gpio: adding interrupt

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

gpio.go

@@ -7,6 +7,9 @@ import "time"
 // The Direction type indicates the direction of a GPIO pin.
 type Direction int
 
+// The Edge trigger for the GPIO Interrupt
+type Edge string
+
 const (
 	// In represents read mode.
 	In Direction = iota
@@ -23,8 +26,27 @@ const (
 	High
 )
 
+const (
+	EdgeNone    Edge = "none"
+	EdgeRising  Edge = "rising"
+	EdgeFalling Edge = "falling"
+	EdgeBoth    Edge = "both"
+)
+
+// InterruptPin implements access to a Interruptable capable GPIO pin.
+type InterruptPin interface {
+
+	// Start watching this pin for interrupt
+	Watch(edge Edge, handler func(DigitalPin)) error
+
+	// Stop watching this pin for interrupt
+	StopWatching() error
+}
+
 // DigitalPin implements access to a digital IO capable GPIO pin.
 type DigitalPin interface {
+	InterruptPin
+
 	// N returns the logical GPIO number.
 	N() int
 

host/generic/digitalpin.go

@@ -228,6 +228,10 @@ func (p *digitalPin) PullDown() error {
 }
 
 func (p *digitalPin) Close() error {
+	if err := p.StopWatching(); err != nil {
+		return err
+	}
+
 	if err := p.drv.Unregister(p.id); err != nil {
 		return err
 	}
@@ -253,3 +257,25 @@ func (p *digitalPin) Close() error {
 
 	return nil
 }
+
+func (p *digitalPin) setEdge(edge embd.Edge) error {
+	file, err := p.openFile(path.Join(p.basePath(), "edge"))
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	_, err = file.Write([]byte(edge))
+	return err
+}
+
+func (p *digitalPin) Watch(edge embd.Edge, handler func(embd.DigitalPin)) error {
+	if err := p.setEdge(edge); err != nil {
+		return err
+	}
+	return registerInterrupt(p, handler)
+}
+
+func (p *digitalPin) StopWatching() error {
+	return unregisterInterrupt(p)
+}

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
+}

samples/gpiointerrupt.go

@@ -0,0 +1,42 @@
+// +build ignore
+
+package main
+
+import (
+	"flag"
+	"fmt"
+
+	"github.com/kidoman/embd"
+
+	_ "github.com/kidoman/embd/host/all"
+)
+
+func main() {
+	flag.Parse()
+
+	if err := embd.InitGPIO(); err != nil {
+		panic(err)
+	}
+	defer embd.CloseGPIO()
+
+	btn, err := embd.NewDigitalPin(10)
+	if err != nil {
+		panic(err)
+	}
+	defer btn.Close()
+
+	if err := btn.SetDirection(embd.In); err != nil {
+		panic(err)
+	}
+	btn.ActiveLow(false)
+
+	quit := make(chan interface{})
+	err = btn.Watch(embd.EdgeFalling, func(btn embd.DigitalPin) {
+		quit <- btn
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("Button %v was pressed.\n", <-quit)
+}