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

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Message-Id: <05EEFFFE-AD93-40A1-8851-92C3053286E1@mac.com>
Date:	Sun, 1 Jul 2007 00:51:15 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
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 Jun 30, 2007, at 12:42:06, Jeff Garzik wrote:
> 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.

With some buggy switches and other hardware you actually *want* to  
bounce the link to get them to properly renegotiate.  I can also see  
wanting to power off and on a single-PoE-port NIC to restart whatever  
device is at the other end, although I don't know if any such devices  
exist.  Currently the tg3 driver turns the PHY off and on during down/ 
up on a few of my systems, which I use to make a buggy no-name switch  
recognize STP changes properly.

Cheers,
Kyle Moffett
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