[vmxnet3] Avoid completely filling the TX descriptor ring

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/vmxnet3.c

 			      struct io_buffer *iobuf ) {
 	struct vmxnet3_nic *vmxnet = netdev_priv ( netdev );
 	struct vmxnet3_tx_desc *tx_desc;
+	unsigned int fill;
 	unsigned int desc_idx;
 	unsigned int generation;
 
 	/* Check that we have a free transmit descriptor */
-	desc_idx = ( vmxnet->count.tx_prod % VMXNET3_NUM_TX_DESC );
-	generation = ( ( vmxnet->count.tx_prod & VMXNET3_NUM_TX_DESC ) ?
-		       0 : cpu_to_le32 ( VMXNET3_TXF_GEN ) );
-	if ( vmxnet->tx_iobuf[desc_idx] ) {
+	fill = ( vmxnet->count.tx_prod - vmxnet->count.tx_cons );
+	if ( fill >= VMXNET3_TX_FILL ) {
 		DBGC ( vmxnet, "VMXNET3 %p out of transmit descriptors\n",
 		       vmxnet );
 		return -ENOBUFS;
 	}
 
+	/* Locate transmit descriptor */
+	desc_idx = ( vmxnet->count.tx_prod % VMXNET3_NUM_TX_DESC );
+	generation = ( ( vmxnet->count.tx_prod & VMXNET3_NUM_TX_DESC ) ?
+		       0 : cpu_to_le32 ( VMXNET3_TXF_GEN ) );
+	assert ( vmxnet->tx_iobuf[desc_idx] == NULL );
+
 	/* Increment producer counter */
 	vmxnet->count.tx_prod++;
 

src/drivers/net/vmxnet3.h

 /** MTU size */
 #define VMXNET3_MTU ( ETH_FRAME_LEN + 4 /* VLAN */ + 4 /* FCS */ )
 
+/** Transmit ring maximum fill level */
+#define VMXNET3_TX_FILL ( VMXNET3_NUM_TX_DESC - 1 )
+
 /** Receive ring maximum fill level */
 #define VMXNET3_RX_FILL 8