Re: ahci: CAP_SSS and parallel scan

[Jeff Garzik <jeff@garzik.org>]

On 05/08/2010 04:40 AM, Tejun Heo wrote:
> On 05/07/2010 11:33 PM, Jeff Garzik wrote:
>> I'm not sure the usefulness is limited, as it definitely avoids power
>> spikes on server BIOSen that care.
>
> So, are there actually machines which get affected by this?  In that
> case, sure, the current behavior is the right one.

The current behavior is the safe, conservative interpretation.  The 
alternative is risking unwanted power spikes for the sake of boot speed.

I would set a high barrier for taking that risk:  polling multiple 
hardware vendors about in-the-field SSS usage.


>> Also, it seems unwise for the Linux SATA driver to do the exact
>> -opposite- of what the SSS bit intends, by default.
>
> Yeah, well, the thing is that SSS in itself doesn't really indicate
> need for staggered spin up.  It just says it knows how to.

A valid point, I agree.  But absent any other method of communicating 
that SSS behavior is desired, activating staggered spin-up based on SSS 
is the only logical interpretation, AFAICS.  SSS bit set implies SSS 
-might- be needed.  If SSS might be needed, then one must stagger 
spin-up or risk failing to meet the requirement.  !SSS is the only case 
where you are -guaranteed- not to need staggered spin-up.

	Jeff