scsi: mpt3sas: Fix Firmware fault state 0x2100 during heavy 4K RR FIO stress test.

Due existence of loop in the IO path our HBA will receive heavy IOs and
also as driver is not updating the Reply Post Host Index frequently, So
there will be a high chance that our Firmware unable to find any free
entry in the Reply Post Descriptor Queue (i.e. Queue overflow occurs)
and can observe 0x2100 firmware fault.  So to fix this, we have defined
a thresh hold value. After continuously processing this thresh hold
number of reply descriptors driver will update the Reply Descriptor Host
Index so that this thresh hold number of reply descriptors entries will
be freed and these entries will be available for firmware and we won't
observe this Firmware fault. We have defined this threshold value as
1/3rd of the hba queue depth.
Signed-off-by: Chaitra P B <chaitra.basappa@broadcom.com>
Signed-off-by: Suganath Prabu S <suganath-prabu.subramani@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/mpt3sas/mpt3sas_base.c

@@ -1040,6 +1040,25 @@ _base_interrupt(int irq, void *bus_id)
 		    reply_q->reply_post_free[reply_q->reply_post_host_index].
 		    Default.ReplyFlags & MPI2_RPY_DESCRIPT_FLAGS_TYPE_MASK;
 		completed_cmds++;
+		/* Update the reply post host index after continuously
+		 * processing the threshold number of Reply Descriptors.
+		 * So that FW can find enough entries to post the Reply
+		 * Descriptors in the reply descriptor post queue.
+		 */
+		if (completed_cmds > ioc->hba_queue_depth/3) {
+			if (ioc->combined_reply_queue) {
+				writel(reply_q->reply_post_host_index |
+						((msix_index  & 7) <<
+						 MPI2_RPHI_MSIX_INDEX_SHIFT),
+				    ioc->replyPostRegisterIndex[msix_index/8]);
+			} else {
+				writel(reply_q->reply_post_host_index |
+						(msix_index <<
+						 MPI2_RPHI_MSIX_INDEX_SHIFT),
+						&ioc->chip->ReplyPostHostIndex);
+			}
+			completed_cmds = 1;
+		}
 		if (request_desript_type == MPI2_RPY_DESCRIPT_FLAGS_UNUSED)
 			goto out;
 		if (!reply_q->reply_post_host_index)