linux-kernel - RE: [RFC net-next] net: phy: Add basic support for Synopsys XPCS using a PHY driver

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Date:   Mon, 20 Jan 2020 11:07:23 +0000
From:   Jose Abreu <Jose.Abreu@...opsys.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
CC:     Andrew Lunn <andrew@...n.ch>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC net-next] net: phy: Add basic support for Synopsys XPCS
 using a PHY driver

From: Russell King - ARM Linux admin <linux@...linux.org.uk>
Date: Jan/20/2020, 10:50:20 (UTC+00:00)

> On Mon, Jan 20, 2020 at 10:31:17AM +0000, Jose Abreu wrote:
> > From: Russell King - ARM Linux admin <linux@...linux.org.uk>
> > Date: Jan/13/2020, 14:18:17 (UTC+00:00)
> > 
> > > I've recently suggested a patch to phylink to add a generic helper to
> > > read the state from a generic 802.3 clause 37 PCS, but I guess that
> > > won't be sufficient for an XPCS.  However, it should give some clues
> > > if you're intending to use phylink.
> > 
> > So, I think for my particular setup (that has no "real" PHY) we can have 
> > something like this in SW PoV:
> > 
> > stmmac -> xpcs -> SW-PHY / Fixed PHY
> > 
> > - stmmac + xpcs state would be handled by phylink (MAC side)
> > - SW-PHY / Fixed PHY state would be handled by phylink (PHY side)
> > 
> > This would need updates for Fixed PHY to support >1G speeds.
> 
> You don't want to do that if you have 1G SFPs.  Yes, you *can* do it
> and make it work, but you miss out completely on the fact that the
> link is supposed to be negotiated across the SFP link for 1G speeds,
> and then you're into the realms of having to provide users ways to
> edit the DT and reboot if the parameters at the link partner change.

You may have missed my answer to Andrew so I'll quote it here:

---
[...]

My current setup is this:

Host PC x86 -> PCI -> XGMAC -> XPCS -> SERDES 10G-BASE-R -> QSFP+

The only piece that needs configuration besides XGMAC is the XPCS hereby 

I "called" it a PHY [...]
---

So, besides not having a DT based setup to test changes, I also don't have 
access to SFP bus neither SERDES ... As you suggested, I would like to 
integrate XPCS with PHYLINK in stmmac but I'm not entirely sure on how to 
implement the remaining connections as the connect_phy() callbacks will 
fail because the only MMD device in the bus will be XPCS. That's why I 
suggested the Fixed PHY approach ...

---
Thanks,
Jose Miguel Abreu