[PATCH 1/3] ata: libata-pmp: fix ata_pmp_qc_defer_cmd_switch()

[Niklas Cassel <cassel@kernel.org>]

When using Port Multipliers (PMPs) with Command-Based Switching (CBS), you
can only issue commands to one link at a time.

This is implemented in sata_pmp_qc_defer_cmd_switch() by setting
ap->excl_link if there is no link currently being exclusive, and setting
qc->flags ATA_QCFLAG_CLEAR_EXCL to clear the exlusive link in
__ata_qc_complete().

ATA_QCFLAG_CLEAR_EXCL is only set if this is the first link to become
active, or if (since CBS can only have one active link) the current link
is active. __ata_qc_complete() clears ap->excl_link as long as
ATA_QCFLAG_CLEAR_EXCL is set, it does not matter if there are still
outstanding requests to the same link. This works because
sata_pmp_qc_defer_cmd_switch() will only allow new commands to be queued
if (ap->nr_active_links == 0 || ata_link_active(link)).

However, since sata_pmp_qc_defer_cmd_switch() does a:
return ata_std_qc_defer(qc); before setting ap->excl_link, this means that
in the case where ap->excl_link == NULL and ata_std_qc_defer() returns 0
(i.e. the QC should not be deferred), we would not set ap->excl_link.

Additionally, think of the following example:
NCQ command1 to drive1 issued:
sets ATA_QCFLAG_CLEAR_EXCL
does not set ap->excl_link

NCQ command2 to drive1 issued:
sets ATA_QCFLAG_CLEAR_EXCL
does not set ap->excl_link

NCQ command3 to drive2 issued:
ap->excl_link gets set
does not set ATA_QCFLAG_CLEAR_EXCL
returns ATA_DEFER_PORT

For command3, it seems wrong to set ap->excl_link while also deferring the
command with ATA_DEFER_PORT.

Modify sata_pmp_qc_defer_cmd_switch() to only set ap->excl_link if we can
actually issue a command (i.e. if we don't return ATA_DEFER_PORT or
ATA_DEFER_LINK).

Fixes: 31f88384443b ("libata-pmp: implement qc_defer for command switching PMP support")
Signed-off-by: Niklas Cassel <cassel@kernel.org>
---
 drivers/ata/libata-pmp.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/ata/libata-pmp.c b/drivers/ata/libata-pmp.c
index e3adc008fed1..0575e1e283d8 100644
--- a/drivers/ata/libata-pmp.c
+++ b/drivers/ata/libata-pmp.c
@@ -113,11 +113,14 @@ int sata_pmp_qc_defer_cmd_switch(struct ata_queued_cmd *qc)
 
 	if (ap->excl_link == NULL || ap->excl_link == link) {
 		if (ap->nr_active_links == 0 || ata_link_active(link)) {
+			int ret;
+
 			qc->flags |= ATA_QCFLAG_CLEAR_EXCL;
-			return ata_std_qc_defer(qc);
+			ret = ata_std_qc_defer(qc);
+			if (!ret)
+				ap->excl_link = link;
+			return ret;
 		}
-
-		ap->excl_link = link;
 	}
 
 	return ATA_DEFER_PORT;
-- 
2.54.0