Re: [PATCH v5 3/4] ata: libata-scsi: do not use the deferred QC feature on PMPs with CBS

[Damien Le Moal <dlemoal@kernel.org>]

On 2026/05/14 9:39, Niklas Cassel wrote:
> When using Port Multipliers (PMPs) with Command-Based Switching (CBS), you
> can only issue commands to one link at a time. For PMPs with CBS, there is
> already code to handle commands being sent to different links in
> sata_pmp_qc_defer_cmd_switch() using ap->excl_link. sata_sil24 also makes
> use of ap->excl_link.
> 
> A user on the list reported that commit 0ea84089dbf6 ("ata: libata-scsi:
> avoid Non-NCQ command starvation") broke PMPs with CBS. The commit
> introduced code that stores a deferred qc in ap->deferred_qc, to later be
> issued via a workqueue. It turns out that this change is incompatible with
> the existing ap->excl_link handling used by PMPs with CBS.
> 
> Thus, modify sata_pmp_qc_defer_cmd_switch() and sil24_qc_defer() to return
> ATA_DEFER_LINK_EXCL, and make sure that the deferred QC handling via
> workqueue is not used for this return value.
> 
> This way, PMPs with CBS will work once again. Note that the starvation
> referenced in commit 0ea84089dbf6 ("ata: libata-scsi: avoid Non-NCQ
> command starvation") can only happen on libsas ports, and libsas does not
> support Port Multipliers, thus there is no harm of reverting back to the
> previous way of deferring commands for PMPs with CBS.
> 
> Non-libsas ports connected to anything but a PMP with CBS (e.g. a normal
> drive or a PMP with FBS) will continue using the deferred workqueue, since
> it does result in lower completion latencies for non-NCQ commands, even
> though the workqueue is not strictly needed to avoid starvation for
> non-libsas ports.
> 
> If we want to modify the scope of the workqueue issuing to also handle
> PMPs with CBS, then we should ensure that we can save both NCQ and non-NCQ
> commands in ap->deferred_qc, while also removing the existing PMP CBS
> handling using ap->excl_link, such that we don't duplicate features.
> 
> While at it, also add a comment explaining how the ap->excl_link mechanism
> works.
> 
> Fixes: 0ea84089dbf6 ("ata: libata-scsi: avoid Non-NCQ command starvation")
> Signed-off-by: Niklas Cassel <cassel@kernel.org>

Looks good.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>


-- 
Damien Le Moal
Western Digital Research