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

[Sebastian Hesselbarth]

On 11/20/2013 10:10 PM, Florian Fainelli wrote:
> 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.

No doubt.

> 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.

Well, I haven't thought about WoL and stuff and how the driver should
leave the PHY for that to work. Suspend support isn't really spread
among ARM SoCs :P

But if you can run some tests on non-ARM platforms you might already
been testing with, I am sure we can work it out. Maybe, we just have
PHY_SUSPENDED and deal with it differently if any regressions pop up?

Also, suspend_unused and auto-suspend on phy_stop can be disabled by
default for some kernel versions until we have enough coverage?

> > 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.

I see it was a good idea to send the RFC early. The savings are
impressive but I was already quite sure that it has the potential to
break (at least) the quirky PHYs.

Sebastian

> > [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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