runtime/hifive1: use CLINT peripheral for timekeeping

The CLINT is implemented both on the fe310-g002 chip and in the sifive_e
QEMU machine type. Therefore, use that peripheral for consistency.

The only difference is the clock speed, which runs at 10MHz in QEMU for
some reason instead of 32.768kHz as on the physical HiFive1 boards.

src/runtime/runtime_fe310.go

@@ -14,8 +14,6 @@ import (
 
 type timeUnit int64
 
-const tickMicros = 32768 // RTC runs at 32.768kHz
-
 //go:extern _sbss
 var _sbss unsafe.Pointer
 
@@ -82,3 +80,26 @@ func putchar(c byte) {
 }
 
 const asyncScheduler = false
+
+func ticks() timeUnit {
+	// Combining the low bits and the high bits yields a time span of over 270
+	// years without counter rollover.
+	highBits := sifive.CLINT.MTIMEH.Get()
+	for {
+		lowBits := sifive.CLINT.MTIME.Get()
+		newHighBits := sifive.CLINT.MTIMEH.Get()
+		if newHighBits == highBits {
+			// High bits stayed the same.
+			return timeUnit(lowBits) | (timeUnit(highBits) << 32)
+		}
+		// Retry, because there was a rollover in the low bits (happening every
+		// 1.5 days).
+		highBits = newHighBits
+	}
+}
+
+func sleepTicks(d timeUnit) {
+	target := ticks() + d
+	for ticks() < target {
+	}
+}

src/runtime/runtime_fe310_baremetal.go

@@ -4,36 +4,13 @@ package runtime
 
 import (
 	"device/riscv"
-	"device/sifive"
 )
 
+const tickMicros = 32768 // RTC clock runs at 32.768kHz
+
 func abort() {
 	// lock up forever
 	for {
 		riscv.Asm("wfi")
 	}
 }
-
-func ticks() timeUnit {
-	// Combining the low bits and the high bits yields a time span of over 270
-	// years without counter rollover.
-	highBits := sifive.RTC.RTCHI.Get()
-	for {
-		lowBits := sifive.RTC.RTCLO.Get()
-		newHighBits := sifive.RTC.RTCHI.Get()
-		if newHighBits == highBits {
-			// High bits stayed the same.
-			println("bits:", highBits, lowBits)
-			return timeUnit(lowBits) | (timeUnit(highBits) << 32)
-		}
-		// Retry, because there was a rollover in the low bits (happening every
-		// 1.5 days).
-		highBits = newHighBits
-	}
-}
-
-func sleepTicks(d timeUnit) {
-	target := ticks() + d
-	for ticks() < target {
-	}
-}

src/runtime/runtime_fe310_qemu.go

@@ -7,6 +7,8 @@ import (
 	"unsafe"
 )
 
+const tickMicros = 100 // CLINT.MTIME increments every 100ns
+
 // Special memory-mapped device to exit tests, created by SiFive.
 var testExit = (*volatile.Register32)(unsafe.Pointer(uintptr(0x100000)))
 
@@ -16,13 +18,3 @@ func abort() {
 	// Signal a successful exit.
 	testExit.Set(0x5555)
 }
-
-func ticks() timeUnit {
-	return timestamp
-}
-
-func sleepTicks(d timeUnit) {
-	// Note: QEMU doesn't seem to support the RTC peripheral at the time of
-	// writing so just simulate sleeping here.
-	timestamp += d
-}