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

[Tejun Heo <htejun@gmail.com>]

Jeff Garzik wrote:
> 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.

Will do.

-- 
tejun