Implement PIT driver

kernel/src/lib.rs

@@ -38,6 +38,7 @@ mod rtc;
 mod sched;
 mod selftest;
 mod sync;
+mod time;
 mod x86_util;
 
 #[cfg(not(test))]
@@ -81,6 +82,7 @@ pub extern "C" fn kinit(_mbinfop: *const multiboot::Info, boot_infop: *const han
     mm::init(mem_map.clone());
     acpi::init();
     interrupts::init();
+    time::init();
     rtc::init();
 
     selftest::run_tests();

kernel/src/time/mod.rs

@@ -0,0 +1,15 @@
+mod pit;
+
+pub fn init() {
+    pit::init();
+}
+
+/// Represents a periodic tick generator.
+trait TickSource {
+    /// Get an estimation of the ticks emitted per second.
+    fn approx_ticks_per_second(&self) -> u64;
+    // Set a function to be called for each tick.
+    fn set_tick_handler(&mut self, fn(u64));
+    // Get the current number of elapsed ticks.
+    fn get_ticks(&self) -> u64;
+}

kernel/src/time/pit.rs

@@ -0,0 +1,104 @@
+use spin::Mutex;
+
+use interrupts::set_irq_handler;
+use x86_util::{inb, outb};
+
+use super::TickSource;
+
+/// A `TickSource` adapter for the programmable interval timer.
+pub struct Pit {
+    data: spin::Mutex<PitData>,
+}
+
+struct PitData {
+    ticks: u64,
+    handler: Option<fn(u64)>,
+}
+
+impl TickSource for Pit {
+    fn approx_ticks_per_second(&self) -> u64 {
+        1193182 / PIT_RELOAD_VALUE as u64
+    }
+
+    fn set_tick_handler(&mut self, tick_handler: fn(u64)) {
+        unsafe {
+            asm!("cli");
+        }
+
+        {
+            let mut data = self.data.lock();
+            data.handler = Some(tick_handler);
+        }
+
+        unsafe {
+            asm!("sti");
+        }
+    }
+
+    fn get_ticks(&self) -> u64 {
+        let ticks;
+
+        unsafe {
+            asm!("cli");
+        }
+
+        {
+            let mut data = self.data.lock();
+            ticks = data.ticks;
+        }
+
+        unsafe {
+            asm!("sti");
+        }
+
+        ticks
+    }
+}
+
+static PIT: Pit = Pit {data: spin::Mutex::new(PitData { ticks: 0, handler: None })};
+
+pub fn init() {
+    // The PIT IRQ is 0. Set our handler for that IRQ.
+    set_irq_handler(0, Some(timer_handler));
+
+    unsafe {
+        // Set channel 0 to lobyte/hibyte access mode and rate
+        // generator operating mode.
+        outb(0x43, 0b00_11_010_0);
+        // Output our desired reload value.
+        outb(0x40, PIT_RELOAD_VALUE as u8);
+        outb(0x40, (PIT_RELOAD_VALUE >> 8) as u8);
+    }
+}
+
+fn timer_handler() {
+    let now_ticks;
+    let maybe_handler;
+
+    {
+        unsafe {
+            asm!("cli");
+        }
+
+        let mut pit_data = PIT.data.lock();
+        pit_data.ticks += 1;
+
+        now_ticks = pit_data.ticks;
+        maybe_handler = pit_data.handler;
+
+        unsafe {
+            asm!("sti");
+        }
+    }
+
+    match maybe_handler {
+        Some(handler) => handler(now_ticks),
+        None => (),
+    };
+}
+
+// The counter value at which we want the PIT to generate an
+// interrupt. The interrupt frequency is
+// 1193182 / `PIT_RELOAD_VALUE`. For an interrupt about every
+// 10 microseconds, we use a value of 20.
+const PIT_RELOAD_VALUE: u16 = 1193;