diff options
authorDan Williams <dan.j.williams@intel.com>2014-04-17 11:48:21 -0700
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-05-13 13:59:43 +0200
commitcfd192c9983ea4e993cabf4f899ef587f31ab51e (patch)
parent43dae355ff1d15be58dd267a07aea6eb31ebfd9f (diff)
libata/ahci: accommodate tag ordered controllers
commit 8a4aeec8d2d6a3edeffbdfae451cdf05cbf0fefd upstream. The AHCI spec allows implementations to issue commands in tag order rather than FIFO order: 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 <dave.jiang@intel.com> Cc: Ed Ciechanowski <ed.ciechanowski@intel.com> Reviewed-by: Matthew Wilcox <matthew.r.wilcox@intel.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2 files changed, 14 insertions, 8 deletions
diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index 8b8cbe9bcb92..9cf616b5210b 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -4765,21 +4765,26 @@ void swap_buf_le16(u16 *buf, unsigned int buf_words)
static struct ata_queued_cmd *ata_qc_new(struct ata_port *ap)
struct ata_queued_cmd *qc = NULL;
- unsigned int i;
+ unsigned int i, tag;
/* no command while frozen */
if (unlikely(ap->pflags & ATA_PFLAG_FROZEN))
return NULL;
- /* the last tag is reserved for internal command. */
- for (i = 0; i < ATA_MAX_QUEUE - 1; i++)
- if (!test_and_set_bit(i, &ap->qc_allocated)) {
- qc = __ata_qc_from_tag(ap, i);
+ for (i = 0; i < ATA_MAX_QUEUE; i++) {
+ tag = (i + ap->last_tag + 1) % ATA_MAX_QUEUE;
+ /* the last tag is reserved for internal command. */
+ if (tag == ATA_TAG_INTERNAL)
+ continue;
+ if (!test_and_set_bit(tag, &ap->qc_allocated)) {
+ qc = __ata_qc_from_tag(ap, tag);
+ qc->tag = tag;
+ ap->last_tag = tag;
- if (qc)
- qc->tag = i;
+ }
return qc;
diff --git a/include/linux/libata.h b/include/linux/libata.h
index f33619d8ac52..eec130af2dfa 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -772,6 +772,7 @@ 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() */