spi: Update spi.Conn to separate app and device configuration. (#123)

- Allow both app and device drive set max bus speed.
- Make DevParams callable only exactly once.
- sysfs-spi: make SPI implement io.Reader.
- sysfs-spi: Fix Tx() with write only or read only.

devices/apa102/apa102.go

@@ -283,7 +283,7 @@ func (d *Dev) Write(pixels []byte) (int, error) {
 // https://en.wikipedia.org/wiki/Flicker_fusion_threshold is a recommended
 // reading.
 func New(s spi.Conn, numLights int, intensity uint8, temperature uint16) (*Dev, error) {
-	if err := s.Configure(spi.Mode3, 8); err != nil {
+	if err := s.DevParams(20000000, spi.Mode3, 8); err != nil {
 		return nil, err
 	}
 	// End frames are needed to be able to push enough SPI clock signals due to

devices/apa102/apa102_test.go

@@ -336,9 +336,9 @@ func TestDevEmpty(t *testing.T) {
 	}
 }
 
-func TestConfigureFail(t *testing.T) {
+func TestDevParamsFail(t *testing.T) {
 	if d, err := New(&configFail{}, 150, 255, 6500); d != nil || err == nil {
-		t.Fatal("Configure() call have failed")
+		t.Fatal("DevParams() call have failed")
 	}
 }
 
@@ -682,7 +682,7 @@ type configFail struct {
 	spitest.Record
 }
 
-func (c *configFail) Configure(mode spi.Mode, bits int) error {
+func (c *configFail) DevParams(maxHz int64, mode spi.Mode, bits int) error {
 	return errors.New("injected error")
 }
 

devices/bme280/bme280.go

@@ -6,7 +6,7 @@
 //
 // Datasheet
 //
-// https://cdn-shop.adafruit.com/datasheets/BST-BME280_DS001-10.pdf
+// https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BME280_DS001-11.pdf
 package bme280
 
 import (
@@ -172,7 +172,7 @@ func NewI2C(b i2c.Bus, opts *Opts) (*Dev, error) {
 // device in the mail.
 func NewSPI(s spi.Conn, opts *Opts) (*Dev, error) {
 	// It works both in Mode0 and Mode3.
-	if err := s.Configure(spi.Mode3, 8); err != nil {
+	if err := s.DevParams(10000000, spi.Mode3, 8); err != nil {
 		return nil, err
 	}
 	d := &Dev{d: s, isSPI: true}

devices/ssd1306/ssd1306.go

@@ -89,9 +89,8 @@ type Dev struct {
 // connection is to connect RES and DC to ground, CS to 3.3v, SDA to MOSI, SCK
 // to SCLK.
 //
-// As per datasheet, maximum clock speed is 1/100ns = 10MHz.
 func NewSPI(s spi.Conn, w, h int, rotated bool) (*Dev, error) {
-	if err := s.Configure(spi.Mode3, 8); err != nil {
+	if err := s.DevParams(3300000, spi.Mode3, 8); err != nil {
 		return nil, err
 	}
 	return newDev(s, w, h, rotated)
@@ -101,10 +100,8 @@ func NewSPI(s spi.Conn, w, h int, rotated bool) (*Dev, error) {
 // controler.
 //
 // If rotated, turns the display by 180°
-//
-// As per datasheet, maximum clock speed is 1/2.5µs = 400KHz. It's worth
-// bumping up from default bus speed of 100KHz if possible.
 func NewI2C(i i2c.Bus, w, h int, rotated bool) (*Dev, error) {
+	// Maximum clock speed is 1/2.5µs = 400KHz.
 	return newDev(&i2c.Dev{Bus: i, Addr: 0x3C}, w, h, rotated)
 }
 

experimental/devices/bitbang/spi.go

@@ -31,6 +31,8 @@ type SPI struct {
 	csn gpio.PinOut // CS
 
 	mu        sync.Mutex
+	maxHzBus  int64
+	maxHzDev  int64
 	mode      spi.Mode
 	bits      int
 	halfCycle time.Duration
@@ -51,21 +53,34 @@ func (s *SPI) Duplex() conn.Duplex {
 	return conn.Full
 }
 
-// Speed implements spi.Conn.
+// Speed implements spi.ConnCloser.
-func (s *SPI) Speed(hz int64) error {
+func (s *SPI) Speed(maxHz int64) error {
+	if maxHz <= 0 {
+		return errors.New("bitbang-spi: invalid maxHz")
+	}
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	s.halfCycle = time.Second / time.Duration(hz) / time.Duration(2)
+	s.maxHzBus = maxHz
+	if s.maxHzDev == 0 || s.maxHzBus < s.maxHzDev {
+		s.halfCycle = time.Second / time.Duration(maxHz) / time.Duration(2)
+	}
 	return nil
 }
 
-// Configure implements spi.Conn.
+// DevParams implements spi.Conn.
-func (s *SPI) Configure(mode spi.Mode, bits int) error {
+func (s *SPI) DevParams(maxHz int64, mode spi.Mode, bits int) error {
-	s.mu.Lock()
+	if maxHz < 0 {
-	defer s.mu.Unlock()
+		return errors.New("bitbang-spi: invalid maxHz")
+	}
 	if mode != spi.Mode3 {
 		return errors.New("bitbang-spi: not implemented")
 	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.maxHzDev = maxHz
+	if s.maxHzDev != 0 && (s.maxHzBus == 0 || s.maxHzDev < s.maxHzBus) {
+		s.halfCycle = time.Second / time.Duration(maxHz) / time.Duration(2)
+	}
 	s.mode = mode
 	s.bits = bits
 	return nil