Date:   Mon, 16 Sep 2019 10:40:24 -0500
From:   George McCollister <george.mccollister@...il.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Florian Fainelli <f.fainelli@...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 Sat, Sep 14, 2019 at 3:49 AM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> 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.

Excellent, this is exactly the information I was looking for. I had
started converting the Ethernet driver to phylink but there was still
a lot of work to do and I fear there was a significant potential for
regressions. I'll abandon that and see if I can get it to work by
making similar changes to the 1G Marvell PHY driver.

I'm assuming I must set the sfp property of the PHY in DT instead of the MAC.

>
> 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

Perfect.

>
> 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.)

Please CC me on these patches as I'll follow your lead for what I do
on the 1G Marvell PHY driver. If you don't remember, that's fine, I
understand.

>
> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
> According to speedtest.net: 11.9Mbps down 500kbps up

Thanks,
George

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