stos/amd64: hpet: Add MdHpetTimeUsec() helper

Signed-off-by: Chloe M. <chloe@mensia.org>

paw/stos/arch/amd64/platform/hpet.c

@@ -14,6 +14,7 @@
 #include <mm/vmm.h>
 #include <ke/knot.h>
 #include <ex/trace.h>
+#include <units.h>
 
 #define DTRACE(Fmt, ...) \
     TRACE("[ HPET ]: " Fmt, ##__VA_ARGS__)
@@ -57,6 +58,20 @@ HpetReadq(UCHAR Register)
     return MMIORead64(RegBase);
 }
 
+USIZE
+MdHpetTimeUsec(VOID)
+{
+    UQUAD Period, Freq, Caps;
+    UQUAD Counter;
+
+    Caps = HpetReadq(HPET_GENERAL_CAP);
+    Period = (Caps >> HPET_PERIOD_SHIFT) & HPET_PERIOD_MASK;
+    Freq = FSEC_PER_SECOND / Period;
+    Counter = HpetReadq(HPET_MAIN_COUNTER);
+
+    return (Counter * USEC_PER_SECOND) / Freq;
+}
+
 VOID
 MdHpetInit(VOID)
 {

paw/stos/head/arch/amd64/hpet.h

@@ -16,4 +16,9 @@
  */
 VOID MdHpetInit(VOID);
 
+/*
+ * Obtain the elapsed time in microseconds
+ */
+USIZE MdHpetTimeUsec(VOID);
+
 #endif  /* !_MACHINE_HPET_H_ */