Function microseconds_monotonic

kernel/src/time.rs:59–86

Monotonic count of he number of microseconds since restart Uses PIT interrupts to calibrate the TSC

()

/// Uses PIT interrupts to calibrate the TSC
/// 
pub fn microseconds_monotonic() -> u64 {
    // Number of PIT ticks
    let pit = PIT_TICKS.load(Ordering::Relaxed);
    // Number of TSC ticks since last PIT interrupt
    let tsc = time_stamp_counter() - LAST_TSC.load(Ordering::Relaxed);

    // Number of TSC counts per PIT tick
    let tsc_per_pit = TSC_PER_PIT.load(Ordering::Relaxed);

    // PIT frequency is 3_579_545 / 3 = 1_193_181.666 Hz
    //                   each PIT tick is 0.83809534452 microseconds
    //             878807 / (1024*1024) = 0.83809566497
    // 
    // Calculate total TSC then divide to get microseconds
    // Note: Don't use TSC directly because jitter in tsc_per_pit would lead to
    // non-monotonic outputs

    // Note! This next expression will overflow in about 2 hours :
    //       2**64 / (1024 * 1024 * 2270) microseconds
    //((pit * tsc_per_pit + tsc) * 878807) / (1024*1024 * tsc_per_pit)

    const SCALED_TSC_RATE: u64 = 16;
    let scaled_tsc = (tsc * SCALED_TSC_RATE) / tsc_per_pit;

    // Factorize 878807 = 437 * 2011
    // This will overflow in about 142 years : 2**64 / 4096 microseconds
    ((((pit * SCALED_TSC_RATE + scaled_tsc) * 2011) / 4096) * 437) / (256 * SCALED_TSC_RATE)
}