gpio: adding interrupt

this is inspired by Dave Cheney's gpio library and his work on EPOLL

host/generic/interrupt.go

@@ -0,0 +1,126 @@
+// Generic Interrupt Pins.
+
+package generic
+
+import (
+	"errors"
+	"fmt"
+	"sync"
+	"syscall"
+
+	"github.com/kidoman/embd"
+)
+
+const (
+	MaxGPIOInterrupt = 64
+)
+
+var ErrorPinAlreadyRegistered = errors.New("pin interrupt already registered")
+
+type interrupt struct {
+	pin            embd.DigitalPin
+	initialTrigger bool
+	handler        func(embd.DigitalPin)
+}
+
+func (i *interrupt) Signal() {
+	if !i.initialTrigger {
+		i.initialTrigger = true
+		return
+	}
+	i.handler(i.pin)
+}
+
+type ePollListener struct {
+	epollFd           int
+	interruptablePins map[int]*interrupt
+	mu                sync.Mutex
+}
+
+var ePollListenerInstance *ePollListener
+
+func getEPollListenerInstance() *ePollListener {
+	if ePollListenerInstance == nil {
+		ePollListenerInstance = initEPollListener()
+	}
+	return ePollListenerInstance
+}
+
+func initEPollListener() *ePollListener {
+	epollFd, err := syscall.EpollCreate1(0)
+	if err != nil {
+		panic(fmt.Sprintf("Unable to create epoll: %v", err))
+	}
+	listener := &ePollListener{epollFd: epollFd, interruptablePins: make(map[int]*interrupt)}
+
+	go func() {
+		var epollEvents [MaxGPIOInterrupt]syscall.EpollEvent
+
+		for {
+			n, err := syscall.EpollWait(listener.epollFd, epollEvents[:], -1)
+			if err != nil {
+				panic(fmt.Sprintf("EpollWait error: %v", err))
+			}
+			for i := 0; i < n; i++ {
+				if irq, ok := listener.interruptablePins[int(epollEvents[i].Fd)]; ok {
+					irq.Signal()
+				}
+			}
+		}
+	}()
+	return listener
+}
+
+func registerInterrupt(pin *digitalPin, handler func(embd.DigitalPin)) error {
+	l := getEPollListenerInstance()
+
+	pinFd := int(pin.val.Fd())
+
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	if _, ok := l.interruptablePins[pinFd]; ok {
+		return ErrorPinAlreadyRegistered
+	}
+
+	var event syscall.EpollEvent
+	event.Events = syscall.EPOLLIN | (syscall.EPOLLET & 0xffffffff) | syscall.EPOLLPRI
+
+	if err := syscall.SetNonblock(pinFd, true); err != nil {
+		return err
+	}
+
+	event.Fd = int32(pinFd)
+
+	if err := syscall.EpollCtl(l.epollFd, syscall.EPOLL_CTL_ADD, pinFd, &event); err != nil {
+		return err
+	}
+
+	l.interruptablePins[pinFd] = &interrupt{pin: pin, handler: handler}
+
+	return nil
+}
+
+func unregisterInterrupt(pin *digitalPin) error {
+	l := getEPollListenerInstance()
+
+	pinFd := int(pin.val.Fd())
+
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	if _, ok := l.interruptablePins[pinFd]; !ok {
+		return nil
+	}
+
+	if err := syscall.EpollCtl(l.epollFd, syscall.EPOLL_CTL_DEL, pinFd, nil); err != nil {
+		return err
+	}
+
+	if err := syscall.SetNonblock(pinFd, false); err != nil {
+		return err
+	}
+
+	delete(l.interruptablePins, pinFd)
+	return nil
+}