netdev - Re: [PATCH RFC v2 6/9] net: phy: add support for probing MMDs >= 8 for devices-in-package

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Date:   Wed, 10 Jun 2020 17:34:17 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Calvin Johnson <calvin.johnson@....nxp.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Jeremy Linton <jeremy.linton@....com>,
        Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH RFC v2 6/9] net: phy: add support for probing MMDs >= 8
 for devices-in-package

On Wed, Jun 10, 2020 at 09:46:33PM +0530, Calvin Johnson wrote:
> Hi Russell,
> 
> On Wed, May 27, 2020 at 11:34:11AM +0100, Russell King wrote:
> > Add support for probing MMDs above 7 for a valid devices-in-package
> > specifier, but only probe the vendor MMDs for this if they also report
> > that there the device is present in status register 2.  This avoids
> > issues where the MMD is implemented, but does not provide IEEE 802.3
> > compliant registers (such as the MV88X3310 PHY.)
> 
> While this patch looks good to me, commit message doesn't seem to align
> with the code changes as it is dealing with MMD at addresses 30 & 31.
> Can you please clarify?

IEEE 802.3 does not define the "device-is-present" two bits in register
8 for all MMDs - it is only present for MMDs 1, 2, 3, 4, 5, 30 and 31.
None of the other MMDs, even those that have been recently defined (at
least in IEEE 802.3-2018) have these bits.

Hence, we can't use them except on the MMDs where they are defined to
be present.

I considered also checking them in MMDs 1, 2, 3, 4, 5, but decided that
the risk of regression was too high for this patch; that's something
which could be added in a separate patch though, to avoid having to
revert the entire thing if a regression is found at a later date.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC for 0.8m (est. 1762m) line in suburbia: sync at 13.1Mbps down 503kbps up