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