From 822e734fef042c98b3c03efd079da4e5fb9bcd0f Mon Sep 17 00:00:00 2001 From: Dan Williams Date: Thu, 17 Apr 2014 11:48:21 -0700 Subject: libata/ahci: accommodate tag ordered controllers commit 8a4aeec8d2d6a3edeffbdfae451cdf05cbf0fefd upstream. The AHCI spec allows implementations to issue commands in tag order rather than FIFO order: 5.3.2.12 P:SelectCmd HBA sets pSlotLoc = (pSlotLoc + 1) mod (CAP.NCS + 1) or HBA selects the command to issue that has had the PxCI bit set to '1' longer than any other command pending to be issued. The result is that commands posted sequentially (time-wise) may play out of sequence when issued by hardware. This behavior has likely been hidden by drives that arrange for commands to complete in issue order. However, it appears recent drives (two from different vendors that we have found so far) inflict out-of-order completions as a matter of course. So, we need to take care to maintain ordered submission, otherwise we risk triggering a drive to fall out of sequential-io automation and back to random-io processing, which incurs large latency and degrades throughput. This issue was found in simple benchmarks where QD=2 seq-write performance was 30-50% *greater* than QD=32 seq-write performance. Tagging for -stable and making the change globally since it has a low risk-to-reward ratio. Also, word is that recent versions of an unnamed OS also does it this way now. So, drives in the field are already experienced with this tag ordering scheme. Cc: Dave Jiang Cc: Ed Ciechanowski Reviewed-by: Matthew Wilcox Signed-off-by: Dan Williams Signed-off-by: Tejun Heo Signed-off-by: Greg Kroah-Hartman --- include/linux/libata.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/linux') diff --git a/include/linux/libata.h b/include/linux/libata.h index bec6dbe939a..3fee55e73e5 100644 --- a/include/linux/libata.h +++ b/include/linux/libata.h @@ -822,6 +822,7 @@ struct ata_port { unsigned long qc_allocated; unsigned int qc_active; int nr_active_links; /* #links with active qcs */ + unsigned int last_tag; /* track next tag hw expects */ struct ata_link link; /* host default link */ struct ata_link *slave_link; /* see ata_slave_link_init() */ -- cgit v1.2.3