netdev - Re: No traffic with Marvell switch and latest linux-next

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Date:   Sun, 24 Feb 2019 15:49:00 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: No traffic with Marvell switch and latest linux-next

On Sun, Feb 24, 2019 at 04:39:30PM +0100, Heiner Kallweit wrote:
> On 24.02.2019 16:34, Russell King - ARM Linux admin wrote:
> > On Sun, Feb 24, 2019 at 04:28:32PM +0100, Heiner Kallweit wrote:
> >> On 24.02.2019 16:15, Russell King - ARM Linux admin wrote:
> >>> On Sun, Feb 24, 2019 at 04:04:03PM +0100, Andrew Lunn wrote:
> >>>>> I think what's not correct is that phydev->autoneg is set
> >>>>> (by phy_device_create) for a fixed link.
> >>>>
> >>>> Fixed-link tries to emulate auto-neg:
> >>>>
> >>>>                 bmsr |= BMSR_LSTATUS | BMSR_ANEGCOMPLETE;
> >>>>
> >>>> Maybe it needs better emulation of auto-neg?
> >>>
> >>> Or maybe it needs to represent a fixed-speed PHY by clearing bit 1.3
> >>> (BMSR_ANEGCAPABLE).  In any case, 0.12 (BMCR_ANENABLE) is not set,
> >>> so according to 802.3-2015, we should not be setting 1.5
> >>> (BMSR_ANEGCOMPLETE).
> >>>
> >>> However, swphy does try to emulate autonegotiation - we do have cases
> >>> where swphy is used in situations where the speed and duplex are not
> >>> fixed.  It returns an emulated link partner advertisement for the
> >>> current speed, which would suggest that we should set BMCR_ANENABLE.
> >>>
> >> If we emulate auto-neg, then it's not needed to set the speed bits
> >> in BMCR. Also what just comes to my mind, certain speeds like 1000BaseT
> >> don't support forced mode. So we may have to go with auto-neg.
> > 
> > Sure.
> > 
> >> To avoid the original issue it should be sufficient to copy
> >> supported -> advertising at a suited place.
> > 
> Sorry, seems this wasn't clear enough. I don't mean to change
> swphy but the user side.
> 
> > Why bother - the software PHY emulation is an emulation to allow
> > existing userspace that pre-dates the ethtool API to get some link
> > parameters.  If we augment the PHY emulation in non-standard ways,
> > userspace will need to be updated to handle those non-standard
> > ways.  If userspace needs to be updated, why not just bite the
> > bullet and update to ethtool APIs rather than adding more
> > complication through an emulation layer?
> > 
> It's not only userspace. Based on my limited knowledge of DSA this
> code also uses e.g. genphy_read_status() with a fixed link.

DSA has support for phylink, which is perfectly capable of supporting
fixed links without using fixed-phy.c, although we have no way to
create that without DT.  Support could be added for non-DT though.

That would avoid using phylib functions to read back from a fixed-link
PHY.  Since phylink presents to the MAC a fixed link in the same
abstract manner as a real PHY, it should result in a more elegant
implementation.

DSA already has phylink support to support SFPs.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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