scsi: hisi_sas: change slot index allocation mode

Currently we find the lowest available empty bit in the IPTT bitmap to
allocate the IPTT for a command.

To reduce possibility of hitting unknown SoC bugs and also aid in the
debugging of those same bugs, change the allocation mode.

The next allocation method is to use the next free slot adjacent to the
most recently allocated slot, in a round-robin fashion.
Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/hisi_sas/hisi_sas.h

@@ -274,6 +274,7 @@ struct hisi_hba {
 	struct workqueue_struct *wq;
 
 	int slot_index_count;
+	int last_slot_index;
 	unsigned long *slot_index_tags;
 	unsigned long reject_stp_links_msk;
 

drivers/scsi/hisi_sas/hisi_sas_main.c

@@ -195,11 +195,18 @@ static int hisi_sas_slot_index_alloc(struct hisi_hba *hisi_hba, int *slot_idx)
 	unsigned int index;
 	void *bitmap = hisi_hba->slot_index_tags;
 
-	index = find_first_zero_bit(bitmap, hisi_hba->slot_index_count);
+	index = find_next_zero_bit(bitmap, hisi_hba->slot_index_count,
+			hisi_hba->last_slot_index + 1);
+	if (index >= hisi_hba->slot_index_count) {
+		index = find_next_zero_bit(bitmap, hisi_hba->slot_index_count,
+					   0);
 		if (index >= hisi_hba->slot_index_count)
 			return -SAS_QUEUE_FULL;
+	}
 	hisi_sas_slot_index_set(hisi_hba, index);
 	*slot_idx = index;
+	hisi_hba->last_slot_index = index;
+
 	return 0;
 }