The start of interrupt and timer support, with some DeviceTree parsing.

1 files changed, 43 insertions, 0 deletions

diff --git a/kernel/src/drivers/riscv_timer.rs b/kernel/src/drivers/riscv_timer.rs
new file mode 100644
index 0000000..a702f7b
--- /dev/null
+++ b/kernel/src/drivers/riscv_timer.rs
@@ -0,0 +1,43 @@
+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));
+    }
+}