netdev - Clause 73 and USXGMII

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Date:   Wed, 7 Aug 2019 15:46:22 +0000
From:   Jose Abreu <Jose.Abreu@...opsys.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Clause 73 and USXGMII

Hello,

I've some sample code for Clause 73 support using Synopsys based XPCS 
but I would like to clarify some things that I noticed.

I'm using USXGMII as interface and a single SERDES that operates at 10G 
rate but MAC side is working at 2.5G. Maximum available bandwidth is 
therefore 2.5Gbps.

So, I configure USXGMII for 2.5G mode and it works but if I try to limit 
the autoneg abilities to 2.5G max then it never finishes:
# ethtool enp4s0
Settings for enp4s0:
	Supported ports: [ ]
	Supported link modes:   1000baseKX/Full 
	                        2500baseX/Full 
	Supported pause frame use: Symmetric Receive-only
	Supports auto-negotiation: Yes
	Supported FEC modes: Not reported
	Advertised link modes:  1000baseKX/Full 
	                        2500baseX/Full 
	Advertised pause frame use: Symmetric Receive-only
	Advertised auto-negotiation: Yes
	Advertised FEC modes: Not reported
	Speed: Unknown!
	Duplex: Unknown! (255)
	Port: MII
	PHYAD: 0
	Transceiver: internal
	Auto-negotiation: on
	Supports Wake-on: ug
	Wake-on: d
	Current message level: 0x0000003f (63)
			       drv probe link timer ifdown ifup
	Link detected: no

When I do not limit autoneg and I say that maximum limit is 10G then I 
get Link Up and autoneg finishes with this outcome:
# ethtool enp4s0
Settings for enp4s0:
	Supported ports: [ ]
	Supported link modes:   1000baseKX/Full 
	                        2500baseX/Full 
	                        10000baseKX4/Full 
	                        10000baseKR/Full 
	Supported pause frame use: Symmetric Receive-only
	Supports auto-negotiation: Yes
	Supported FEC modes: Not reported
	Advertised link modes:  1000baseKX/Full 
	                        2500baseX/Full 
	                        10000baseKX4/Full 
	                        10000baseKR/Full 
	Advertised pause frame use: Symmetric Receive-only
	Advertised auto-negotiation: Yes
	Advertised FEC modes: Not reported
	Link partner advertised link modes:  1000baseKX/Full 
	                                     2500baseX/Full 
	                                     10000baseKX4/Full 
	                                     10000baseKR/Full 
	Link partner advertised pause frame use: Symmetric Receive-only
	Link partner advertised auto-negotiation: Yes
	Link partner advertised FEC modes: Not reported
	Speed: 2500Mb/s
	Duplex: Full
	Port: MII <- Never mind this, it's a SW issue
	PHYAD: 0
	Transceiver: internal
	Auto-negotiation: on
	Supports Wake-on: ug
	Wake-on: d
	Current message level: 0x0000003f (63)
			       drv probe link timer ifdown ifup
	Link detected: yes

I was expecting that, as MAC side is limited to 2.5G, I should set in 
phylink the correct capabilities and then outcome of autoneg would only 
have up to 2.5G modes. Am I wrong ?

---
Thanks,
Jose Miguel Abreu