Re: [PATCH net] net: stmmac: intel: don't reconfigure SerDes on unchanged mode

[Andrew Lunn <andrew@lunn.ch>]

On Mon, Jul 06, 2026 at 08:19:54AM +0200, Markus Breitenberger wrote:
> From: Markus Breitenberger <bre@keba.com>
> 
> intel_mac_finish() is registered as the phylink mac_finish() callback
> for the Elkhart Lake SGMII ports. phylink calls mac_finish() at the end
> of every major link reconfiguration, including the initial one during
> probe, before any interface mode has actually changed.
> 
> The callback reprograms the shared ModPHY LCPLL through the PMC IPC and
> then power-cycles the SerDes. On Elkhart Lake that ModPHY is also used
> by the on-die AHCI SATA PHY. Running the reconfiguration during the
> initial boot-time link-up disturbs the shared analog block while it is
> still driving SATA, so the SATA link fails to train:
> 
>   ata1: SATA link down (SStatus 1 SControl 300)
> 
> The disk carrying the root filesystem is never detected and the system
> hangs at rootwait. Ethernet itself comes up normally, which makes the
> failure look unrelated to the network driver.
> 
> Firmware already programs the ModPHY for the configured interface, so
> the reconfiguration is redundant unless the interface mode really
> changes. Return early when the requested mode equals the current one.
> This avoids touching the shared ModPHY (and the SATA PHY) during boot
> while preserving runtime SGMII to 2500BASE-X switching, which still
> sees a genuine mode change and reconfigures as before.

What happens to the disk at runtime, rather than boot time, if it is
necessary to reconfigure the ModPHY?

I think i would prefer the machine fails to boot, rather than corrupt
its disk when i plug it into a different network switch.

	Andrew