usb/msc: erase blocks before writing to them

We could perhaps check whether they've been erased already (via unmap or
something) but the default of most OSes seems to be to _not_ unmap
blocks so we will have to erase in most cases anyway.

Author: aykevl

src/machine/usb/msc/msc.go

@@ -73,7 +73,7 @@ func newMSC(dev machine.BlockDevice) *msc {
 	m := &msc{
 		// Some platforms require reads/writes to be aligned to the full underlying hardware block
 		blockCache:    make([]byte, dev.WriteBlockSize()),
-		blockSizeUSB:  512,
+		blockSizeUSB:  max(uint32(dev.EraseBlockSize()), 512),
 		buf:           make([]byte, dev.WriteBlockSize()),
 		cswBuf:        make([]byte, csw.MsgLen),
 		cbw:           &CBW{Data: make([]byte, 31)},

src/machine/usb/msc/scsi_readwrite.go

@@ -91,6 +91,13 @@ func (m *msc) writeBlock(b []byte, lba, offset uint32) (n int, err error) {
 		return 0, invalidWriteError
 	}
 
+	// Erase the block first if needed.
+	if uint32(blockStart)%m.blockSizeUSB == 0 {
+		firstBlock := uint32(blockStart) / uint32(m.dev.EraseBlockSize())
+		numBlocks := (m.blockSizeUSB + uint32(m.dev.EraseBlockSize()) - 1) / uint32(m.dev.EraseBlockSize())
+		m.dev.EraseBlocks(int64(firstBlock), int64(numBlocks))
+	}
+
 	// Write the full block to the underlying device
 	n, err = m.dev.WriteAt(b, blockStart)
 	n -= int(blockOffset)