[EXT] Re: [PATCH v1 2/5] net: phy: marvell: extend 88E2110 to use both 2.5GHz modes

[Marek Behun]

On Wed, 24 Mar 2021 09:55:04 +0000
Kostya Porotchkin <kostap at marvell.com> wrote:

> Hi, Marek,
> 
> > -----Original Message-----
> > From: Marek Behun <marek.behun at nic.cz>
> > Sent: Wednesday, March 24, 2021 12:44
> > To: Stefan Roese <sr at denx.de>
> > Cc: u-boot at lists.denx.de; Kostya Porotchkin <kostap at marvell.com>; Stefan
> > Chulski <stefanc at marvell.com>; Ramon Fried <rfried.dev at gmail.com>;
> > Nadav Haklai <nadavh at marvell.com>; Joe Hershberger
> > <joe.hershberger at ni.com>; Marcin Wojtas <mw at semihalf.com>; sa_ip-sw-
> > jenkins <sa_ip-sw-jenkins at marvell.com>; Igal Liberman <igall at marvell.com>;
> > Simon Glass <sjg at chromium.org>; Yan Markman <ymarkman at marvell.com>
> > Subject: [EXT] Re: [PATCH v1 2/5] net: phy: marvell: extend 88E2110 to use
> > both 2.5GHz modes
> > 
> > External Email
> > 
> > ----------------------------------------------------------------------
> > On Wed, 24 Mar 2021 10:20:05 +0100
> > Stefan Roese <sr at denx.de> wrote:
> >   
> > > PHY_INTERFACE_MODE_SGMII_2500  
> > 
> > What the hell is this mode???
> > 
> > AFAIK something like this does not actually exist.  
> [KP] I think you are wrong. These modes are definitely exist
> https://en.wikipedia.org/wiki/2.5GBASE-T_and_5GBASE-T
> 
> Regards
> Kosta

Hi Kosta,

the wikipedia page you linked specifies copper modes, not PHY modes.

We are discussing PHY modes here.

What I am confused about is that this patch adds check for
  PHY_INTERFACE_MODE_SGMII_2500 
in addition to
  PHY_INTERFACE_MODE_2500BASEX

But what is the difference between these two?

Marvell named this protocol HS-SGMII in some of their datasheets
and code. I guess this was done because of the similarities with
1000base-x and SGMII. Marvell uses the names SGMII and 1000base-x
interchangably, although this is not correct. I guess they are
similarily using names 2500base-x and HS-SGMII (and now SGMII_2500)
interchangably, which is also not correct.

SGMII uses the same coding as 1000base-x, but the latter works only
with one speed (1000mbps), while the former can also work in 10mbps and
100mbps (by repeating each byte 100 or 10 times, respectively).

Then there is 2500base-x, which is the same as 1000base-x, but with the
clock being at 2.5x the speed of 1000base-x clock.

But there is no analogue of the SGMII protocol (i.e. the repearing of
bytes in order to achieve lower speed) for the 2500base-x.

So what I am confused about here is what is supposed to be the
difference between
  PHY_INTERFACE_MODE_SGMII_2500 
and
  PHY_INTERFACE_MODE_2500BASEX
?

Marek