Re: [PATCH net-next v5 01/11] net: stmmac: move XPCS lifetime management to platform drivers

Thank you for your contribution! Sashiko AI review found 3 potential
issue(s) to consider:

Critical: 0 · High: 0 · Medium: 2 · Low: 1

- [Medium] Commit message mischaracterizes the removal of the generic
  XPCS creation as removing dead code.
- [Medium] DT binding vs implementation mismatch: after this patch
  nothing in the stmmac core reads the generic 'pcs-handle' property,…
- [Low] The commit message describes two defects, one framed as "more
  critically" (the common code overwriting priv->hw->xpcs after a…

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net: stmmac: move XPCS lifetime management to platform drivers

The generic XPCS creation in stmmac_pcs_setup() (pcs-handle lookup and
mdio_bus_data->pcs_mask based xpcs_create_mdiodev()) is removed, leaving
only the plat->pcs_init() / plat->pcs_exit() dispatch. The Intel mGbE glue
grows its own pcs_init()/pcs_exit() that create and destroy the XPCS, and
the renesas-gbeth, rzn1 and socfpga pcs_exit() callbacks now clear
priv->hw->phylink_pcs after destroying the PCS.

A few questions on the commit message and on the removal of the generic
path.

    First, if a device tree specifies a "pcs-handle" but no select_pcs()
    callback is provided by the platform driver, the created XPCS is never
    used. The phylink framework requires select_pcs() to actually return
    the PCS to the core, so the pcs-handle property becomes effectively
    useless without the matching callback.

[Severity: Medium]
Is "the created XPCS is never used" accurate?  An XPCS stored in
priv->hw->xpcs has two consumers that do not go through select_pcs().

stmmac_phylink_setup() in stmmac_main.c uses it to widen the phylink
interface sets:

	/* If we have an xpcs, it defines which PHY interfaces are supported. */
	if (priv->hw->xpcs)
		pcs = xpcs_to_phylink_pcs(priv->hw->xpcs);
	else
		pcs = priv->hw->phylink_pcs;

	if (pcs)
		phy_interface_or(config->supported_interfaces, ...

and stmmac_init_phy() uses it to decide whether to attach a PHY at all:

	if (priv->hw->xpcs &&
	    xpcs_get_an_mode(priv->hw->xpcs, mode) == DW_AN_C73)
		return 0;

So for a dwmac instance with a pcs-handle but no plat->pcs_init, does this
change narrow config->supported_interfaces (and config->lpi_interfaces) and
alter the PHY-attach decision, rather than just dropping something unused?
Would it be worth spelling that behavioural change out in the commit
message?

    Second, and more critically, when a platform driver sets pcs_init()
    and creates an XPCS inside that callback, the common code afterwards
    still runs unconditionally and overwrites priv->hw->xpcs with the
    local xpcs variable, which stays NULL. The platform driver has no way
    to prevent this override because the common code runs after the
    platform-specific initialization.

[Severity: Low]
This isn't a bug, but could this paragraph say that no in-tree platform is
affected today?

All three pre-existing pcs_init() implementations store their PCS in
priv->hw->phylink_pcs, not priv->hw->xpcs:

  renesas_gmac_pcs_init()  -> priv->hw->phylink_pcs = miic_create(...)
  rzn1_dwmac_pcs_init()    -> priv->hw->phylink_pcs = miic_create(...)
  socfpga_dwmac_pcs_init() -> priv->hw->phylink_pcs = pcs

so the old "priv->hw->xpcs = xpcs;" on the pcs_init branch overwrote NULL
with NULL.  intel_mgbe_pcs_init() added here is the first in-tree pcs_init()
that stores an XPCS, which makes this a refactor that removes a latent trap
rather than a fix for observable breakage.

[ ... ]

[Severity: Medium]
After this change, does anything in the stmmac core still read the generic
"pcs-handle" property?

The property is still documented generically in
Documentation/devicetree/bindings/net/ethernet-controller.yaml:

  pcs-handle:
    $ref: /schemas/types.yaml#/definitions/phandle-array
    items:
      maxItems: 1
    description:
      Specifies a reference to a node representing a PCS PHY device on a MDIO
      bus to link with an external PHY (phy-handle) if exists.

and snps,dwmac.yaml $refs ethernet-controller.yaml and ends with
"additionalProperties: true", so pcs-handle stays schema-valid on every
dwmac node regardless of which glue is bound.

On a glue that does not set plat->pcs_init, stmmac_pcs_setup() now returns 0
without looking at the property, priv->hw->xpcs stays NULL, and nothing is
logged - whereas the removed code would at least fail probe with
"No xPCS found" for a bad phandle.  Should the binding be updated to say
pcs-handle is only honoured by specific glue drivers, or should the core
keep a generic path (or emit a warning) so the two agree?

The same question applies to struct stmmac_mdio_bus_data.pcs_mask.
stmmac_mdio_register() still treats it as "a PCS lives at these MDIO
addresses" and masks them out of the bus scan:

	new_bus->phy_mask = mdio_bus_data->phy_mask | mdio_bus_data->pcs_mask;

but no core code creates a PCS for those addresses any more, only
dwmac-intel's private intel_mgbe_pcs_init().  Does a glue that sets
pcs_mask without pcs_init now end up with that address neither scanned as a
PHY nor instantiated as a PCS?

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