netdev - Re: [PATCH 2/5] net: phy: hook up clause 45 autonegotiation restart

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Message-ID: <20170601162417.GA14541@n2100.armlinux.org.uk>
Date:   Thu, 1 Jun 2017 17:24:18 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 2/5] net: phy: hook up clause 45 autonegotiation restart

On Thu, Jun 01, 2017 at 04:47:35PM +0100, Russell King - ARM Linux wrote:
> On Thu, Jun 01, 2017 at 03:19:55PM +0200, Andrew Lunn wrote:
> > On Thu, Jun 01, 2017 at 02:09:00PM +0100, Russell King - ARM Linux wrote:
> > > On Thu, Jun 01, 2017 at 03:05:27PM +0200, Andrew Lunn wrote:
> > > > So you are saying a 10G PHY driver always needs to have a aneg_done
> > > > callback, even if it just needs to call phygen_c45_aneg_done?
> > > > 
> > > > This seems a bit error prone. I can see somebody writing a 10G driver,
> > > > leaving out aneg_done() and having the c22 version called. Is the read
> > > > of MII_BMSR likely to return 0xffff, since the register does not
> > > > exist? If so, genphy_aneg_done() is likely to always return
> > > > BMSR_ANEGCOMPLETE.
> > > 
> > > Don't forget that the read will fail, so phy_read() will return a
> > > negative number.
> > 
> > By fail, you mean return something like -EIO or -ETIMEOUT? Is this
> > guaranteed in the code somewhere? This particular Marvell PHY only
> > does c45. But i could imagine some other PHYs answering a c22 request
> > with 0xffff.
> 
> Yes, C45 allows the PHYs to answer C22 as well, but then they have to
> implement the C22 register set.  Such a PHY would be out of spec,
> especially as what you're suggesting is that it answers C22 cycles
> and fails to implement MII_BMSR.  I also think that there's a comment
> in the 802.3 specs that says that unimplemented registers are to
> return zero, not 0xffff.

Checking 802.3-2015, in "22.2.4 Management functions", the first
sentence requires all PHYs that respond to Clause 22 cycles to
implement BMCR and BMSR.  However, my statement about unimplemented
registers returning zero seems to be a C45 thing, not a C22 thing,
according to the C22 PICS and "45.2 MDIO Interface Registers"

However, digging a bit further, "22.2.4.2.10 Auto-Negotiation complete"
states that bit 5 shall be zero if aneg has not completed or if aneg is
unimplemented.

-- 
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