bitbang/spi cleanup (#268)

- precalculate clock idle polarity and remove clockOn()/clockOff()
 - check for higher (unknown) modes and reject them
 - move internal functions down, to be grouped together
- idle clock on Connect

CONTRIBUTORS

@@ -33,5 +33,6 @@ Marc-Antoine Ruel <maruel@chromium.org> <maruel@gmail.com>
 Matt Aimonetti <mattaimonetti@gmail.com>
 Max Ekman <max@looplab.se>
 Matias Insaurralde <matias@insaurral.de>
+Seán C McCord <ulexus@gmail.com> <scm@cycoresys.com>
 Stephan Sperber <sperberstephan@googlemail.com>
 Thorsten von Eicken <tve@voneicken.com>

experimental/devices/bitbang/spi.go

@@ -76,6 +76,9 @@ func (s *SPI) Connect(f physic.Frequency, mode spi.Mode, bits int) (spi.Conn, er
 	if mode&spi.LSBFirst == spi.LSBFirst {
 		return nil, errors.New("bitbang-spi: LSBFirst mode not supported")
 	}
+	if mode >= 0x20 {
+		return nil, fmt.Errorf("bitbang-spi: unhandled mode %d(%s)", mode, mode.String())
+	}
 	s.spiConn.mu.Lock()
 	defer s.spiConn.mu.Unlock()
 	s.spiConn.freqDev = f
@@ -84,6 +87,14 @@ func (s *SPI) Connect(f physic.Frequency, mode spi.Mode, bits int) (spi.Conn, er
 	}
 	s.spiConn.mode = mode
 	s.spiConn.bits = bits
+	s.spiConn.readAfterClockPulse = mode&spi.Mode1 == spi.Mode1
+
+	// Set clock idle polarity, ensuring an idle clock to start
+	s.spiConn.clockIdle = gpio.Level(mode&spi.Mode2 == spi.Mode2)
+	if err := s.spiConn.sck.Out(s.spiConn.clockIdle); err != nil {
+		return nil, fmt.Errorf("bitbang-spi: failed to idle clock: %v", err)
+	}
+
 	return &s.spiConn, nil
 }
 
@@ -135,6 +146,8 @@ type spiConn struct {
 	mu                  sync.Mutex
 	freqPort            physic.Frequency
 	freqDev             physic.Frequency
+	clockIdle           gpio.Level
+	readAfterClockPulse bool
 	mode                spi.Mode
 	bits                int
 	halfCycle           time.Duration
@@ -150,46 +163,6 @@ func (s *spiConn) Duplex() conn.Duplex {
 	return conn.Full
 }
 
-func (s *spiConn) clockOn() error {
-	if s.mode&spi.Mode2 == spi.Mode2 {
-		return s.sck.Out(gpio.Low)
-	}
-	return s.sck.Out(gpio.High)
-}
-
-func (s *spiConn) clockOff() error {
-	if s.mode&spi.Mode2 == spi.Mode2 {
-		return s.sck.Out(gpio.High)
-	}
-	return s.sck.Out(gpio.Low)
-}
-
-func (s *spiConn) readAfterClockPulse() bool {
-	return s.mode&spi.Mode1 == spi.Mode1
-}
-
-func (s *spiConn) assertCS() error {
-	if s.csn == nil || s.mode&spi.NoCS == spi.NoCS {
-		return nil
-	}
-	if err := s.csn.Out(gpio.Low); err != nil {
-		return err
-	}
-	s.sleepHalfCycle()
-	return nil
-}
-
-func (s *spiConn) unassertCS() error {
-	if s.csn == nil || s.mode&spi.NoCS == spi.NoCS {
-		return nil
-	}
-	if err := s.csn.Out(gpio.High); err != nil {
-		return err
-	}
-	s.sleepHalfCycle()
-	return nil
-}
-
 // Tx implements spi.Conn.
 //
 // BUG(maruel): Implement mode (HalfDuplex and LSBFirst remain to be done).
@@ -213,14 +186,14 @@ func (s *spiConn) Tx(w, r []byte) (err error) {
 		}
 
 		s.sleepHalfCycle()
-		if err = s.clockOn(); err != nil {
+		if err = s.sck.Out(!s.clockIdle); err != nil {
 			return fmt.Errorf("bitbang-spi: failed to assert clock: %v", err)
 		}
 		s.sleepHalfCycle()
 
-		if s.readAfterClockPulse() {
+		if s.readAfterClockPulse {
-			if err = s.clockOff(); err != nil {
+			if err = s.sck.Out(s.clockIdle); err != nil {
-				return fmt.Errorf("bitbang-spi: failed to unassert clock: %v", err)
+				return fmt.Errorf("bitbang-spi: failed to idle clock: %v", err)
 			}
 			s.sleepHalfCycle()
 		}
@@ -231,9 +204,9 @@ func (s *spiConn) Tx(w, r []byte) (err error) {
 			}
 		}
 
-		if !s.readAfterClockPulse() {
+		if !s.readAfterClockPulse {
-			if err = s.clockOff(); err != nil {
+			if err = s.sck.Out(s.clockIdle); err != nil {
-				return fmt.Errorf("bitbang-spi: failed to unassert clock: %v", err)
+				return fmt.Errorf("bitbang-spi: failed to idle clock: %v", err)
 			}
 		}
 	}
@@ -284,5 +257,27 @@ func (s *spiConn) sleepHalfCycle() {
 	cpu.Nanospin(s.halfCycle)
 }
 
+func (s *spiConn) assertCS() error {
+	if s.csn == nil || s.mode&spi.NoCS == spi.NoCS {
+		return nil
+	}
+	if err := s.csn.Out(gpio.Low); err != nil {
+		return err
+	}
+	s.sleepHalfCycle()
+	return nil
+}
+
+func (s *spiConn) unassertCS() error {
+	if s.csn == nil || s.mode&spi.NoCS == spi.NoCS {
+		return nil
+	}
+	if err := s.csn.Out(gpio.High); err != nil {
+		return err
+	}
+	s.sleepHalfCycle()
+	return nil
+}
+
 var _ spi.PortCloser = &SPI{}
 var _ fmt.Stringer = &SPI{}