At least some versions of Emulex SNP driver differ from other vendors…

… by never returning a NULL txbuf in GetStatus.

To tolerate this, modify the transmit poll behavior to loop until all iobufs iPXE wanted to transmit had their addresses returned.
If iPXE has no iobufs that it thinks should be transmitting, don't bother calling GetStatus anymore and proceed to receive loop.

src/drivers/net/efi/snpnet.c

@@ -43,6 +43,9 @@ struct snpnet_device {
 
 	/** State that the SNP should be in after close */
 	UINT32 close_state;
+
+	/** Count of packets awaiting confirmed transmit */
+	UINT32 txpending;
 };
 
 /**
@@ -58,6 +61,7 @@ static int snpnet_transmit ( struct net_device *netdev,
 	EFI_SIMPLE_NETWORK_PROTOCOL *snp = snpnetdev->snp;
 	EFI_STATUS efirc;
 	size_t len = iob_len ( iobuf );
+	snpnetdev->txpending++;
 
 	efirc = snp->Transmit ( snp, 0, len, iobuf->data, NULL, NULL, NULL );
 	return EFIRC_TO_RC ( efirc );
@@ -75,6 +79,8 @@ static void snpnet_complete ( struct net_device *netdev, void *txbuf ) {
 
 	list_for_each_entry_safe ( iobuf, tmp, &netdev->tx_queue, list ) {
 		if ( iobuf->data == txbuf ) {
+			struct snpnet_device *snpnetdev = netdev->priv;
+			snpnetdev->txpending--;
 			netdev_tx_complete ( netdev, iobuf );
 			break;
 		}
@@ -95,18 +101,16 @@ static void snpnet_poll ( struct net_device *netdev ) {
 	void *txbuf;
 
 	/* Process Tx completions */
-	while ( 1 ) {
+	while ( snpnetdev->txpending ) {
 		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 );
+		if ( txbuf != NULL )
+			snpnet_complete ( netdev, txbuf );
 	}
 
 	/* Process received packets */