runtime/esp32c3: use TIMG0 alarm interrupt for sleepTicks instead of busy-waiting

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/machine/machine_esp32c3_usb.go

@@ -17,7 +17,7 @@ import (
 // CDC-ACM serial port. The USB protocol and enumeration are handled entirely
 // in hardware; software only reads/writes the EP1 FIFO.
 
-const cpuInterruptFromUSB = 8
+const cpuInterruptFromUSB = 10
 
 // flushTimeout is the maximum number of busy-wait iterations in flush().
 // Prevents hanging when no USB host is connected.

src/runtime/interrupt/interrupt_esp32c3.go

@@ -55,6 +55,9 @@ func (i Interrupt) Enable() error {
 //go:linkname callHandlers runtime/interrupt.callHandlers
 func callHandlers(num int)
 
+//go:linkname signalInterrupt runtime.signalInterrupt
+func signalInterrupt()
+
 const (
 	IRQNUM_1 = 1 + iota
 	IRQNUM_2
@@ -203,6 +206,9 @@ func handleInterrupt() {
 		// Call registered interrupt handler(s)
 		callHandler(int(interruptNumber))
 
+		// Signal to sleepTicks that an interrupt has occurred.
+		signalInterrupt()
+
 		// disable CPU interrupts
 		riscv.MSTATUS.ClearBits(riscv.MSTATUS_MIE)
 

src/runtime/runtime_esp32.go

@@ -65,6 +65,14 @@ var _sbss [0]byte
 //go:extern _ebss
 var _ebss [0]byte
 
+// sleepTicks busy-waits until the given number of ticks have passed.
+func sleepTicks(d timeUnit) {
+	sleepUntil := ticks() + d
+	for ticks() < sleepUntil {
+		// TODO: suspend the CPU to not burn power here unnecessarily.
+	}
+}
+
 func abort() {
 	for {
 		device.Asm("waiti 0")

src/runtime/runtime_esp32c3.go

@@ -6,6 +6,7 @@ import (
 	"device/esp"
 	"device/riscv"
 	"machine"
+	"runtime/interrupt"
 	"runtime/volatile"
 	"unsafe"
 )
@@ -57,6 +58,9 @@ func main() {
 	// Initialize main system timer used for time.Now.
 	initTimer()
 
+	// Initialize timer alarm interrupt for the scheduler.
+	initTimerInterrupt()
+
 	// Initialize the heap, call main.main, etc.
 	run()
 
@@ -98,5 +102,75 @@ func interruptInit() {
 	riscv.EnableInterrupts(mie)
 }
 
+// CPU interrupt number used for the TIMG0 timer alarm.
+const timerAlarmCPUInterrupt = 9
+
+var interruptPending volatile.Register8
+
+func signalInterrupt() {
+	interruptPending.Set(1)
+}
+
+// initTimerInterrupt routes the TIMG0 timer 0 alarm interrupt to a CPU
+// interrupt and registers a handler that signals timerWakeup.
+func initTimerInterrupt() {
+	// Map the TIMG0 T0 peripheral interrupt to a CPU interrupt line.
+	esp.INTERRUPT_CORE0.TG_T0_INT_MAP.Set(timerAlarmCPUInterrupt)
+
+	// Enable T0 interrupt at the timer group level.
+	esp.TIMG0.INT_ENA_TIMERS.SetBits(1)
+
+	// Register the interrupt handler (compile-time wiring).
+	interrupt.New(timerAlarmCPUInterrupt, func(interrupt.Interrupt) {
+		// Clear the timer interrupt at the peripheral level.
+		esp.TIMG0.INT_CLR_TIMERS.Set(1)
+	})
+
+	// Manually enable the CPU interrupt with correct ordering:
+	// 1) clear any stale pending bit first
+	// 2) set edge-triggered
+	// 3) set priority above threshold
+	// 4) enable the interrupt last
+	mie := riscv.DisableInterrupts()
+
+	esp.INTERRUPT_CORE0.CPU_INT_CLEAR.SetBits(1 << timerAlarmCPUInterrupt)
+	esp.INTERRUPT_CORE0.CPU_INT_CLEAR.ClearBits(1 << timerAlarmCPUInterrupt)
+
+	esp.INTERRUPT_CORE0.CPU_INT_TYPE.SetBits(1 << timerAlarmCPUInterrupt)
+
+	priReg := (*volatile.Register32)(unsafe.Add(unsafe.Pointer(&esp.INTERRUPT_CORE0.CPU_INT_PRI_0), timerAlarmCPUInterrupt*4))
+	priReg.Set(10)
+
+	riscv.Asm("fence")
+
+	esp.INTERRUPT_CORE0.CPU_INT_ENABLE.SetBits(1 << timerAlarmCPUInterrupt)
+
+	riscv.EnableInterrupts(mie)
+}
+
+// sleepTicks spins until the given number of ticks have elapsed, using the
+// TIMG0 alarm interrupt to avoid busy-waiting for the entire duration.
+func sleepTicks(d timeUnit) {
+	target := ticks() + d
+	for ticks() < target {
+		// Set the alarm to fire at the target tick count (or as close
+		// as the 54-bit counter allows).
+		interruptPending.Set(0)
+
+		esp.TIMG0.T0ALARMLO.Set(uint32(target))
+		esp.TIMG0.T0ALARMHI.Set(uint32(target >> 32))
+
+		// Enable the alarm (auto-clears when alarm fires).
+		esp.TIMG0.T0CONFIG.SetBits(esp.TIMG_T0CONFIG_ALARM_EN)
+
+		// Wait for any interrupt (timer alarm or other) or a timeout.
+		for interruptPending.Get() == 0 {
+			if ticks() >= target {
+				return
+			}
+		}
+	}
+}
+
 //go:extern _vector_table
 var _vector_table [0]uintptr

src/runtime/runtime_esp32xx.go

@@ -55,14 +55,6 @@ func ticksToNanoseconds(ticks timeUnit) int64 {
 	return int64(ticks) * 25
 }
 
-// sleepTicks busy-waits until the given number of ticks have passed.
-func sleepTicks(d timeUnit) {
-	sleepUntil := ticks() + d
-	for ticks() < sleepUntil {
-		// TODO: suspend the CPU to not burn power here unnecessarily.
-	}
-}
-
 func exit(code int) {
 	abort()
 }