Re: [PATCH net v3] net: stmmac: intel: skip SerDes reconfig when rate is unchanged

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi Markus,

On 7/13/26 19:16, 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 it at
> the end of every major link reconfiguration, including the
> initial one during probe.
> 
> The callback selects the PMC ModPHY LCPLL programming for the
> requested MAC-side interface and then power-cycles the SerDes.
> On Elkhart Lake that ModPHY is also used by the on-die AHCI
> SATA PHY. Reapplying the programming 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.
> 
> Before mac_finish() runs, the legacy SerDes power-up path has
> already programmed SERDES_GCR0 for the current interface. The
> 1G and 2.5G ModPHY tables selected by mac_finish() correspond
> to the SerDes lane rate, so read that rate back from SERDES_GCR0
> and skip the PMC reprogramming and SerDes power-cycle when it
> already matches the selected interface.
> 
> This keeps the disruptive reprogramming out of the boot path
> when the SerDes is configured correctly, while preserving the
> previous behavior when a real SGMII/1000BASE-X to 2500BASE-X
> rate change is needed. If the register read fails, reconfigure
> as before.
> 
> Fixes: a42f6b3f1cc1 ("net: stmmac: configure SerDes according to the interface mode")
> Cc: stable@vger.kernel.org
> Assisted-by: GitHub-Copilot:claude-opus-4.8
> Signed-off-by: Markus Breitenberger <bre@keba.com>

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

Maxime