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Date:   Mon, 18 Mar 2019 15:19:13 +0100
From:   Andrew Lunn <>
To:     Jose Abreu <>
Cc:     Heiner Kallweit <>,
        Florian Fainelli <>,,,
        "David S. Miller" <>,
        Joao Pinto <>
Subject: Re: [PATCH net] net: phy: Don't assume loopback is supported

> We provide PHYs to our customers and in the documentation I have
> this can be an optional feature that HW team can choose to have
> or not, making the bit read-only or r/w.
> Heiner, can you please confirm there is no Clause 22 "pitfalls" /
> "hidden comments" that allow this bitfield to be read-only ?

Hi Jose

I have the 802.3 standard from 2015. It should be free to download
from the IEEE. So you can go get it yourself.

Section defines loopback. I don't see anything which makes
it optional. The only wiggle room you have is where in the PHY the
loopback actually takes place. That is implementation specific, but it
recommends you make it as late as possible in the path so as to test
as much as possible.

If your PHY does not implement loopback, i would say it breaks the
standard. We try to keep workarounds for brokenness in the specific
PHY driver, not the generic code.


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