Re: [PATCH #upstream-fixes] sata_mv: don't issue two DMA commands concurrently

[Mark Lord <liml@rtr.ca>]

Tejun Heo wrote:
> Gwendal Grignou wrote:
>> What happen if there is a SATA port multiplier behing the marvell
>> controller and NCQ is disabled?
>>
>> I have the impression that with this patch we are able to only send
>> one DMA command at a time to all the drives behind the PM, which has
>> an impact on performance.
> 
> Yeah, right. :-( The simplest way to implement this would be just
> calling ata_std_qc_defer() which computes per-link restrictions after
> ruling out all non-standard restrictions.  I wasn't really sure how
> EDMA/NCQ_EN flags are supposed to work so just sticked to the original
> condition tests.  Mark, can you please help us out here?  :-)
..

The hardware is both tricky and buggy, depending upon the version,
and my memory is fading -- I haven't looked at the code since early June,
when Marvell stopped paying invoices (administrative "issues").

The only 100% safe scenario is to issue only a single non-NCQ command at a time.
But there are many situations where it is safe to allow one command/port,
provided that there are no PIO commands anywhere in the mix.

Messy.  I'll have another walk through it whenever Marvell gets over
their "administrative issues" on this.

Cheers