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

["Pali Rohár" <pali@kernel.org>]

Hello!

On Friday 04 June 2021 07:35:33 Madalin Bucur wrote:
> > -----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.

SGMII supports 1 Gbps speed and also 100 / 10 Mbps by repeating frame 10
or 100 times.

So... if this HW has 2.5G SGMII (sgmii-2500) as 2.5x overclocked SGMII,
does it mean that 2.5G SGMII supports 25 Mbps and 250 Mbps speeds by
repeating frame 10 and 100 times (like for 1G SGMII)?

> 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.

Yea, I understand. If at that time there was no sw support, "something"
was chosen.

> 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.

1000base-x and SGMII are not same modes. E.g. SGMII support 10 Mbps
while 1000base-x not. So in my opinion 1000base-x and SGMII should not
be treated as the same mode (in SW).

I'm not sure how what exactly SGMII-2500 supports, but as 2500base-x
does not support 25 Mbps speed I do not think that SGMII-2500 is same as
2500base-x.

But now I'm totally confused by all these modes, so I hope that somebody
else tries to explain what kernel expects and how kernel treats these
modes.

> 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