linux-kernel - Re: [PATCH net-next RFC v2 1/6] net: ethtool: common: Make BaseT a 4-lanes mode

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Message-ID: <20250123114702.2c69f49f@fedora.home>
Date: Thu, 23 Jan 2025 11:47:02 +0100
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: davem@...emloft.net, netdev@...r.kernel.org,
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Subject: Re: [PATCH net-next RFC v2 1/6] net: ethtool: common: Make BaseT a
 4-lanes mode

Hello Russell,

On Wed, 22 Jan 2025 18:55:17 +0000
"Russell King (Oracle)" <linux@...linux.org.uk> wrote:

> On Wed, Jan 22, 2025 at 06:42:46PM +0100, Maxime Chevallier wrote:
> > When referring to BaseT ethernet, we are most of the time thinking of
> > BaseT4 ethernet on Cat5/6/7 cables. This is therefore BaseT4, although
> > BaseT4 is also possible for 100BaseTX. This is even more true now that
> > we have a special __LINK_MODE_LANES_T1 mode especially for Single Pair
> > ethernet.
> > 
> > Mark BaseT as being a 4-lanes mode.  
> 
> This is a problem:
> 
> 1.4.50 10BASE-T: IEEE 802.3 Physical Layer specification for a 10 Mb/s
> CSMA/CD local area network over two pairs of twisted-pair telephone
> wire. (See IEEE Std 802.3, Clause 14.)
> 
> Then we have the 100BASE-T* family, which can be T1, T2, T4 or TX.
> T1 is over a single balanced twisted pair. T2 is over two pairs of
> Cat 3 or better. T4 is over four pairs of Cat3/4/5.
> 
> The common 100BASE-T* type is TX, which is over two pairs of Cat5.
> This is sadly what the ethtool 100baseT link modes are used to refer
> to.
> 
> We do have a separate link mode for 100baseT1, but not 100baseT4.
> 
> So, these ethtool modes that are of the form baseT so far are
> describing generally two pairs, one pair in each direction. (T1 is
> a single pair that is bidirectional.)
> 
> It's only once we get to 1000BASE-T (1000baseT) that we get to an
> ethtool link mode that has four lanes in a bidirectional fashion.
> 
> So, simply redefining this ends up changing 10baseT and 100baseT from
> a single lane in each direction to four lanes (and is a "lane" here
> defined as the total number of pairs used for communication in both
> directions, or the total number of lanes used in either direction.
> 
> Hence, I'm not sure this makes sense.
> 

I'm fine with your justification, so let's simplify and drop that
patch then. That should also avoid the lanes/pairs confusion as well.

Thanks for the feedback !

Maxime