsctimer_setup(). Specifically, the code calculates the number of tsc
ticks in n PIT intervals and then calculates
"tsc_hz=pit_hz*elapsed_tsc/n". The complicated math is to avoid 64bit
divides. I recommend against moving that low-level math to another
function, as I fear the relationship would then be harder to
understand.
-Kevin
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