linux-kernel - Re: [PATCH] net: ethernet: mvneta: add support for 2.5G DRSGMII mode

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Date:   Tue, 9 Jun 2020 14:11:31 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     netdev@...r.kernel.org, devicetree@...r.kernel.org,
        kernel@...gutronix.de, linux-kernel@...r.kernel.org,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] net: ethernet: mvneta: add support for 2.5G DRSGMII mode

On Mon, Jun 08, 2020 at 05:08:01PM +0100, Russell King - ARM Linux admin wrote:
> On Mon, Jun 08, 2020 at 09:47:16AM +0200, Sascha Hauer wrote:
> > The Marvell MVNETA Ethernet controller supports a 2.5 Gbps SGMII mode
> > called DRSGMII.
> > 
> > This patch adds a corresponding phy-mode string 'drsgmii' and parses it
> > from DT. The MVNETA then configures the SERDES protocol value
> > accordingly.
> > 
> > It was successfully tested on a MV78460 connected to a FPGA.
> 
> Digging around, this is Armada XP?  Which SoCs is this mode supported?
> There's no mention of DRSGMII in the A38x nor A37xx documentation which
> are later than Armada XP.

It's an Armada XP MV78460 in my case. I have no idea what other SoCs
this mode is supported on.

> 
> What exactly is "drsgmii"?  It can't be "double-rate" SGMII because that
> would give you 2Gbps max instead of the 1Gbps, but this gives 2.5Gbps,
> so I'm really not sure using "drsgmii" is a good idea.  It may be what
> Marvell call it, but we really need to know if there's some vendor
> neutral way to refer to it.

The abbreviation really is for "Double Rated SGMII". It seems it has 2.5
times the clock rate than ordinary SGMII. Another term I found is HSGMII
(High serial gigabit media-independent interface) which also has
2.5Gbps.

Anyway, I just learned from the paragraph you added to
Documentation/networking/phy.rst that 1000BASEX differs from SGMII in
the format of the control word. As we have a fixed link to a FPGA the
control word seems to be unused, at least the Port MAC Control Register0
PortType setting bit doesn't change anything. So I can equally well use
the existing 2500BASEX mode.

Sascha

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