netdev - Re: SFP support with RGMII MAC via RGMII to SERDES/SGMII PHY?

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Date:   Sat, 14 Sep 2019 09:48:56 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     George McCollister <george.mccollister@...il.com>,
        netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: SFP support with RGMII MAC via RGMII to SERDES/SGMII PHY?

On Fri, Sep 13, 2019 at 08:31:18PM -0700, Florian Fainelli wrote:
> +Russell, Andrew, Heiner,
> 
> On 9/13/2019 9:44 AM, George McCollister wrote:
> > Every example of phylink SFP support I've seen is using an Ethernet
> > MAC with native SGMII.
> > Can phylink facilitate support of Fiber and Copper SFP modules
> > connected to an RGMII MAC if all of the following are true?
> 
> I don't think that use case has been presented before, but phylink
> sounds like the tool that should help solve it. From your description
> below, it sounds like all the pieces are there to support it. Is the
> Ethernet MAC driver upstream?

It has been presented, and it's something I've been trying to support
for the last couple of years - in fact, I have patches in my tree that
support a very similar scenario on the Macchiatobin with the 88x3310
PHYs.

> > 1) The MAC is connected via RGMII to a transceiver/PHY (such as
> > Marvell 88E1512) which then connects to the SFP via SERDER/SGMII. If
> > you want to see a block diagram it's the first one here:
> > https://www.marvell.com/transceivers/assets/Alaska_88E1512-001_product_brief.pdf

As mentioned above, this is no different from the Macchiatobin,
where we have:

                  .-------- RJ45
MAC ---- 88x3310 PHY
                  `-------- SFP+

except instead of the MAC to PHY link being 10GBASE-R, it's RGMII,
and the PHY to SFP+ link is 10GBASE-R instead of 1000BASE-X.

Note that you're abusing the term "SGMII".  SGMII is a Cisco
modification of the IEEE 802.3 1000BASE-X protocol.  Fiber SFPs
exclusively use 1000BASE-X protocol.  However, some copper SFPs
(with a RJ45) do use SGMII.

> > 2) The 1G Ethernet driver has been converted to use phylink.

This is not necessary for this scenario.  The PHY driver needs to
be updated to know about SFP though.

See:

http://git.armlinux.org.uk/cgit/linux-arm.git/commit/?h=phy&id=ece56785ee0e9df40dc823fdc39ee74b4a7cd1c4

as an example of the 88x3310 supporting a SFP+ cage.  This patch is
also necessary:

http://git.armlinux.org.uk/cgit/linux-arm.git/commit/?h=phy&id=ef2d699397ca28c7f89e01cc9e5037989096a990

and if anything is going to stand in the way of progress on this, it
is likely to be that patch.  I'll be attempting to post these after
the next merge window (i.o.w. probably posting them in three weeks
time.)

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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