Give up entirely on the transmit queue if using snpnet.c.

My first attempt would have looped infinitely with iPXE SNP, and my second would leak memory with iPXE SNP and potentially others.

Practically speaking, it seems the convention is to only have one packet pending and not rely upon any mechanism to associate returned
txbuf with txqueue.

src/drivers/net/efi/snpnet.c

@@ -57,28 +57,25 @@ static int snpnet_transmit ( struct net_device *netdev,
 	struct snpnet_device *snpnetdev = netdev->priv;
 	EFI_SIMPLE_NETWORK_PROTOCOL *snp = snpnetdev->snp;
 	EFI_STATUS efirc;
+	void *txbuf=NULL;
 	size_t len = iob_len ( iobuf );
 
 	efirc = snp->Transmit ( snp, 0, len, iobuf->data, NULL, NULL, NULL );
-	return EFIRC_TO_RC ( efirc );
-}
-
-/**
- * Find a I/O buffer on the list of outstanding Tx buffers and complete it.
- *
- * @v snpnetdev		SNP network device
- * @v txbuf		Buffer address
- */
-static void snpnet_complete ( struct net_device *netdev, void *txbuf ) {
-	struct io_buffer *tmp;
-	struct io_buffer *iobuf;
-
-	list_for_each_entry_safe ( iobuf, tmp, &netdev->tx_queue, list ) {
-		if ( iobuf->data == txbuf ) {
-			netdev_tx_complete ( netdev, iobuf );
+	if (efirc) {
+		return EFIRC_TO_RC ( efirc );
+	}
+	/* since GetStatus is so inconsistent, don't try more than one outstanding transmit at a time */
+	while ( txbuf == NULL ) {
+		efirc = snp->GetStatus ( snp, NULL, &txbuf );
+		if ( efirc ) {
+			DBGC ( snp, "SNP %p could not get status %s\n", snp,
+			       efi_strerror ( efirc ) );
 			break;
 		}
+
 	}
+	netdev_tx_complete ( netdev, iobuf );
+	return 0;
 }
 
 /**
@@ -92,22 +89,6 @@ static void snpnet_poll ( struct net_device *netdev ) {
 	EFI_STATUS efirc;
 	struct io_buffer *iobuf = NULL;
 	UINTN len;
-	void *txbuf;
-
-	/* Process Tx completions */
-	while ( 1 ) {
-		efirc = snp->GetStatus ( snp, NULL, &txbuf );
-		if ( efirc ) {
-			DBGC ( snp, "SNP %p could not get status %s\n", snp,
-			       efi_strerror ( efirc ) );
-			break;
-		}
-
-		if ( txbuf == NULL )
-			break;
-
-		snpnet_complete ( netdev, txbuf );
-	}
 
 	/* Process received packets */
 	while ( 1 ) {