[kernel] Add HPET support, create clock class

Create a clock class which can be queried for current timestamp in
nanoseconds. Also implements a simple HPET class as one possible clock
source.

Tags: time

src/kernel/clock.h

@@ -0,0 +1,44 @@
+#pragma once
+/// \file clock.h
+/// The kernel time keeping interface
+
+#include <stdint.h>
+
+class clock
+{
+public:
+	/// A source is a function that returns the current
+	/// value of some clock source.
+	using source = uint64_t (*)(void*);
+
+	/// Constructor.
+	/// \arg rate   Number of source ticks per ns
+	/// \arg source Function for the clock source
+	/// \arg data   Data to pass to the source function
+	clock(uint64_t rate, source source_func, void *data);
+
+	/// Get the current value of the clock.
+	/// \returns Current value of the source, in ns
+	/// TODO: optimize divison by finding a multiply and
+	/// shift value instead
+	inline uint64_t value() const { return m_source(m_data) / m_rate; }
+
+	/// Update the internal state via the source
+	/// \returns Current value of the clock
+	inline void update() { m_current = value(); return m_current; }
+
+	/// Wait in a tight loop
+	/// \arg interval  Time to wait, in ns
+	void spinwait(uint64_t ns) const;
+
+	/// Get the master clock
+	static clock & get() { return *s_instance; }
+
+private:
+	uint64_t m_current; ///< current ns count
+	uint64_t m_rate; ///< source ticks per ns
+	void *m_data;
+	source m_source;
+
+	static clock *s_instance;
+};