[phantom] Guard against partially-written status descriptors

Conjecture: The hardware issues 64-bit DMA writes of status descriptors,
which some PCI bridges seem to split into two 32-bit writes in reverse
order (i.e. dword 1 first).  This means that we sometimes observe a
partial status descriptor.  Add an explicit check to ensure that the
descriptor is complete before processing it.

Also ensure that the RDS consumer counter is incremented only when we
know that we have actually consumed an RX descriptor.

src/drivers/net/phantom/phantom.c

 
 		/* Check received opcode */
 		sds_opcode = NX_GET ( sds, opcode );
-		switch ( sds_opcode ) {
-		case UNM_RXPKT_DESC:
-		case UNM_SYN_OFFLOAD:
+		if ( ( sds_opcode == UNM_RXPKT_DESC ) ||
+		     ( sds_opcode == UNM_SYN_OFFLOAD ) ) {
+
+			/* Sanity check: ensure that all of the SDS
+			 * descriptor has been written.
+			 */
+			if ( NX_GET ( sds, total_length ) == 0 ) {
+				DBGC ( phantom, "Phantom %p port %d SDS %d "
+				       "incomplete; deferring\n", phantom,
+				       phantom_port->port, sds_consumer_idx );
+				/* Leave for next poll() */
+				break;
+			}
+
 			/* Process received packet */
 			sds_handle = NX_GET ( sds, handle );
 			iobuf = phantom_port->rds_iobuf[sds_handle];
 				phantom, phantom_port->port, sds_handle );
 			netdev_rx ( netdev, iobuf );
 			phantom_port->rds_iobuf[sds_handle] = NULL;
-			break;
-		default:
+
+			/* Update RDS consumer counter.  This is a
+			 * lower bound for the number of descriptors
+			 * that have been read by the hardware, since
+			 * the hardware must have read at least one
+			 * descriptor for each completion that we
+			 * receive.
+			 */
+			rds_consumer_idx =
+				( ( rds_consumer_idx + 1 ) % PHN_NUM_RDS );
+			phantom_port->rds_consumer_idx = rds_consumer_idx;
+
+		} else {
+
 			DBGC ( phantom, "Phantom %p port %d unexpected SDS "
 			       "opcode %02x\n",
 			       phantom, phantom_port->port, sds_opcode );
 			DBGC_HDA ( phantom, virt_to_bus ( sds ),
 				   sds, sizeof ( *sds ) );
-			break;
 		}
 			
-		/* Update RDS consumer counter.  This is a lower bound
-		 * for the number of descriptors that have been read
-		 * by the hardware, since the hardware must have read
-		 * at least one descriptor for each completion that we
-		 * receive.
-		 */
-		rds_consumer_idx = ( ( rds_consumer_idx + 1 ) % PHN_NUM_RDS );
-		phantom_port->rds_consumer_idx = rds_consumer_idx;
-
 		/* Clear status descriptor */
 		memset ( sds, 0, sizeof ( *sds ) );