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

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Message-ID: <20070131094858.GA23842@srcf.ucam.org>
Date:	Wed, 31 Jan 2007 09:48:58 +0000
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Amit Kucheria <amit.kucheria@...ia.com>
Cc:	ipw2100-devel@...ts.sourceforge.net, netdev@...r.kernel.org,
	linux-pm@...ts.osdl.org
Subject: Re: [linux-pm] [RFC] Runtime power management on ipw2100

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@...f.ucam.org
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