s390/qdio: add sanity checks to the fast-requeue path

If the device driver were to send out a full queue's worth of SBALs,
current code would end up discovering the last of those SBALs as PRIMED
and erroneously skip the SIGA-w. This immediately stalls the queue.

Add a check to not attempt fast-requeue in this case. While at it also
make sure that the state of the previous SBAL was successfully extracted
before inspecting it.
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

drivers/s390/cio/qdio_main.c

@@ -1558,13 +1558,13 @@ static int handle_outbound(struct qdio_q *q, unsigned int callflags,
 		rc = qdio_kick_outbound_q(q, phys_aob);
 	} else if (need_siga_sync(q)) {
 		rc = qdio_siga_sync_q(q);
+	} else if (count < QDIO_MAX_BUFFERS_PER_Q &&
+		   get_buf_state(q, prev_buf(bufnr), &state, 0) > 0 &&
+		   state == SLSB_CU_OUTPUT_PRIMED) {
+		/* The previous buffer is not processed yet, tack on. */
+		qperf_inc(q, fast_requeue);
 	} else {
-		/* try to fast requeue buffers */
-		get_buf_state(q, prev_buf(bufnr), &state, 0);
-		if (state != SLSB_CU_OUTPUT_PRIMED)
 		rc = qdio_kick_outbound_q(q, 0);
-		else
-			qperf_inc(q, fast_requeue);
 	}
 
 	/* in case of SIGA errors we must process the error immediately */