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/internal/task/waiter.go

@@ -0,0 +1,99 @@
+package task
+
+import (
+	"runtime/interrupt"
+	"unsafe"
+)
+
+// A waiter waits for an interrupt to fire. Call Wait() from a goroutine to
+// pause it, and call Resume() from an interrupt handler to resume the
+// goroutine.
+type Waiter struct {
+	waiting *Task
+	lock    PMutex
+}
+
+var resumeTaskSentinel = (*Task)(unsafe.Pointer(uintptr(2)))
+var workingTaskSentinel = (*Task)(unsafe.Pointer(uintptr(1)))
+
+// Wait from a main goroutine until an interrupt calls Resume(). It will also
+// return if the interrupt called Resume() before the Wait() call (to avoid a
+// race condition).
+func (w *Waiter) Wait() {
+	if interrupt.In() {
+		runtimePanic("Waiter: called Wait from interrupt")
+	}
+
+	mask := interrupt.Disable()
+	w.lock.Lock()
+	switch w.waiting {
+	case nil, workingTaskSentinel:
+		w.waiting = Current()
+		w.lock.Unlock()
+		interrupt.Restore(mask)
+		Pause()
+	case resumeTaskSentinel:
+		// Marked as 'resume now', can return immediately.
+		w.waiting = workingTaskSentinel
+		w.lock.Unlock()
+		interrupt.Restore(mask)
+	default:
+		w.lock.Unlock()
+		interrupt.Restore(mask)
+		runtimePanic("Waiter: task is waiting already")
+	}
+}
+
+// Resume a waiting goroutine from an interrupt handler. If there is a goroutine
+// waiting, it will resume. If not, the next one that calls Wait() will
+// immediately resume.
+func (w *Waiter) Resume() {
+	if !interrupt.In() {
+		runtimePanic("Waiter: called Resume outside interrupt")
+	}
+
+	waiting := w.waiting
+	switch waiting {
+	case nil, workingTaskSentinel:
+		w.waiting = resumeTaskSentinel
+	case resumeTaskSentinel:
+		// Called Resume() twice in a row, this may indicate a bug at the caller
+		// site.
+	default:
+		// Schedule the given task to resume.
+		// If it's not yet paused, it will immediately resume on the next call
+		// to Pause().
+		w.waiting = workingTaskSentinel
+		scheduleTask(waiting)
+	}
+}
+
+// Return true if Done has not been called after a Resume call.
+// This is typically used to indicate the task outside the interrupt is still
+// being worked on.
+func (w *Waiter) Working() bool {
+	switch w.waiting {
+	case workingTaskSentinel, resumeTaskSentinel:
+		return true
+	default: // nil, <*task.Task>
+		return false
+	}
+}
+
+// Done can be called outside interrupt context to indicate the task this waiter
+// was waiting for is done, and Working() can return false. It is entirely
+// optional, if not called Working will continue to return true until it is
+// blocked in Wait() but the waiter will function normally otherwise.
+func (w *Waiter) Done() {
+	if interrupt.In() {
+		runtimePanic("Waiter: called Done from interrupt")
+	}
+
+	mask := interrupt.Disable()
+	w.lock.Lock()
+	if w.waiting == workingTaskSentinel {
+		w.waiting = nil
+	}
+	w.lock.Unlock()
+	interrupt.Restore(mask)
+}

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
@@ -26,14 +26,14 @@ const (
 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
-	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
-	respStatus    csw.Status // Response status for the last command
-	sendZLP       bool       // Flag to indicate if a zero-length packet should be sent before sending CSW
+	buf           []byte      // Buffer for incoming/outgoing data
+	blockCache    []byte      // Buffer for block read/write data
+	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
+	respStatus    csw.Status  // Response status for the last command
+	sendZLP       bool        // Flag to indicate if a zero-length packet should be sent before sending CSW
 
 	cbw           *CBW   // Last received Command Block Wrapper
 	queuedBytes   uint32 // Number of bytes queued for sending
@@ -120,21 +120,21 @@ func newMSC(dev machine.BlockDevice) *msc {
 func (m *msc) processTasks() {
 	// Process tasks that cannot be done in an interrupt context
 	for {
-		if m.taskQueued {
-			cmd := m.cbw.SCSICmd()
-			switch cmd.CmdType() {
-			case scsi.CmdWrite:
-				m.scsiWrite(cmd, m.buf)
-			case scsi.CmdUnmap:
-				m.scsiUnmap(m.buf)
-			}
+		// Wait for the next task to arrive.
+		m.taskWaiter.Wait()
 
-			// Acknowledge the received data from the host
-			m.queuedBytes = 0
-			m.taskQueued = false
-			machine.AckUsbOutTransfer(usb.MSC_ENDPOINT_OUT)
+		cmd := m.cbw.SCSICmd()
+		switch cmd.CmdType() {
+		case scsi.CmdWrite:
+			m.scsiWrite(cmd, m.buf)
+		case scsi.CmdUnmap:
+			m.scsiUnmap(m.buf)
 		}
-		time.Sleep(100 * time.Microsecond)
+
+		// Acknowledge the received data from the host
+		m.queuedBytes = 0
+		m.taskWaiter.Done()
+		machine.AckUsbOutTransfer(usb.MSC_ENDPOINT_OUT)
 	}
 }
 

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) {