Re: [PATCH] libata: Add an additional LPM policy to match Intel recommendations

[Matthew Garrett]

On Mon, Apr 13, 2015 at 7:32 AM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> On Fri, Apr 10, 2015 at 01:15:29PM -0700, Matthew Garrett wrote:
>> Intel publish a document on designing energy efficient SATA devices at
>> http://www.intel.com/content/dam/doc/reference-guide/sata-devices-implementation-recommendations.pdf
>> which recommends that ALPE be set, ASPE be cleared and that DIPM be enabled
>> on the device. Right now we have no policy that matches that - medium_power
>> does not enable DIPM and min_power sets ASPE. Add a new low_power policy to
>> implement these recommendations. I've left devslp disabled because I'm not
>> yet sure what Intel do in that case. With luck this will provide reasonable
>> power savings without causing the device breakages we occasionally see with
>> the min_power policy.
>
> I suspect the aggressive methods actually don't buy us much, except
> for the obsessive mode switchings, in terms of power saving compared
> to something more reasonable provided via DIPM and I'm not sure it's a
> good idea to introduce yet another mode.  I'd be all in for converting
> min_power mode to DIPM for cases where we know this actually works
> rather than introducing yet another mode.  Or if there's a clear power
> consumption disadvantage to DIPM, let's conver medium_power to that.

Ok - my only real concern there was that we might end up changing the
semantics of medium_power enough that it would break somebody. I'll
post a patch to do that instead and I guess we can see what happens -
worst case we revert it and add a new one.
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