netdev - Re: PM policy, hotplug, power saving (was Re: [PATCH] b44: power down PHY when interface down)

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Date:	Sat, 30 Jun 2007 14:10:58 -0400
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Török Edvin <edwintorok@...il.com>,
	netdev@...r.kernel.org, power@...host.org, zambrano@...adcom.com,
	linux-kernel@...r.kernel.org
Subject: Re: PM policy, hotplug, power saving (was Re: [PATCH] b44: power
 down PHY when interface down)

On Sat, 30 Jun 2007 12:42:06 -0400
Jeff Garzik <jeff@...zik.org> wrote:

> Arjan van de Ven wrote:
> > Matthew Garrett wrote:
> >> Do you still get link beat detection when the phy is powered down?
> 
> > does that matter?
> > If the interface is down, nic drivers aren't expected to detect link... 
> > if userspace wants to find link status it should have the interface up.
> 
> 
> Definitely matters.  Switch renegotiation can take a while, and you must 
> take into account the common case of interface bouncing (immediate down, 
> then up).
> 
> Hoards actively complained the few times we experimented with this, 
> because of e.g. DHCP's habit of bouncing the interface, which resulted 
> in PHY power bouncing, which resulted in negotiation, which resulted in 
> an excrutiating wait on various broken or stupid switches.
> 
> Overall, this may be classed with other problems of a similar sort:  we 
> can power down a PHY, but that removes hotplug capability and extends 
> partner/link negotiation time.
> 
> Like SATA, we actually want to support BOTH -- active hotplug and PHY 
> power-down -- and so this wanders into power management policy.
> 
> Give me a knob, and we can program plenty of ethernet|SATA|USB|... 
> drivers to power down the PHY and save power.
> 
> 	Jeff

We do have IFF_DORMANT, but almost no driver uses it. And most
certainly, the common applications wouldn't know how to use it.
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