Re: [PATCH RFC v1 0/7] net: phy: Ethernet PHY powerdown optimization

[Florian Fainelli]

2013/11/20 Sebastian Hesselbarth <sebastian.hesselbarth@xxxxxxxxx>:
> On 11/20/2013 09:36 PM, David Miller wrote:
>>
>> From: Sebastian Hesselbarth <sebastian.hesselbarth@xxxxxxxxx>
>> Date: Wed, 20 Nov 2013 21:21:46 +0100
>>
>>> Ethernet PHYs consume a significant amount of power when link is
>>> detected.
>>> Especially, for embedded systems it can be easily 20-40% of total system
>>> power. Now, currently most likely all ethernet drivers leave PHYs powered
>>> on, even if the device is taken down. Also, some stupid boot loaders
>>> power
>>> on all PHYs available.
>>>
>>> This RFC deals with saving power consumed by ethernet PHYs, that have no
>>> corresponding ethernet device driver or are attached to ethernet devices
>>> which are taken down by user request, i.e. ifconfig ethN down. Ports with
>>> no link, i.e. cable removed, are already quite good at power saving due
>>> to
>>> PHY internal link detection.
>>
>>
>> The idea is sound and the goal is of course valuable, but it brings up
>> a chronically reoccurring issue as of late.
>>
>> You cannot reset the PHY or take it down without somehow retaining the
>> settings the PHY had when you bring it back up.
>
>
> Right, as far as I understand BMCR powerdown, i.e. what is called in
> genphy_suspend/resume, powers down the PHY but _does_ retain PHY config.
> It is not resetting the device.

Right that's also my understanding of how BMCR powerdown works. That
said, I am relatively sure that we can find PHY devices for which this
is not true.

As for the PHY state machine, I think we need a new state
PHY_SUSPENDED  and upon calling phy_start() we make sure that we treat
PHY_SUSPENDED just like we treat PHY_HALTED today since that would
make sure that the PHY parameters are applied correctly upon resume
(interrupt configuration, autoneg and and such).

There is still some discussion on how we should deal with
auto-suspending the PHY when phy_stop() is called, and how does that
differ from the PHY_HALTED state? So this also raises the question of
whether PHY_HALTED is really different from PHY_SUSPENDED. The only
difference with your patches would be that we have put the PHY into a
low-power mode.

>
> I haven't checked a lot of datasheets but [1] notes that "registers will
> preserve their configuration". Even if we have PHYs that do not preserve
> it, they should have a device specific callback for suspend/resume that
> takes care of preserving it.

Right, but the PHY driver should only take care of restoring "state
less" PHY context, while the PHY state machine has to restore a "state
aware" PHY device context. So for quirky PHY chips, or those having
advanced power management features, we definitively need the two to be
helping each other.

>
> [1] http://www.ti.com/lit/an/snoa463a/snoa463a.pdf
>
>
>> If I ifdown/ifup a device, my ethtool link configuration better be
>> retained.
>>
>> This means the PHY layer must have a way to reprogram the device when
>> it is brought back up, with whatever settings the software state
>> things are there.
>>
>
>
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-- 
Florian
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