netdev - Re: [PATCH net-next 0/3] net: stmmac: phylink PCS conversion part 3 (dodgy stuff)

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Message-ID: <aQhusPX0Hw9ZuLNR@oss.qualcomm.com>
Date: Mon, 3 Nov 2025 14:28:24 +0530
From: Mohd Ayaan Anwar <mohd.anwar@....qualcomm.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Alexis Lothoré <alexis.lothore@...tlin.com>,
        Andrew Lunn <andrew+netdev@...n.ch>,
        Boon Khai Ng <boon.khai.ng@...era.com>,
        Daniel Machon <daniel.machon@...rochip.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Furong Xu <0x1207@...il.com>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "Jan Petrous (OSS)" <jan.petrous@....nxp.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-stm32@...md-mailman.stormreply.com,
        Maxime Chevallier <maxime.chevallier@...tlin.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>, netdev@...r.kernel.org,
        Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
        Vladimir Oltean <olteanv@...il.com>,
        Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH net-next 0/3] net: stmmac: phylink PCS conversion part 3
 (dodgy stuff)

On Thu, Oct 30, 2025 at 03:22:12PM +0000, Russell King (Oracle) wrote:
> On Thu, Oct 30, 2025 at 03:19:27PM +0000, Russell King (Oracle) wrote:
> > > 
> > > This is probably fine since Bit(9) is self-clearing and its value just
> > > after this is 0x00041000.
> > 
> > Yes, and bit 9 doesn't need to be set at all. SGMII isn't "negotiation"
> > but the PHY says to the MAC "this is how I'm operating" and the MAC says
> > "okay". Nothing more.
> > 
> > I'm afraid the presence of snps,ps-speed, this disrupts the test.
> 
> Note also that testing a 10M link, 100M, 1G and finally 100M again in
> that order would also be interesting given my question about the RGMII
> register changes that configure_sgmii does.
> 

Despite several attempts, I couldn't get 10M to work. There is a link-up
but the data path is broken. I checked the net-next tip and it's broken
there as well.

Oddly enough, configure_sgmii is called with its speed argument set to
1000:
[   12.305488] qcom-ethqos 23040000.ethernet eth0: phy link up sgmii/10Mbps/Half/pause/off/nolpi
[   12.315233] qcom-ethqos 23040000.ethernet eth0: major config, requested phy/sgmii
[   12.322965] qcom-ethqos 23040000.ethernet eth0: interface sgmii inband modes: pcs=00 phy=03
[   12.331586] qcom-ethqos 23040000.ethernet eth0: major config, active phy/outband/sgmii
[   12.339738] qcom-ethqos 23040000.ethernet eth0: phylink_mac_config: mode=phy/sgmii/pause adv=0000000,00000000,00000000,00000000 pause=00
[   12.355113] qcom-ethqos 23040000.ethernet eth0: ethqos_configure_sgmii : Speed = 1000
[   12.363196] qcom-ethqos 23040000.ethernet eth0: Link is Up - 10Mbps/Half - flow control off

Nevertheless, I manually updated RGMII_CONFIG_SGMII_CLK_DVDR to 0x31 and
did not observe any issues with 100M and 1G (and 10M was still broken).

I tried to dig around for information about the particular register
update and found basically the same thing as Konrad.
1. B(18:10) - RGMII_CONFIG_SGMII_CLK_DVDR - It defines programming value
for Divider 20. This field is used for 10Mbs mode operation in RMII and
set value of 9'd 19.
2. The programming guide for this IOMACRO core mentions that the field
needs to be set to 0x31 for 10M link.

I am inclined to believe that the register description is a typo (as the
reset value of this field is anyways 0x13). The 0x31 value is
recommended for only 10M. For other speeds, it mentions the default
value of 0x13.

However, that does raise the question of why setting the field to 0x31
is not impacting 100M/1G. I will try to investigate more on this. But
right now I am trying to prioritize on verifying 100M/1G/2.5G links as
those should be more common. After that, there's still the issue of IQ8
only advertising support for 2.5G.

	Ayaan

> -- 
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!