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

[Markus Breitenberger <bre@breiti.cc>]

Hi Andrew,

Thanks for looking at this, and you're right - the runtime case is the
more dangerous one. If a genuine interface change (SGMII <-> 2500BASE-X)
happened at runtime while the disk was live, reprogramming the shared
ModPHY LCPLL would disturb the SATA PHY under an active filesystem, and
a failed boot would be preferable to that.

Two points of clarification:

- A plain switch change does not reprogram the ModPHY on my fixed-PHY
  setup. mac_finish() only runs a real reconfiguration when the
  MAC-side interface mode changes (e.g. a multi-rate SFP moving between
  SGMII and 2500BASE-X). On a fixed copper PHY the interface mode does
  not change, so changing the link partner / switch does not trigger
  the reconfiguration.

- The runtime reconfiguration path is not introduced by this patch. It
  came in with the Fixes: commit a42f6b3f1cc1 ("net: stmmac: configure
  SerDes according to the interface mode"), which added
  intel_mac_finish()/intel_set_reg_access() and the PMC LCPLL
  reprogramming. v2 will only read the current SerDes rate back from
  SERDES_GCR0 and skip the reconfiguration when it already matches the
  selected interface. At boot that suppresses the redundant reprogram
  that breaks SATA; for a real rate change, v2 leaves the
  reconfiguration unchanged from mainline.

So for the runtime case you are worried about - a real ModPHY rate
change while SATA is live - this patch does not make things safer or
more dangerous; it only removes the spurious boot-time reprogramming.
The broader question of protecting a live SATA disk against a real
runtime ModPHY change is pre-existing, and I don't have a board that
combines a multi-rate SFP with SATA on the same ModPHY, so I can't
exercise or safely test a guard for that topology.

Given that, I'd like to keep this patch scoped to the boot regression
and leave the pre-existing shared-ModPHY-with-live-SATA question to the
maintainers, who have the hardware knowledge to decide whether a
stronger guard is warranted.

Thanks,
Markus