Re: [PATCH 0/3] ata: fix deferred QC handling for port multipliers

[John Garry <john.g.garry@oracle.com>]

On 01/05/2026 13:54, Niklas Cassel wrote:
> Hello all,
> 
> Tommy Kelly reported a regression with PMP that use CBS:
> https://lore.kernel.org/linux-ide/ce09cc21-a8e9-4845-b205-35411e22fba9@tkel.ly/
> 
> To me, this appears to be a problem in ata_pmp_qc_defer_cmd_switch() that
> has been there since this function was introduced. ap->excl_link did not
> always get set. This is fixed in patch 1/3.
> 
> While looking at the code, when using a PMP with CBS, it turns out that we
> are incorrectly using the deferred_qc feature that issues a QC via a
> workqueue, even for non-NCQ commands that are issued to another link
> (a link that is not the active link). This workqueue feature was meant to
> avoid dealing with non-NCQ vs NCQ commands for a single drive, not to
> duplicate the existing ap->excl_link logic. This is fixed in patch 2/3.
> 
> While looking at the code, it turns out that the deferred qc issuing via
> workqueue is misdesigned. It assumed that we can't mix NCQ and non-NCQ
> commands on the same port. The limitation is that you can not mix NCQ and
> non-NCQ commands on the same drive. However, with a PMP with FBS, you can
> issue (mixed NCQ and non-NCQ commands) to the different drives. Thus, move
> the saved deferred QC from struct ata_port to struct ata_link. This is
> fixed in patch 3/3.
> 
> Tommy, it would be nice if you could apply this series and see if it solves
> your problem. Right now the series is compile tested only.
> 
> 

Hi Niklas,

As I understand, this deferred QC scheme was intended to solve 
starvation problems for multiqueue SAS HBAs (using libsas-based 
drivers). libsas has no port multiplier support, so I wonder why we make 
deferred QC scheme work for port multiplier, i.e. non-libsas scenario.

Thanks,
John

> Niklas Cassel (3):
>    ata: libata-pmp: fix ata_pmp_qc_defer_cmd_switch()
>    ata: libata-scsi: do not use the deferred QC feature on PMPs with CBS
>    ata: libata-scsi: do not needlessly defer commands when using PMP with
>      FBS
> 
>   drivers/ata/libata-core.c | 16 +++++++---
>   drivers/ata/libata-eh.c   |  8 ++---
>   drivers/ata/libata-pmp.c  | 13 +++++---
>   drivers/ata/libata-scsi.c | 66 ++++++++++++++++++++++-----------------
>   include/linux/libata.h    |  6 ++--
>   5 files changed, 64 insertions(+), 45 deletions(-)
>