ata: libata-scsi: do not use the deferred QC feature on PMPs with CBS

ata: libata-scsi: do not use the deferred QC feature on PMPs with CBS

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")
Tested-by: Tommy Kelly <linux@tkel.ly>
Reported-by: Tommy Kelly <linux@tkel.ly>
Closes: https://lore.kernel.org/linux-ide/ce09cc21-a8e9-4845-b205-35411e22fba9@tkel.ly/
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>