netdev - Re: [PATCH net-next 3/6] net: dsa: add support for retrieving the interface mode

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Message-ID: <20220722145936.497ac73f@dellmb>
Date:   Fri, 22 Jul 2022 14:59:36 +0200
From:   Marek Behún <kabel@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Alvin __ipraga <alsi@...g-olufsen.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Daniel Scally <djrscally@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        DENG Qingfang <dqfext@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        George McCollister <george.mccollister@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        Matthias Brugger <matthias.bgg@...il.com>,
        netdev@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Sean Wang <sean.wang@...iatek.com>,
        UNGLinuxDriver@...rochip.com,
        Vivien Didelot <vivien.didelot@...il.com>,
        Woojung Huh <woojung.huh@...rochip.com>
Subject: Re: [PATCH net-next 3/6] net: dsa: add support for retrieving the
 interface mode

On Thu, 21 Jul 2022 21:22:16 +0300
Vladimir Oltean <olteanv@...il.com> wrote:

> On Thu, Jul 21, 2022 at 07:21:45PM +0200, Marek Behún wrote:
> > Marvell documentation says that 2500base-x does not implement inband
> > AN.  
> 
> Does Marvell documentation actually call it 2500base-x when it says it
> doesn't support in-band autoneg?

Yes, it does.

> > But when it was first implemented, for some reason it was thought that
> > 2500base-x is just 1000base-x at 2.5x speed, and 1000base-x does
> > support inband AN. Also it worked during tests for both switches and
> > SOC NICs, so it was enabled.
> > 
> > At the time 2500base-x was not standardized. Now 2500base-x is
> > stanradrized, and the standard says that 2500base-x does not support
> > clause 37 AN. I guess this is because where it is used, it is intended
> > to work with clause 73 AN somehow.  
> 
> When you say 2500base-x is standardized, do you mean there is a document
> somewhere which I could use to read more about this?

IEEE Std 802.3cb-2018: Amendment 1: Physical Layer Specifications and
Management Parameters for 2.5 Gb/s and 5 Gb/s Operation over Backplane.

Annex 127A (informative): Compatibility of 2.5GBASE-X PCS/PMA with
1000BASE-X PCS/PMA running 2.5 times faster

  ...
  This annex discusses the restrictions when operating 2.5GBASE-X
  PCS/PMA with a 1000BASE-X PCS/PMA link partner running 2.5 times
  faster. Compatibility of the PMD is outside the scope of this annex.
  In this annex when 1000BASE-X PCS/PMA is referred to, the 2.5 times
  speed up is implied.
  ...
  The 2.5GBASE-X PCS does not support Clause 37 Auto-Negotiation.
  Hence, the 1000BASE-X PCS is expected to have its Clause 37
  Auto-Negotiation functionality disabled so that the /C/ ordered set
  will not be transmitted. If a 2.5GBASE-X PCS receives /C/ ordered
  set, then undefined behavior may occur.
  ...

Marek