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

From: "Marek Behún" <kabel@kernel.org>
To: Vladimir Oltean <olteanv@gmail.com>
Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>,
	Andrew Lunn <andrew@lunn.ch>,
	Heiner Kallweit <hkallweit1@gmail.com>,
	Alexandre Belloni <alexandre.belloni@bootlin.com>,
	Alvin __ipraga <alsi@bang-olufsen.dk>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Claudiu Manoil <claudiu.manoil@nxp.com>,
	Daniel Scally <djrscally@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	DENG Qingfang <dqfext@gmail.com>,
	Eric Dumazet <edumazet@google.com>,
	Florian Fainelli <f.fainelli@gmail.com>,
	George McCollister <george.mccollister@gmail.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Hauke Mehrtens <hauke@hauke-m.de>,
	Heikki Krogerus <heikki.krogerus@linux.intel.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Kurt Kanzenbach <kurt@linutronix.de>,
	Landen Chao <Landen.Chao@mediatek.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	linux-acpi@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org,
	Matthias Brugger <matthias.bgg@gmail.com>,
	netdev@vger.kernel.org, Paolo Abeni <pabeni@redhat.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Sakari Ailus <sakari.ailus@linux.intel.com>,
	Sean Wang <sean.wang@mediatek.com>,
	UNGLinuxDriver@microchip.com,
	Vivien Didelot <vivien.didelot@gmail.com>,
	Woojung Huh <woojung.huh@microchip.com>
Subject: Re: [PATCH net-next 3/6] net: dsa: add support for retrieving the interface mode
Date: Fri, 22 Jul 2022 14:59:36 +0200	[thread overview]
Message-ID: <20220722145936.497ac73f@dellmb> (raw)
In-Reply-To: <20220721182216.z4vdaj4zfb6w3emo@skbuf>

On Thu, 21 Jul 2022 21:22:16 +0300
Vladimir Oltean <olteanv@gmail.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

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