system(gpio): add edge polling function

adaptor.go

@@ -27,6 +27,12 @@ type DigitalPinOptioner interface {
 	// of events for all the lines in this line request, lseqno is the same but for this line
 	SetEventHandlerForEdge(handler func(lineOffset int, timestamp time.Duration, detectedEdge string, seqno uint32,
 		lseqno uint32), edge int) (changed bool)
+	// SetPollForEdgeDetection use a discrete input polling method to detect edges. A poll interval of zero or smaller will
+	// deactivate this function. Please note: Using this feature is CPU consuming and less accurate than using cdev event
+	// handler (gpiod implementation) and should be done only if the former is not implemented or not working for the
+	// adaptor. E.g. sysfs driver in gobot has not implemented edge detection yet. The function is only useful together with
+	// SetEventHandlerForEdge() and its corresponding With*() functions.
+	SetPollForEdgeDetection(pollInterval time.Duration, pollQuitChan chan struct{}) (changed bool)
 }
 
 // DigitalPinOptionApplier is the interface to apply options to change pin behavior immediately

system/GPIO.md

@@ -142,6 +142,28 @@ Connect the input header pin26 to +3.3V with an resistor (e.g. 1kOhm).
 1
 ```
 
+### Test edge detection behavior of gpio251 (sysfs Tinkerboard)
+
+investigate status:
+
+```sh
+# cat /sys/class/gpio/gpio251/edge
+none
+```
+
+The file exists only if the pin can be configured as an interrupt generating input pin. To activate edge detection,
+"rising", "falling", or "both" needs to be set.
+
+```sh
+# cat /sys/class/gpio/gpio251/value
+1
+```
+
+If edge detection is activated, a poll will return only when the interrupt was triggered. The new value is written to
+the beginning of the file.
+
+> Not tested yet, not supported by gobot yet.
+
 ### Test output behavior of gpio251 (sysfs Tinkerboard)
 
 Connect the output header pin26 to +3.3V with an resistor (e.g. 1kOhm leads to ~0.3mA, 300Ohm leads to ~10mA).

system/digitalpin_config.go

@@ -53,6 +53,8 @@ type digitalPinConfig struct {
 	debouncePeriod   time.Duration
 	edge             int
 	edgeEventHandler func(lineOffset int, timestamp time.Duration, detectedEdge string, seqno uint32, lseqno uint32)
+	pollInterval     time.Duration
+	pollQuitChan     chan struct{}
 }
 
 func newDigitalPinConfig(label string, options ...func(gobot.DigitalPinOptioner) bool) *digitalPinConfig {
@@ -140,6 +142,13 @@ func WithPinEventOnBothEdges(handler func(lineOffset int, timestamp time.Duratio
 	}
 }
 
+// WithPinPollForEdgeDetection initializes a discrete input pin polling function to use for edge detection.
+func WithPinPollForEdgeDetection(pollInterval time.Duration, pollQuitChan chan struct{}) func(gobot.DigitalPinOptioner) bool {
+	return func(d gobot.DigitalPinOptioner) bool {
+		return d.SetPollForEdgeDetection(pollInterval, pollQuitChan)
+	}
+}
+
 // SetLabel sets the label to use for next reconfigure. The function is intended to use by WithPinLabel().
 func (d *digitalPinConfig) SetLabel(label string) bool {
 	if d.label == label {
@@ -221,3 +230,18 @@ func (d *digitalPinConfig) SetEventHandlerForEdge(handler func(int, time.Duratio
 	d.edgeEventHandler = handler
 	return true
 }
+
+// SetPollForEdgeDetection use a discrete input polling method to detect edges. A poll interval of zero or smaller will
+// deactivate this function. Please note: Using this feature is CPU consuming and less accurate than using cdev event
+// handler (gpiod implementation) and should be done only if the former is not implemented or not working for the
+// adaptor. E.g. sysfs driver in gobot has not implemented edge detection yet. The function is only useful together with
+// SetEventHandlerForEdge() and its corresponding With*() functions.
+// The function is intended to use by WithPinPollForEdgeDetection().
+func (d *digitalPinConfig) SetPollForEdgeDetection(pollInterval time.Duration, pollQuitChan chan struct{}) (changed bool) {
+	if d.pollInterval == pollInterval {
+		return false
+	}
+	d.pollInterval = pollInterval
+	d.pollQuitChan = pollQuitChan
+	return true
+}

system/digitalpin_config_test.go

@@ -413,3 +413,36 @@ func TestWithPinEventOnBothEdges(t *testing.T) {
 		})
 	}
 }
+
+func TestWithPinPollForEdgeDetection(t *testing.T) {
+	const (
+		oldVal = time.Duration(1)
+		newVal = time.Duration(3)
+	)
+	tests := map[string]struct {
+		oldPollInterval time.Duration
+		want            bool
+		wantVal         time.Duration
+	}{
+		"no_change": {
+			oldPollInterval: newVal,
+		},
+		"change": {
+			oldPollInterval: oldVal,
+			want:            true,
+		},
+	}
+	for name, tc := range tests {
+		t.Run(name, func(t *testing.T) {
+			// arrange
+			dpc := &digitalPinConfig{pollInterval: tc.oldPollInterval}
+			stopChan := make(chan struct{})
+			defer close(stopChan)
+			// act
+			got := WithPinPollForEdgeDetection(newVal, stopChan)(dpc)
+			// assert
+			assert.Equal(t, tc.want, got)
+			assert.Equal(t, newVal, dpc.pollInterval)
+		})
+	}
+}

system/digitalpin_gpiod.go

@@ -215,7 +215,8 @@ func digitalPinGpiodReconfigureLine(d *digitalPinGpiod, forceInput bool) error {
 			opts = append(opts, gpiod.WithDebounce(d.debouncePeriod))
 		}
 		// edge detection
-		if d.edgeEventHandler != nil {
+		if d.edgeEventHandler != nil && d.pollInterval <= 0 {
+			// use edge detection provided by gpiod
 			wrappedHandler := digitalPinGpiodGetWrappedEventHandler(d.edgeEventHandler)
 			switch d.edge {
 			case digitalPinEventOnFallingEdge:
@@ -277,10 +278,20 @@ func digitalPinGpiodReconfigureLine(d *digitalPinGpiod, forceInput bool) error {
 	}
 	d.line = gpiodLine
 
+	// start discrete polling function and wait for first read is done
+	if (d.direction == IN || forceInput) && d.pollInterval > 0 {
+		if err := startEdgePolling(d.label, d.Read, d.pollInterval, d.edge, d.edgeEventHandler,
+			d.pollQuitChan); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }
 
-func digitalPinGpiodGetWrappedEventHandler(handler func(int, time.Duration, string, uint32, uint32)) func(gpiod.LineEvent) {
+func digitalPinGpiodGetWrappedEventHandler(
+	handler func(int, time.Duration, string, uint32, uint32),
+) func(gpiod.LineEvent) {
 	return func(evt gpiod.LineEvent) {
 		detectedEdge := "none"
 		switch evt.Type {

system/digitalpin_poll.go

@@ -0,0 +1,81 @@
+package system
+
+import (
+	"fmt"
+	"sync"
+	"time"
+)
+
+func startEdgePolling(
+	pinLabel string,
+	pinReadFunc func() (int, error),
+	pollInterval time.Duration,
+	wantedEdge int,
+	eventHandler func(offset int, t time.Duration, et string, sn uint32, lsn uint32),
+	quitChan chan struct{},
+) error {
+	if eventHandler == nil {
+		return fmt.Errorf("an event handler is mandatory for edge polling")
+	}
+	if quitChan == nil {
+		return fmt.Errorf("the quit channel is mandatory for edge polling")
+	}
+
+	const allEdges = "all"
+
+	triggerEventOn := "none"
+	switch wantedEdge {
+	case digitalPinEventOnFallingEdge:
+		triggerEventOn = DigitalPinEventFallingEdge
+	case digitalPinEventOnRisingEdge:
+		triggerEventOn = DigitalPinEventRisingEdge
+	case digitalPinEventOnBothEdges:
+		triggerEventOn = allEdges
+	default:
+		return fmt.Errorf("unsupported edge type %d for edge polling", wantedEdge)
+	}
+
+	wg := sync.WaitGroup{}
+	wg.Add(1)
+
+	go func() {
+		var oldState int
+		var readStart time.Time
+		var firstLoopDone bool
+		for {
+			select {
+			case <-quitChan:
+				return
+			default:
+				// note: pure reading takes between 30us and 1ms on rasperry Pi1, typically 50us, with sysfs also 500us
+				// can happen, so we use the time stamp before start of reading to reduce random duration offset
+				readStart = time.Now()
+				readValue, err := pinReadFunc()
+				if err != nil {
+					fmt.Printf("edge polling error occurred while reading the pin %s: %v", pinLabel, err)
+					readValue = oldState // keep the value
+				}
+				if readValue != oldState {
+					detectedEdge := DigitalPinEventRisingEdge
+					if readValue < oldState {
+						detectedEdge = DigitalPinEventFallingEdge
+					}
+					if firstLoopDone && (triggerEventOn == allEdges || triggerEventOn == detectedEdge) {
+						eventHandler(0, time.Duration(readStart.UnixNano()), detectedEdge, 0, 0)
+					}
+					oldState = readValue
+				}
+				// the real poll interval is increased by the reading time, see also note above
+				// negative or zero duration causes no sleep
+				time.Sleep(pollInterval - time.Since(readStart))
+				if !firstLoopDone {
+					wg.Done()
+					firstLoopDone = true
+				}
+			}
+		}
+	}()
+
+	wg.Wait()
+	return nil
+}