machine: only enable USB-CDC when needed

src/machine/machine_esp32c3.go

@@ -519,6 +519,10 @@ type Serialer interface {
 	RTS() bool
 }
 
+func initUSB() {
+	// nothing to do here
+}
+
 // USB Serial/JTAG Controller
 // See esp32-c3_technical_reference_manual_en.pdf
 // pg. 736

src/machine/serial-usb.go

@@ -6,5 +6,6 @@ package machine
 var Serial Serialer
 
 func InitSerial() {
+	initUSB()
 	Serial = USBCDC
 }

src/machine/usb.go

@@ -19,6 +19,17 @@ var (
 	USBCDC Serialer
 )
 
+func initUSB() {
+	enableUSBCDC()
+	USBDev.Configure(UARTConfig{})
+}
+
+// Using go:linkname here because there's a circular dependency between the
+// machine package and the machine/usb/cdc package.
+//
+//go:linkname enableUSBCDC machine/usb/cdc.EnableUSBCDC
+func enableUSBCDC()
+
 type Serialer interface {
 	WriteByte(c byte) error
 	Write(data []byte) (n int, err error)

src/runtime/runtime_atsamd21.go

@@ -6,7 +6,7 @@ import (
 	"device/arm"
 	"device/sam"
 	"machine"
-	"machine/usb/cdc"
+	_ "machine/usb/cdc"
 	"runtime/interrupt"
 	"runtime/volatile"
 	"unsafe"
@@ -26,8 +26,6 @@ func init() {
 	initUSBClock()
 	initADCClock()
 
-	cdc.EnableUSBCDC()
-	machine.USBDev.Configure(machine.UARTConfig{})
 	machine.InitSerial()
 }
 

src/runtime/runtime_atsamd51.go

@@ -6,7 +6,7 @@ import (
 	"device/arm"
 	"device/sam"
 	"machine"
-	"machine/usb/cdc"
+	_ "machine/usb/cdc"
 	"runtime/interrupt"
 	"runtime/volatile"
 )
@@ -27,8 +27,6 @@ func init() {
 	initADCClock()
 	enableCache()
 
-	cdc.EnableUSBCDC()
-	machine.USBDev.Configure(machine.UARTConfig{})
 	machine.InitSerial()
 }
 

src/runtime/runtime_nrf.go

@@ -1,4 +1,4 @@
-//go:build nrf && !nrf52840
+//go:build nrf
 
 package runtime
 

src/runtime/runtime_nrf52840.go

@@ -2,150 +2,6 @@
 
 package runtime
 
-import (
-	"device/arm"
-	"device/nrf"
-	"machine"
-	"machine/usb/cdc"
-	"runtime/interrupt"
-	"runtime/volatile"
-)
-
-//go:linkname systemInit SystemInit
-func systemInit()
-
-//export Reset_Handler
-func main() {
-	if nrf.FPUPresent {
-		arm.SCB.CPACR.Set(0) // disable FPU if it is enabled
-	}
-	systemInit()
-	preinit()
-	run()
-	exit(0)
-}
-
-func init() {
-	cdc.EnableUSBCDC()
-	machine.USBDev.Configure(machine.UARTConfig{})
-	machine.InitSerial()
-	initLFCLK()
-	initRTC()
-}
-
-func initLFCLK() {
-	if machine.HasLowFrequencyCrystal {
-		nrf.CLOCK.LFCLKSRC.Set(nrf.CLOCK_LFCLKSTAT_SRC_Xtal)
-	}
-	nrf.CLOCK.TASKS_LFCLKSTART.Set(1)
-	for nrf.CLOCK.EVENTS_LFCLKSTARTED.Get() == 0 {
-	}
-	nrf.CLOCK.EVENTS_LFCLKSTARTED.Set(0)
-}
-
-func initRTC() {
-	nrf.RTC1.TASKS_START.Set(1)
-	intr := interrupt.New(nrf.IRQ_RTC1, func(intr interrupt.Interrupt) {
-		if nrf.RTC1.EVENTS_COMPARE[0].Get() != 0 {
-			nrf.RTC1.EVENTS_COMPARE[0].Set(0)
-			nrf.RTC1.INTENCLR.Set(nrf.RTC_INTENSET_COMPARE0)
-			nrf.RTC1.EVENTS_COMPARE[0].Set(0)
-			rtc_wakeup.Set(1)
-		}
-		if nrf.RTC1.EVENTS_OVRFLW.Get() != 0 {
-			nrf.RTC1.EVENTS_OVRFLW.Set(0)
-			rtcOverflows.Set(rtcOverflows.Get() + 1)
-		}
-	})
-	nrf.RTC1.INTENSET.Set(nrf.RTC_INTENSET_OVRFLW)
-	intr.SetPriority(0xc0) // low priority
-	intr.Enable()
-}
-
-func putchar(c byte) {
-	machine.Serial.WriteByte(c)
-}
-
-func getchar() byte {
-	for machine.Serial.Buffered() == 0 {
-		Gosched()
-	}
-	v, _ := machine.Serial.ReadByte()
-	return v
-}
-
-func buffered() int {
-	return machine.Serial.Buffered()
-}
-
-func sleepTicks(d timeUnit) {
-	for d != 0 {
-		ticks := uint32(d) & 0x7fffff // 23 bits (to be on the safe side)
-		if d > 0x7fffff {
-			ticks = 0x7fffff
-		}
-		rtc_sleep(ticks)
-		d -= timeUnit(ticks)
-	}
-}
-
-var rtcOverflows volatile.Register32 // number of times the RTC wrapped around
-
-// ticksToNanoseconds converts RTC ticks (at 32768Hz) to nanoseconds.
-func ticksToNanoseconds(ticks timeUnit) int64 {
-	// The following calculation is actually the following, but with both sides
-	// reduced to reduce the risk of overflow:
-	//     ticks * 1e9 / 32768
-	return int64(ticks) * 1953125 / 64
-}
-
-// nanosecondsToTicks converts nanoseconds to RTC ticks (running at 32768Hz).
-func nanosecondsToTicks(ns int64) timeUnit {
-	// The following calculation is actually the following, but with both sides
-	// reduced to reduce the risk of overflow:
-	//     ns * 32768 / 1e9
-	return timeUnit(ns * 64 / 1953125)
-}
-
-// Monotonically increasing number of ticks since start.
-func ticks() timeUnit {
-	// For some ways of capturing the time atomically, see this thread:
-	// https://www.eevblog.com/forum/microcontrollers/correct-timing-by-timer-overflow-count/msg749617/#msg749617
-	// Here, instead of re-reading the counter register if an overflow has been
-	// detected, we simply try again because that results in (slightly) smaller
-	// code and is perhaps easier to prove correct.
-	for {
-		mask := interrupt.Disable()
-		counter := uint32(nrf.RTC1.COUNTER.Get())
-		overflows := rtcOverflows.Get()
-		hasOverflow := nrf.RTC1.EVENTS_OVRFLW.Get() != 0
-		interrupt.Restore(mask)
-
-		if hasOverflow {
-			// There was an overflow. Try again.
-			continue
-		}
-
-		// The counter is 24 bits in size, so the number of overflows form the
-		// upper 32 bits (together 56 bits, which covers 71493 years at
-		// 32768kHz: I'd argue good enough for most purposes).
-		return timeUnit(overflows)<<24 + timeUnit(counter)
-	}
-}
-
-var rtc_wakeup volatile.Register8
-
-func rtc_sleep(ticks uint32) {
-	nrf.RTC1.INTENSET.Set(nrf.RTC_INTENSET_COMPARE0)
-	rtc_wakeup.Set(0)
-	if ticks == 1 {
-		// Race condition (even in hardware) at ticks == 1.
-		// TODO: fix this in a better way by detecting it, like the manual
-		// describes.
-		ticks = 2
-	}
-	nrf.RTC1.CC[0].Set((nrf.RTC1.COUNTER.Get() + ticks) & 0x00ffffff)
-	for rtc_wakeup.Get() == 0 {
-		waitForEvents()
-	}
-}
+// This package needs to be present so that the machine package can go:linkname
+// EnableUSBCDC from it.
+import _ "machine/usb/cdc"

src/runtime/runtime_rp2.go

@@ -7,7 +7,7 @@ import (
 	"device/rp"
 	"internal/task"
 	"machine"
-	"machine/usb/cdc"
+	_ "machine/usb/cdc"
 	"runtime/interrupt"
 	"runtime/volatile"
 	"unsafe"
@@ -360,8 +360,6 @@ func machineInit()
 func init() {
 	machineInit()
 
-	cdc.EnableUSBCDC()
-	machine.USBDev.Configure(machine.UARTConfig{})
 	machine.InitSerial()
 }