netdev - Re: [PATCH net-next 02/10] net: phy: phylink: allow 10GKR interface to use in-band negotiation

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Date:   Mon, 19 Mar 2018 13:52:27 +0100
From:   Antoine Tenart <antoine.tenart@...tlin.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Antoine Tenart <antoine.tenart@...tlin.com>, davem@...emloft.net,
        kishon@...com, gregory.clement@...tlin.com, andrew@...n.ch,
        jason@...edaemon.net, sebastian.hesselbarth@...il.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        thomas.petazzoni@...tlin.com, maxime.chevallier@...tlin.com,
        miquel.raynal@...tlin.com, nadavh@...vell.com, stefanc@...vell.com,
        ymarkman@...vell.com, mw@...ihalf.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net-next 02/10] net: phy: phylink: allow 10GKR interface
 to use in-band negotiation

Hi Russell,

On Mon, Mar 19, 2018 at 11:12:05AM +0000, Russell King - ARM Linux wrote:
> On Mon, Mar 19, 2018 at 09:52:52AM +0100, Antoine Tenart wrote:
> > 
> > Anyway, the reason to use in-band negotiation was also to avoid using
> > fixed-link. It would work but always report the link is up, which for
> > the user isn't a great experience as we have a way to detect this.
> > 
> > What would you suggest to achieve this in a reasonable way?
> 
> The intention of this test in phylink_of_phy_connect() is to avoid
> failing when there is no requirement for a PHY to be present (such as
> a fixed link, or an 802.3z link.)  However, with 10G PHYs such as the
> 3310, we need the PHY so we can read the speed from it, and so know
> whether to downgrade the MAC to SGMII mode, or having downgraded the
> MAC, upgrade it back to 10G mode when the PHY switches to 10G.
> 
> I'm guessing that you're wanting this for the DB boards, but I don't
> see why.  Do they not have PHYs?

You guessed right, that's exactly my use case. The DB boards (7k and 8k)
have 10G interfaces without PHYs. I could describe them as fixed-link
(it works), but it would be better not to require a PHY in
phylink_of_phy_connect() for such interfaces.

That's why I used in-band AN, which is wrong, but we still probably
need to add a check to allow such setups. I'm all ears for suggestions
as I do not have the full picture of all the supported modes and their
requirements.

Thanks!
Antoine

-- 
Antoine Ténart, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com