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