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

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Message-ID: <20170601154735.GB22219@n2100.armlinux.org.uk>
Date:   Thu, 1 Jun 2017 16:47:35 +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 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.

So, if there is a non-compliant PHY like that, I think the time to
address it is when such a broken situation appears, rather than
engineering additional complexity before we know we need it.

-- 
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