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

From: "Russell King (Oracle)" <[email-masked]>
Date: Wed, 22 Jan 2025 18:55:17 +0000

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.

-- 
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