Re: [linux-pm] [RFC] Runtime power management on ipw2100

Matthew Garrett
2007-01-31T09:48:58Z

On Wed, Jan 31, 2007 at 11:13:07AM +0200, Amit Kucheria wrote:

> What is the latency in changing between different PCI power states for
> peripherals?

I'm not sure in the general case, but the power-down path for the
ipw2100 involves a static wait of 100ms in ipw2100_hw_stop_adapter().

> Would it be possible e.g. to put the peripheral into a low power state
> after each Tx/Rx (with reasonable hyteresis)?

Most wireless drivers support some degree of power management at this
scale, but (in ipw2100 at least) it's implemented in the firmware so I
have absolutely no idea what it's actually doing.

> <snip>
>
> > The situation is slightly more complicated for wired interfaces. As
> > previously discussed, we potentially want three interface states (on,
> > low power, off) with the intermediate one powering down as much of the
> > hardware as possible while still implementing link detection.
>
> And this low power state is what the HW should be in all the time,
> except when it has work to do.

PCI seems to require a delay of 10ms when sequencing from D3 to D0,
which probably isn't acceptable latency for an "up" state. While there's
definitely a benefit to the sort of PM you're describing (it's a model
we've already started using on the desktop as far as the CPU goes), I
think we still want to be able to expose as much power saving as
possible.

--
Matthew Garrett | mjg59@srcf.ucam.org

-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys - and earn cash