RE: Unsupported phy-connection-type sgmii-2500 in arch/powerpc/boot/dts/fsl/t1023rdb.dts

[Madalin Bucur <madalin.bucur@nxp.com>]

> -----Original Message-----
> From: Pali Rohár <pali@kernel.org>
> Sent: 03 June 2021 22:49
> To: Andrew Lunn <andrew@lunn.ch>
> Cc: Igal Liberman <Igal.Liberman@freescale.com>; Shruti Kanetkar
> <Shruti@freescale.com>; Emil Medve <Emilian.Medve@freescale.com>; Scott
> Wood <oss@buserror.net>; Rob Herring <robh+dt@kernel.org>; Michael
> Ellerman <mpe@ellerman.id.au>; Benjamin Herrenschmidt
> <benh@kernel.crashing.org>; Madalin Bucur <madalin.bucur@nxp.com>; Russell
> King <rmk+kernel@armlinux.org.uk>; netdev@vger.kernel.org;
> devicetree@vger.kernel.org; linux-kernel@vger.kernel.org
> Subject: Re: Unsupported phy-connection-type sgmii-2500 in
> arch/powerpc/boot/dts/fsl/t1023rdb.dts
> 
> On Thursday 03 June 2021 17:12:31 Andrew Lunn wrote:
> > On Thu, Jun 03, 2021 at 04:34:53PM +0200, Pali Rohár wrote:
> > > Hello!
> > >
> > > In commit 84e0f1c13806 ("powerpc/mpc85xx: Add MDIO bus muxing support
> to
> > > the board device tree(s)") was added following DT property into DT
> node:
> > > arch/powerpc/boot/dts/fsl/t1023rdb.dts fm1mac3: ethernet@e4000
> > >
> > >     phy-connection-type = "sgmii-2500";
> > >
> > > But currently kernel does not recognize this "sgmii-2500" phy mode.
> See
> > > file include/linux/phy.h. In my opinion it should be "2500base-x" as
> > > this is mode which operates at 2.5 Gbps.
> > >
> > > I do not think that sgmii-2500 mode exist at all (correct me if I'm
> > > wrong).
> >
> > Kind of exist, unofficially. Some vendors run SGMII over clocked at
> > 2500. But there is no standard for it, and it is unclear how inband
> > signalling should work. Whenever i see code saying 2.5G SGMII, i
> > always ask, are you sure, is it really 2500BaseX? Mostly it gets
> > changed to 2500BaseX after review.
> 
> So this is question for authors of that commit 84e0f1c13806. But it
> looks like I cannot send them emails because of following error:
> 
> <Minghuan.Lian@freescale.com>: connect to freescale.com[192.88.156.33]:25:
> Connection timed out
> 
> Do you have other way how to contact maintainers of that DTS file?
> arch/powerpc/boot/dts/fsl/t1023rdb.dts
> 
> > PHY mode sgmii-2500 does not exist in mainline.
> 
> Yes, this is reason why I sent this email. In DTS is specified this mode
> which does not exist.
> 
> > 	Andrew

Hi, the Freescale emails no longer work, years after Freescale joined NXP.
Also, the first four recipients no longer work for NXP.

In regards to the sgmii-2500 you see in the device tree, it describes SGMII
overclocked to 2.5Gbps, with autonegotiation disabled. 

A quote from a long time ago, from someone from the HW team on this:

	The industry consensus is that 2.5G SGMII is overclocked 1G SGMII
	using XAUI electricals. For the PCS and MAC layers, it looks exactly
	like 1G SGMII, just with a faster clock.

The statement that it does not exist is not accurate, it exists in HW, and
it is described as such in the device tree. Whether or not it is properly
treated in SW it's another discussion. In 2015, when this was submitted,
there were no other 2.5G compatibles in use, if I'm not mistaken.
2500Base-X started to be added to device trees four years later, it should
be compatible/interworking but it is less specific on the actual implementation
details (denotes 2.5G speed, 8b/10b coding, which is true for this overclocked
SGMII). If they are compatible, SW should probably treat them in the same manner.

There were some discussions a while ago about the mix or even confusion between
the actual HW description (that's what the dts is supposed to do) and the settings
one wants to represent in SW (i.e. speed) denoted loosely by denominations like
10G Base-R. 

Regards,
Madalin