s390/qdio: optimize state inspection of HW-owned SBALs

When get_buf_states() gets called with count > 1, it scans the
corresponding number of SBAL states until it encounters a mismatch.

But when these SBALs are in a HW-owned state, the callers don't actually
care _how many_ such SBALs are on the queue. If we can't process the
first SBAL, we can't process any of the following SBALs either. So when
the first SBAL is HW-owned, skip the scan of the remaining SBALs and
thus save some CPU time.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/drivers/s390/cio/qdio_main.c b/drivers/s390/cio/qdio_main.c
index be93172..7b7620d 100644 (file)
--- a/drivers/s390/cio/qdio_main.c
+++ b/drivers/s390/cio/qdio_main.c
@@ -205,17 +205,22 @@ static inline int get_buf_states(struct qdio_q *q, unsigned int bufnr,
                                 int auto_ack, int merge_pending)
 {
        unsigned char __state = 0;
-       int i;
+       int i = 1;
 
        if (is_qebsm(q))
                return qdio_do_eqbs(q, state, bufnr, count, auto_ack);
 
        /* get initial state: */
        __state = q->slsb.val[bufnr];
+
+       /* Bail out early if there is no work on the queue: */
+       if (__state & SLSB_OWNER_CU)
+               goto out;
+
        if (merge_pending && __state == SLSB_P_OUTPUT_PENDING)
                __state = SLSB_P_OUTPUT_EMPTY;
 
-       for (i = 1; i < count; i++) {
+       for (; i < count; i++) {
                bufnr = next_buf(bufnr);
 
                /* merge PENDING into EMPTY: */
@@ -228,6 +233,8 @@ static inline int get_buf_states(struct qdio_q *q, unsigned int bufnr,
                if (q->slsb.val[bufnr] != __state)
                        break;
        }
+
+out:
        *state = __state;
        return i;
 }