Re: [ANNOUNCE] libata EH/NCQ/hotplug/PM git tree

[Tejun Heo <htejun@gmail.com>]

Jeff Garzik wrote:
> Tejun Heo wrote:
>> Jeff Garzik wrote:
>>> Tejun Heo wrote:
>>>> tested.  I implemented AHCI PM support half-way but then realized I 
>>>> don't have any AHCI controller which supports PM (I have ICH7R).  Is 
>>>> there any AHCI controller which reports PM support?
>>>
>>> You can always use SCR-based PM support...
>>>
>>
>> Can you elaborate a bit?
> 
> Sections A.1 and 6.1 of the PM spec.  You set the PMP field of the 
> SControl register.  This has obvious parallelism limitations, which is 
> why "hardware context switching support" aka native PM support is 
> greatly preferred.
> 

That cmd-switching PM support is what AHCI SPM bit indicates.  As of 
v1.1, AHCI doesn't define FIS-switching PM support yet.

Supports Port Multiplier (SPM): Indicates whether the HBA can support a 
Port Multiplier. When set, a Port Multiplier using command-based 
switching is supported. When cleared to ‘0’, a Port Multiplier is not 
supported, and a Port Multiplier may not be attached to this HBA.

And. the SControl section says...

19:16 RO 0h  Port Multiplier Port (PMP): This field is not used by AHCI.

Which is understandable considering AHCI doesn't construct FISes itself. 
  I don't think PM requires more hardware support than being capable for 
setting PMP in the FISes, which AHCI certainly can (standing down on 
collision for SDB notify aside).  I can give it a shot but it clearly 
violates the spec, and even if it works, we would need to print big fat 
warning message while attaching PM.

-- 
tejun