[3.13.y.z extended stable] Patch "libata/ahci: accommodate tag ordered controllers" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    libata/ahci: accommodate tag ordered controllers

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.3.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From f3b732755f0fdccc1a45939cfe29d7724be8daf2 Mon Sep 17 00:00:00 2001
From: Dan Williams <dan.j.williams at intel.com>
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 <dave.jiang at intel.com>
Cc: Ed Ciechanowski <ed.ciechanowski at intel.com>
Reviewed-by: Matthew Wilcox <matthew.r.wilcox at intel.com>
Signed-off-by: Dan Williams <dan.j.williams at intel.com>
Signed-off-by: Tejun Heo <tj at kernel.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/ata/libata-core.c | 21 +++++++++++++--------
 include/linux/libata.h    |  1 +
 2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index 2897dbe..0a79c54 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -4794,21 +4794,26 @@ 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;
 			break;
 		}
-
-	if (qc)
-		qc->tag = i;
+	}

 	return qc;
 }
diff --git a/include/linux/libata.h b/include/linux/libata.h
index bec6dbe..3fee55e 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() */
--
1.9.1