fix: enable usb flow control on nrf52840

Author: mikesmitty

src/machine/machine_nrf52840_usb.go

@@ -22,6 +22,15 @@ var (
 	epinen      uint32
 	epouten     uint32
 	easyDMABusy volatile.Register8
+	// epOutFlowControl contains the flow control state of the USB OUT endpoints.
+	epOutFlowControl [NumberOfUSBEndpoints]struct {
+		// nak indicates that we are NAKing any further OUT packets because the rxHandler isn't ready yet.
+		// When this is true, we do not restart the DMA for the endpoint, effectively pausing it.
+		nak bool
+		// dataPending indicates that we have data in the hardware buffer that hasn't been handled yet.
+		// Having one in the buffer is what generates the NAK responses, this is a signal to handle it.
+		dataPending bool
+	}
 
 	endPoints = []uint32{
 		usb.CONTROL_ENDPOINT:  usb.ENDPOINT_TYPE_CONTROL,
@@ -196,7 +205,16 @@ func handleUSBIRQ(interrupt.Interrupt) {
 				nrf.USBD.EPOUT[i].PTR.Set(uint32(uintptr(unsafe.Pointer(&udd_ep_out_cache_buffer[i]))))
 				count := nrf.USBD.SIZE.EPOUT[i].Get()
 				nrf.USBD.EPOUT[i].MAXCNT.Set(count)
-				nrf.USBD.TASKS_STARTEPOUT[i].Set(1)
+				if !epOutFlowControl[i].nak {
+					// Normal case: We want data, so start DMA immediately
+					nrf.USBD.TASKS_STARTEPOUT[i].Set(1)
+					epOutFlowControl[i].dataPending = false
+				} else {
+					// NAK case: We want to NAK, so DO NOT start DMA.
+					// The data stays in HW buffer. Host receives NAKs.
+					// Mark that we have data waiting so we can fetch it later.
+					epOutFlowControl[i].dataPending = true
+				}
 			}
 		}
 	}
@@ -208,6 +226,10 @@ func handleUSBIRQ(interrupt.Interrupt) {
 			buf := handleEndpointRx(uint32(i))
 			if usbRxHandler[i] == nil || usbRxHandler[i](buf) {
 				AckUsbOutTransfer(uint32(i))
+			} else {
+				// usbRxHandler returned false, so NAK further OUT packets until we're ready
+				epOutFlowControl[i].nak = true
+				nrf.USBD.SIZE.EPOUT[i].Set(0)
 			}
 			exitCriticalSection()
 		}
@@ -308,8 +330,26 @@ func handleEndpointRx(ep uint32) []byte {
 }
 
 // AckUsbOutTransfer is called to acknowledge the completion of a USB OUT transfer.
+// It also clears the NAK state and resumes data flow if it was paused.
 func AckUsbOutTransfer(ep uint32) {
-	// set ready for next data
+	epOutFlowControl[ep].nak = false
+
+	// If we ignored a packet earlier (Buffer Full strategy), we must manually
+	// trigger the DMA now to pull it from the HW buffer.
+	if epOutFlowControl[ep].dataPending {
+		epOutFlowControl[ep].dataPending = false
+
+		// Prepare DMA to move data from HW Buffer -> RAM
+		nrf.USBD.EPOUT[ep].PTR.Set(uint32(uintptr(unsafe.Pointer(&udd_ep_out_cache_buffer[ep]))))
+		count := nrf.USBD.SIZE.EPOUT[ep].Get()
+		nrf.USBD.EPOUT[ep].MAXCNT.Set(count)
+
+		// Kick the DMA
+		nrf.USBD.TASKS_STARTEPOUT[ep].Set(1)
+		return
+	}
+
+	// Otherwise, just re-arm the endpoint to accept the NEXT packet
 	nrf.USBD.SIZE.EPOUT[ep].Set(0)
 }
 

src/machine/usb/msc/msc.go

@@ -302,8 +302,7 @@ func (m *msc) run(b []byte, isEpOut bool) bool {
 func (m *msc) RegisterBlockDevice(dev machine.BlockDevice) {
 	m.dev = dev
 
-	maxPacketSize := descriptor.EndpointMSCIN.GetMaxPacketSize()
-	bufSize := max(dev.WriteBlockSize(), int64(maxPacketSize))
+	bufSize := max(dev.WriteBlockSize(), int64(m.maxPacketSize))
 
 	if cap(m.blockCache) != int(bufSize) {
 		m.blockCache = make([]byte, bufSize)

src/machine/usb/msc/scsi.go

@@ -267,7 +267,8 @@ func (m *msc) scsiQueueTask(cmdType scsi.CmdType, b []byte) bool {
 	switch cmdType {
 	case scsi.CmdWrite:
 		// If we're writing data wait until we have a full write block of data that can be processed.
-		if m.queuedBytes == uint32(cap(m.blockCache)) {
+		// Or if we've received all the data we're expecting.
+		if m.queuedBytes == uint32(cap(m.blockCache)) || m.sentBytes+m.queuedBytes >= m.totalBytes {
 			m.taskQueued = true
 		}
 	case scsi.CmdUnmap: