Date:   Wed, 5 Jun 2019 11:27:59 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Ioana Ciornei <ioana.ciornei@....com>
Subject: Re: Cutting the link on ndo_stop - phy_stop or phy_disconnect?

On Wed, 5 Jun 2019 at 06:06, Florian Fainelli <f.fainelli@...il.com> wrote:
>
>
>
> On 6/4/2019 4:46 PM, Vladimir Oltean wrote:
> > On Wed, 5 Jun 2019 at 02:24, Russell King - ARM Linux admin
> > <linux@...linux.org.uk> wrote:
> >>
> >> On Wed, Jun 05, 2019 at 02:03:19AM +0300, Vladimir Oltean wrote:
> >>> On Wed, 5 Jun 2019 at 01:59, Russell King - ARM Linux admin
> >>> <linux@...linux.org.uk> wrote:
> >>>>
> >>>> On Wed, Jun 05, 2019 at 01:44:08AM +0300, Vladimir Oltean wrote:
> >>>>> You caught me.
> >>>>>
> >>>>> But even ignoring the NIC case, isn't the PHY state machine
> >>>>> inconsistent with itself? It is ok with callink phy_suspend upon
> >>>>> ndo_stop, but it won't call phy_suspend after phy_connect, when the
> >>>>> netdev is implicitly stopped?
> >>>>
> >>>> The PHY state machine isn't inconsistent with itself, but it does
> >>>> have strange behaviour.
> >>>>
> >>>> When the PHY is attached, the PHY is resumed and the state machine
> >>>> is in PHY_READY state.  If it goes through a start/stop cycle, the
> >>>> state machine transitions to PHY_HALTED and attempts to place the
> >>>> PHY into a low power state.  So the PHY state is consistent with
> >>>> the state machine state (we don't end up in the same state but with
> >>>> the PHY in a different state.)
> >>>>
> >>>> What we do have is a difference between the PHY state (and state
> >>>> machine state) between the boot scenario, and the interface up/down
> >>>> scenario, the latter behaviour having been introduced by a commit
> >>>> back in 2013:
> >>>>
> >>>>     net: phy: suspend phydev when going to HALTED
> >>>>
> >>>>     When phydev is going to HALTED state, we can try to suspend it to
> >>>>     safe more power. phy_suspend helper will check if PHY can be suspended,
> >>>>     so just call it when entering HALTED state.
> >>>>
> >>>> --
> >>>> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> >>>> FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
> >>>> According to speedtest.net: 11.9Mbps down 500kbps up
> >>>
> >>> I am really not into the PHYLIB internals, but basically what you're
> >>> telling me is that running "ip link set dev eth0 down" is a
> >>> stronger/more imperative condition than not running "ip link set dev
> >>> eth0 up"... Does it also suspend the PHY if I put the interface down
> >>> while it was already down?
> >>
> >> No - but that has nothing to do with phylib internals, more to do with
> >> the higher levels of networking.  ndo_stop() will not be called unless
> >> ndo_open() has already been called.  In other words, setting an already
> >> down device down via "ip link set dev eth0 down" is a no-op.
> >>
> >> So, let's a common scenario.  You power up a board.  The PHY comes up
> >> and establishes a link.  The boot loader runs, loads the kernel, which
> >
> > This may or may not be the case. As you pointed out a few emails back,
> > this is a system-level issue that requires a system-level solution -
> > so cutting the link in U-boot is not out of the question.
> >
> >> then boots.  Your network driver is a module, and hasn't been loaded
> >> yet.  The link is still up.
> >>
> >> The modular network driver gets loaded, and initialises.  Userspace
> >> does not bring the network device up, and the network driver does not
> >> attach or connect to the PHY (which is actually quite common).  So,
> >> the link is still up.
> >>
> >> The modular PHY driver gets loaded, and binds to the PHY.  The link
> >> is still up.
> >
> > I would rather say, 'even if the link is not up, Linux brings it up
> > (possibly prematurely) via phy_resume'.
> > But let's consider the case where the link *was* up. The general idea
> > is 'implement your workarounds in whatever other way, that link is
> > welcome!'.
>
> With the systems that I work with, we have enforced the following
> behavior to happen: the boot loader and kernel only turn on what they
> needs, at the time they need it, and nothing more, once done, they put
> the blocks back into lowest power mode (clock and power gated if
> available). So yes, there are multiple link re-negotiations throughput
> the boot process, but when there is no device bound to a driver the
> system conserves power by default which is deemed a higher goal than
> speed. Your mileage may vary of course.
>
> There is not exactly a simple way of enforcing that kind (or another
> kind for that matter) of policy kernel wide, so it's unfortunately up to
> the driver writer to propose something that is deemed sensible.
>
> We could however, extend existing tools like iproute2 to offer the
> ability to control whether the PHY should be completely powered off, in
> a low power state allowing WoL, or remain UP when the network device is
> brought down. That would not cover the case that Russell explained, but
> it would be another monkey wrench you can throw at the system.
> --
> Florian

Hi Florian,

By going to HALTED on phy_stop, the system already achieves what I am
looking after - although maybe it is an unintended consequence for
you.
I'm only trying to make an argument for removing the phy_resume from
phy_attach_direct now. If there was a link, and it doesn't need
re-negociation, fine, use it in phy_start, but at most leave U-boot to
put that link down and don't force it up prior to the netdev's
ndo_open.

Regards,
-Vladimir

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