rp2040: remove SPI deadline

Removing it improves SPI performance by about 20% for me (updating the
display of a Gopher Badge).

src/machine/machine_rp2040_spi.go

@@ -47,9 +47,6 @@ type SPI struct {
 	Bus *rp.SPI0_Type
 }
 
-// time to wait on a transaction before dropping. Unit in Microseconds for compatibility with ticks().
-const _SPITimeout = 10 * 1000 // 10 ms
-
 // Tx handles read/write operation for SPI interface. Since SPI is a syncronous write/read
 // interface, there must always be the same number of bytes written as bytes read.
 // The Tx method knows about this, and offers a few different ways of calling it.
@@ -95,19 +92,12 @@ func (spi SPI) Tx(w, r []byte) (err error) {
 
 // Write a single byte and read a single byte from TX/RX FIFO.
 func (spi SPI) Transfer(w byte) (byte, error) {
-	var deadline = ticks() + _SPITimeout
 	for !spi.isWritable() {
-		if ticks() > deadline {
-			return 0, ErrSPITimeout
-		}
 	}
 
 	spi.Bus.SSPDR.Set(uint32(w))
 
 	for !spi.isReadable() {
-		if ticks() > deadline {
-			return 0, ErrSPITimeout
-		}
 	}
 	return uint8(spi.Bus.SSPDR.Get()), nil
 }
@@ -298,15 +288,11 @@ func (spi SPI) isBusy() bool {
 
 // tx writes buffer to SPI ignoring Rx.
 func (spi SPI) tx(tx []byte) error {
-	var deadline = ticks() + _SPITimeout
 	// Write to TX FIFO whilst ignoring RX, then clean up afterward. When RX
 	// is full, PL022 inhibits RX pushes, and sets a sticky flag on
 	// push-on-full, but continues shifting. Safe if SSPIMSC_RORIM is not set.
 	for i := range tx {
 		for !spi.isWritable() {
-			if ticks() > deadline {
-				return ErrSPITimeout
-			}
 		}
 		spi.Bus.SSPDR.Set(uint32(tx[i]))
 	}
@@ -316,9 +302,6 @@ func (spi SPI) tx(tx []byte) error {
 		spi.Bus.SSPDR.Get()
 	}
 	for spi.isBusy() {
-		if ticks() > deadline {
-			return ErrSPITimeout
-		}
 	}
 	for spi.isReadable() {
 		spi.Bus.SSPDR.Get()
@@ -333,7 +316,6 @@ func (spi SPI) tx(tx []byte) error {
 // Generally this can be 0, but some devices require a specific value here,
 // e.g. SD cards expect 0xff
 func (spi SPI) rx(rx []byte, txrepeat byte) error {
-	var deadline = ticks() + _SPITimeout
 	plen := len(rx)
 	const fifoDepth = 8 // see txrx
 	var rxleft, txleft = plen, plen
@@ -345,10 +327,7 @@ func (spi SPI) rx(rx []byte, txrepeat byte) error {
 		if rxleft != 0 && spi.isReadable() {
 			rx[plen-rxleft] = uint8(spi.Bus.SSPDR.Get())
 			rxleft--
-			continue // if reading succesfully in rx there is no need to check deadline.
+			continue
-		}
-		if ticks() > deadline {
-			return ErrSPITimeout
 		}
 	}
 	return nil
@@ -357,7 +336,6 @@ func (spi SPI) rx(rx []byte, txrepeat byte) error {
 // Write len bytes from src to SPI. Simultaneously read len bytes from SPI to dst.
 // Note this function is guaranteed to exit in a known amount of time (bits sent * time per bit)
 func (spi SPI) txrx(tx, rx []byte) error {
-	var deadline = ticks() + _SPITimeout
 	plen := len(tx)
 	if plen != len(rx) {
 		return ErrTxInvalidSliceSize
@@ -366,7 +344,7 @@ func (spi SPI) txrx(tx, rx []byte) error {
 	// else FIFO will overflow if this code is heavily interrupted.
 	const fifoDepth = 8
 	var rxleft, txleft = plen, plen
-	for (txleft != 0 || rxleft != 0) && ticks() <= deadline {
+	for txleft != 0 || rxleft != 0 {
 		if txleft != 0 && spi.isWritable() && rxleft < txleft+fifoDepth {
 			spi.Bus.SSPDR.Set(uint32(tx[plen-txleft]))
 			txleft--