usb/msc: wait for interrupt instead of polling a flag

This should make usb/msc a whole lot more efficient by pausing the
worker goroutine and waiting for an interrupt to unpause it instead of
waiting in a loop and sleeping for 0.1ms each cycle.

In other words, this should make it both faster (no unnecessary delay
due to the time.Sleep) and more efficient (no polling).

src/machine/usb/msc/msc.go

@@ -1,12 +1,12 @@
 package msc
 
 import (
+	"internal/task"
 	"machine"
 	"machine/usb"
 	"machine/usb/descriptor"
 	"machine/usb/msc/csw"
 	"machine/usb/msc/scsi"
-	"time"
 )
 
 type mscState uint8
@@ -28,7 +28,7 @@ var MSC *msc
 type msc struct {
 	buf           []byte      // Buffer for incoming/outgoing data
 	blockCache    []byte      // Buffer for block read/write data
-	taskQueued    bool       // Flag to indicate if the buffer has a task queued
+	taskWaiter    task.Waiter // Waiter for events outside interrupt context
 	rxStalled     bool        // Flag to indicate if the RX endpoint is stalled
 	txStalled     bool        // Flag to indicate if the TX endpoint is stalled
 	maxPacketSize uint32      // Maximum packet size for the IN endpoint
@@ -120,7 +120,9 @@ func newMSC(dev machine.BlockDevice) *msc {
 func (m *msc) processTasks() {
 	// Process tasks that cannot be done in an interrupt context
 	for {
-		if m.taskQueued {
+		// Wait for the next task to arrive.
+		m.taskWaiter.Wait()
+
 		cmd := m.cbw.SCSICmd()
 		switch cmd.CmdType() {
 		case scsi.CmdWrite:
@@ -131,11 +133,9 @@ func (m *msc) processTasks() {
 
 		// Acknowledge the received data from the host
 		m.queuedBytes = 0
-			m.taskQueued = false
+		m.taskWaiter.Done()
 		machine.AckUsbOutTransfer(usb.MSC_ENDPOINT_OUT)
 	}
-		time.Sleep(100 * time.Microsecond)
-	}
 }
 
 func (m *msc) ready() bool {

src/machine/usb/msc/scsi.go

@@ -251,7 +251,7 @@ func (m *msc) scsiQueueTask(cmdType scsi.CmdType, b []byte) bool {
 	}
 
 	// Save the incoming data in our buffer for processing outside of interrupt context.
-	if m.taskQueued {
+	if m.taskWaiter.Working() {
 		// If we already have a full task queue we can't accept this data
 		m.sendScsiError(csw.StatusFailed, scsi.SenseAbortedCommand, scsi.SenseCodeMsgReject)
 		return true
@@ -268,14 +268,14 @@ func (m *msc) scsiQueueTask(cmdType scsi.CmdType, b []byte) bool {
 	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)) || (m.sentBytes+m.queuedBytes >= m.transferBytes) {
-			m.taskQueued = true
+			m.taskWaiter.Resume()
 		}
 	case scsi.CmdUnmap:
-		m.taskQueued = true
+		m.taskWaiter.Resume()
 	}
 
 	// Don't acknowledge the incoming data until we can process it.
-	return !m.taskQueued
+	return !m.taskWaiter.Working()
 }
 
 func (m *msc) sendScsiError(status csw.Status, key scsi.Sense, code scsi.SenseCode) {