nrzled.NewSPI: also add padding in the front

Some SPI devices take a while to set up for transfer, and while doing
so, show the first bit they're about to send.

This causes the first LED to always interpret the first symbol as a
logical 1. Zapping out the whole strip will leave the first LED green,
and the first pixel will always be slightly green-ish.

Prevent this, by applying the same 3 bit padding we have at the end to
the front.

Signed-off-by: Florian Klink <flokli@flokli.de>

nrzled/nrzled.go

@@ -81,10 +81,13 @@ func NewSPI(p spi.Port, opts *Opts) (*Dev, error) {
 	}
 	// 4 symbol bytes per byte, 3/4 bytes per pixel.
 	streamLen := 4 * (opts.Channels * opts.NumPixels)
-	// 3 bytes for latch. 24*400ns = 9600ns. In practice this could be skipped,
+	// 3 bytes for latch. 24*400ns = 9600ns. In practice this could be skipped at
-	// as the overhead for SPI Tx() tear down and the next one is likely at least
+	// the end as the overhead for SPI Tx() tear down and the next one is likely
-	// 10µs.
+	// at least 10µs.
-	bufSize := streamLen + 3
+	// In the front, we definitely want it, as SPIs might otherwise be high for
+	// quite a while while setting themselves up, causing the first symbol to be
+	// interpreted as a logical 1.
+	bufSize := 3 + streamLen + 3
 	if l, ok := p.(conn.Limits); ok {
 		if s := l.MaxTxSize(); s < bufSize {
 			return nil, errors.New("spi port buffer is too short for the specified number of pixels")
@@ -101,7 +104,7 @@ func NewSPI(p spi.Port, opts *Opts) (*Dev, error) {
 		numPixels: opts.NumPixels,
 		channels:  opts.Channels,
 		b:         gpiostream.BitStream{Freq: opts.Freq, Bits: buf, LSBF: false},
-		rawBuf:    buf[:streamLen],
+		rawBuf:    buf[3 : bufSize-3],
 		rect:      image.Rect(0, 0, opts.NumPixels, 1),
 	}, nil
 }

nrzled/nrzled_spi_test.go

@@ -36,7 +36,7 @@ func TestSPI_Empty(t *testing.T) {
 	s := spitest.Playback{
 		Playback: conntest.Playback{
 			Count: 1,
-			Ops:   []conntest.IO{{W: []byte{0x00, 0x00, 0x00}}},
+			Ops:   []conntest.IO{{W: []byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00}}},
 		},
 	}
 	d, err := NewSPI(spitest.NewRecordRaw(&buf), &o)
@@ -105,6 +105,7 @@ var writeTests = []struct {
 			{0xFF, 0xFF, 0xFF, 0x00},
 		}),
 		want: []byte{
+			/*NUL*/ 0x00, 0x00, 0x00,
 			/*FF*/ 0xEE, 0xEE, 0xEE, 0xEE /*FF*/, 0xEE, 0xEE, 0xEE, 0xEE /*FF*/, 0xEE, 0xEE, 0xEE, 0xEE,
 			/*EOF*/ 0x00, 0x00, 0x00,
 		},
@@ -120,6 +121,7 @@ var writeTests = []struct {
 			{0xFE, 0xFE, 0xFE, 0x00},
 		}),
 		want: []byte{
+			/*NUL*/ 0x00, 0x00, 0x00,
 			/*FE*/ 0xEE, 0xEE, 0xEE, 0xE8 /*FE*/, 0xEE, 0xEE, 0xEE, 0xE8 /*FE*/, 0xEE, 0xEE, 0xEE, 0xE8,
 			/*EOF*/ 0x00, 0x00, 0x00,
 		},
@@ -135,6 +137,7 @@ var writeTests = []struct {
 			{0xF0, 0xF0, 0xF0, 0x00},
 		}),
 		want: []byte{
+			/*NUL*/ 0x00, 0x00, 0x00,
 			/*F0*/ 0xEE, 0xEE, 0x88, 0x88 /*F0*/, 0xEE, 0xEE, 0x88, 0x88 /*F0*/, 0xEE, 0xEE, 0x88, 0x88,
 			/*EOF*/ 0x00, 0x00, 0x00,
 		},
@@ -150,6 +153,7 @@ var writeTests = []struct {
 			{0x80, 0x80, 0x80, 0x00},
 		}),
 		want: []byte{
+			/*NUL*/ 0x00, 0x00, 0x00,
 			/*80*/ 0xE8, 0x88, 0x88, 0x88 /*80*/, 0xE8, 0x88, 0x88, 0x88 /*80*/, 0xE8, 0x88, 0x88, 0x88,
 			/*EOF*/ 0x00, 0x00, 0x00,
 		},
@@ -165,6 +169,7 @@ var writeTests = []struct {
 			{0x80, 0xFF, 0x00, 0x00},
 		}),
 		want: []byte{
+			/*NUL*/ 0x00, 0x00, 0x00,
 			/*FF*/ 0xEE, 0xEE, 0xEE, 0xEE /*80*/, 0xE8, 0x88, 0x88, 0x88 /*00*/, 0x88, 0x88, 0x88, 0x88,
 			/*EOF*/ 0x00, 0x00, 0x00,
 		},
@@ -180,6 +185,7 @@ var writeTests = []struct {
 			{0x80, 0x00, 0x00, 0x00},
 		}),
 		want: []byte{
+			/*NUL*/ 0x00, 0x00, 0x00,
 			/*00*/ 0x88, 0x88, 0x88, 0x88 /*80*/, 0xE8, 0x88, 0x88, 0x88 /*00*/, 0x88, 0x88, 0x88, 0x88,
 			/*EOF*/ 0x00, 0x00, 0x00,
 		},
@@ -195,6 +201,7 @@ var writeTests = []struct {
 			{0x00, 0x80, 0x00, 0x00},
 		}),
 		want: []byte{
+			/*NUL*/ 0x00, 0x00, 0x00,
 			/*80*/ 0xE8, 0x88, 0x88, 0x88 /*00*/, 0x88, 0x88, 0x88, 0x88 /*00*/, 0x88, 0x88, 0x88, 0x88,
 			/*EOF*/ 0x00, 0x00, 0x00,
 		},
@@ -210,6 +217,7 @@ var writeTests = []struct {
 			{0x00, 0x00, 0x80, 0x00},
 		}),
 		want: []byte{
+			/*NUL*/ 0x00, 0x00, 0x00,
 			/*00*/ 0x88, 0x88, 0x88, 0x88 /*00*/, 0x88, 0x88, 0x88, 0x88 /*80*/, 0xE8, 0x88, 0x88, 0x88,
 			/*EOF*/ 0x00, 0x00, 0x00,
 		},
@@ -236,6 +244,7 @@ var writeTests = []struct {
 			{0x00, 0x00, 0x00, 0x00},
 		}),
 		want: []byte{
+			/*NUL*/ 0x00, 0x00, 0x00,
 			/*FF*/ 0xEE, 0xEE, 0xEE, 0xEE /*FF*/, 0xEE, 0xEE, 0xEE, 0xEE /*FF*/, 0xEE, 0xEE, 0xEE, 0xEE,
 			/*FE*/ 0xEE, 0xEE, 0xEE, 0xE8 /*FE*/, 0xEE, 0xEE, 0xEE, 0xE8 /*FE*/, 0xEE, 0xEE, 0xEE, 0xE8,
 			/*F0*/ 0xEE, 0xEE, 0x88, 0x88 /*F0*/, 0xEE, 0xEE, 0x88, 0x88 /*F0*/, 0xEE, 0xEE, 0x88, 0x88,
@@ -302,14 +311,15 @@ func TestSPI_Long(t *testing.T) {
 	if n, err := d.Write(toRGB(colors)); n != len(colors)*3 || err != nil {
 		t.Fatalf("%d %v", n, err)
 	}
-	expected := make([]byte, 12*o.NumPixels+3)
+	expected := make([]byte, 3+12*o.NumPixels+3)
-	for i := 0; i < 12*o.NumPixels; i += 12 {
+	// leave three bytes of padding
+	for i := 3; i < 12*o.NumPixels; i += 12 {
 		//Each channel should be 0x00
 		for j := 0; j < 12; j++ {
 			expected[i+j] = 0x88
 		}
 	}
-	trailer := expected[12*o.NumPixels:]
+	trailer := expected[3+12*o.NumPixels:]
 	for i := range trailer {
 		trailer[i] = 0x00
 	}
@@ -350,11 +360,16 @@ var drawTests = []struct {
 		}(),
 		want: func() []byte {
 			var b []byte
+			//padding
+			for i := 0; i < 3; i++ {
+				b = append(b, 0x00)
+			}
 			for i := 0; i < 4; i++ {
 				b = append(b, 0xE8, 0x88, 0x88, 0x88) //0x80
 				b = append(b, 0x88, 0x88, 0x88, 0x88) //0x00
 				b = append(b, 0xEE, 0xEE, 0xEE, 0xEE) //0xFF
 			}
+			//padding
 			for i := 0; i < 3; i++ {
 				b = append(b, 0x00)
 			}
@@ -427,6 +442,9 @@ func TestSPI_Halt(t *testing.T) {
 			Ops: []conntest.IO{
 				{},
 				{W: []byte{
+					//Begin of frame
+					0x00, 0x00, 0x00,
+
 					0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88,
 					0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88,
 					0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88,