//! Support for RISC-V's timers, using the Sstc extension.
#![cfg(target_arch = "riscv64")]
#![no_std]

use core::{arch::asm, ops::Add, time::Duration};

/// The number of `Instant` "ticks" in a second. Initialized by the early-boot DeviceTree parser.
pub static mut TIMEBASE_FREQUENCY: u32 = 0;

/// A moment in time.
#[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
pub struct Instant(u64);

impl Instant {
    /// Returns the current time as an `Instant`.
    pub fn now() -> Instant {
        let out;
        // SAFETY: We require the Sstc extension, enable it before jumping to Rust, and never
        // disable it.
        unsafe {
            asm!("rdtime {out}", out = out(reg) out, options(nomem, nostack));
        }
        Instant(out)
    }
}

impl Add<Duration> for Instant {
    type Output = Instant;

    #[allow(clippy::suspicious_arithmetic_impl)]
    fn add(self, duration: Duration) -> Instant {
        // SAFETY: TIMEBASE_FREQUENCY is never concurrently written to.
        let ticks_per_second = unsafe { TIMEBASE_FREQUENCY };
        let ticks = duration.as_nanos().wrapping_mul(ticks_per_second as u128) / 1_000_000_000;
        Instant(self.0.wrapping_add(ticks as u64))
    }
}

/// Sets the timer interrupt to fire at the given instant. Note that this sets a global value,
/// rather than interacting with the scheduler in any way.
pub fn set_timer(instant: Instant) {
    // SAFETY: We require the Sstc extension, enable it before jumping to Rust, and never
    // disable it.
    unsafe {
        asm!("csrw stimecmp, {instant}", instant = in(reg) instant.0, options(nomem, nostack));
    }
}