esp32: optimize SPI transmit

For write-only operations (in SPI displays for example), the transmit
speed is doubled with this relatively small change.

In the future, we should try to use DMA instead for larger buffers. But
this is already a significant improvement and will always be an
improvement for small buffer sizes.

src/machine/machine_esp32.go

@@ -478,16 +478,35 @@ func (spi SPI) Tx(w, r []byte) error {
 
 		// Fill tx buffer.
 		transferWords := (*[16]volatile.Register32)(unsafe.Pointer(uintptr(unsafe.Pointer(&spi.Bus.W0))))
-		var outBuf [16]uint32
-		txSize := 64
-		if txSize > len(w) {
-			txSize = len(w)
-		}
-		for i := 0; i < txSize; i++ {
-			outBuf[i/4] = outBuf[i/4] | uint32(w[i])<<((i%4)*8)
-		}
-		for i, word := range outBuf {
-			transferWords[i].Set(word)
+		if len(w) >= 64 {
+			// We can fill the entire 64-byte transfer buffer with data.
+			// This loop is slightly faster than the loop below.
+			for i := 0; i < 16; i++ {
+				word := uint32(w[i*4])<<0 | uint32(w[i*4+1])<<8 | uint32(w[i*4+2])<<16 | uint32(w[i*4+3])<<24
+				transferWords[i].Set(word)
+			}
+		} else {
+			// We can't fill the entire transfer buffer, so we need to be a bit
+			// more careful.
+			// Note that parts of the transfer buffer that aren't used still
+			// need to be set to zero, otherwise we might be transferring
+			// garbage from a previous transmission if w is smaller than r.
+			for i := 0; i < 16; i++ {
+				var word uint32
+				if i*4+3 < len(w) {
+					word |= uint32(w[i*4+3]) << 24
+				}
+				if i*4+2 < len(w) {
+					word |= uint32(w[i*4+2]) << 16
+				}
+				if i*4+1 < len(w) {
+					word |= uint32(w[i*4+1]) << 8
+				}
+				if i*4+0 < len(w) {
+					word |= uint32(w[i*4+0]) << 0
+				}
+				transferWords[i].Set(word)
+			}
 		}
 
 		// Do the transfer.