Re: [PATCH 5/6] libata-pmp-prep: implement ops->qc_defer()

[Jeff Garzik <jgarzik@pobox.com>]

Tejun Heo wrote:
> Controllers which support PMP have various restrictions on which
> combinations of commands are allowed to what number of devices
> concurrently.  This patch implements ops->qc_defer() which determines
> whether a qc can be issued at the moment or should be deferred.
> 
> If the function returns ATA_DEFER_LINK, the qc will be deferred until
> a qc completes on the link.  If ATA_DEFER_PORT, until a qc completes
> on any link.  The defer conditions are advisory and in general
> ATA_DEFER_LINK can be considered as lower priority deferring than
> ATA_DEFER_PORT.
> 
> ops->qc_defer() replaces fixed ata_scmd_need_defer().  For standard
> NCQ/non-NCQ exclusion, ata_std_qc_defer() is implemented.  ahci and
> sata_sil24 are converted to use ata_std_qc_defer().
> 
> ops->qc_defer() is heavier than the original mechanism because full qc
> is prepped before determining to defer it, but various information is
> needed to determine defer conditinos and fully translating a qc is the
> only way to supply such information in generic manner.
> 
> IMHO, this shouldn't cause any noticeable performance issues as
> 
> * for most cases deferring occurs rarely (except for NCQ-aware
>   cmd-switching PMP)
> * translation itself isn't that expensive
> * once deferred the command won't be repeated until another command
>   completes which usually is a very long time cpu-wise.
> 
> Signed-off-by: Tejun Heo <htejun@gmail.com>

ACK patches 1-4.

For this patch (#5), I would much rather prefer to update ->qc_issue() 
or ->qc_prep() to return a DEFER return value.  That matches more 
closely other APIs in Linux.

	Jeff