machine: changed to block if no read data exists on UART and USB

src/machine/machine_nxpmk66f18_uart.go

@@ -60,9 +60,6 @@ var (
 	ErrNotConfigured  = errors.New("device has not been configured")
 )
 
-//go:linkname gosched runtime.Gosched
-func gosched()
-
 // PutcharUART writes a byte to the UART synchronously, without using interrupts
 // or calling the scheduler
 func PutcharUART(u *UART, c byte) {

src/machine/uart.go

@@ -3,7 +3,9 @@
 
 package machine
 
-import "errors"
+import (
+	"errors"
+)
 
 var errUARTBufferEmpty = errors.New("UART buffer empty")
 
@@ -40,12 +42,16 @@ const (
 
 // Read from the RX buffer.
 func (uart *UART) Read(data []byte) (n int, err error) {
-	// check if RX buffer is empty
-	size := uart.Buffered()
-	if size == 0 {
+	if len(data) == 0 {
 		return 0, nil
 	}
 
+	size := uart.Buffered()
+	for size == 0 {
+		gosched()
+		size = uart.Buffered()
+	}
+
 	// Make sure we do not read more from buffer than the data slice can hold.
 	if len(data) < size {
 		size = len(data)
@@ -89,3 +95,6 @@ func (uart *UART) Buffered() int {
 func (uart *UART) Receive(data byte) {
 	uart.Buffer.Put(data)
 }
+
+//go:linkname gosched runtime.Gosched
+func gosched() int

src/machine/usb.go

@@ -606,12 +606,16 @@ func newUSBSetup(data []byte) usbSetup {
 
 // Read from the RX buffer.
 func (usbcdc *USBCDC) Read(data []byte) (n int, err error) {
-	// check if RX buffer is empty
-	size := usbcdc.Buffered()
-	if size == 0 {
+	if len(data) == 0 {
 		return 0, nil
 	}
 
+	size := usbcdc.Buffered()
+	for size == 0 {
+		gosched()
+		size = usbcdc.Buffered()
+	}
+
 	// Make sure we do not read more from buffer than the data slice can hold.
 	if len(data) < size {
 		size = len(data)