Subject: Re: [net-next,05/14] net: stmmac: add stmmac core serdes support

From: "Russell King (Oracle)" <[email-masked]>
Date: Wed, 21 Jan 2026 14:46:42 +0000

On Tue, Jan 20, 2026 at 02:11:14PM +0200, Vladimir Oltean wrote:
> On Tue, Jan 20, 2026 at 10:12:46AM +0000, Russell King (Oracle) wrote:
> > First, I'll say I'm on a very short fuse today; no dinner last night,
> > at the hospital up until 5:30am, and a fucking cold caller rang the door
> > bell at 10am this morning. Just fucking our luck.
> 
> Sorry to hear that.
> 
> > On Tue, Jan 20, 2026 at 10:18:44AM +0200, Vladimir Oltean wrote:
> > > Isn't it sufficient to set pl->pcs to NULL when pcs_enable() fails and
> > > after calling pcs_disable(), though?
> >
> > No. We've already called mac_prepare(), pcs_pre_config(),
> > pcs_post_config() by this time, we're past the point of being able to
> > unwind.
> 
> I'm set out to resolve a much smaller problem.
> 
> Calling it a full "unwind" is perhaps a bit much, because pcs_pre_config()
> and pcs_post_config() don't have unwinding equivalents, unlike how
> pcs_enable() has pcs_disable(). I don't see what API convention would be
> violated if phylink decided to drop a PCS whose enable() returned an error.

While pcs_pre_config() and pcs_post_config() do not have unwinding
equivalents (what would they be?) the issue here is that these could
have changed any state that isn't simply undone by calling
pcs_disable().

For example, pcs_pre_config() could have reprogrammed signal routing,
clocking, or power supplies to blocks.

This already applies to Marvell DSA pcs-639x.c, where the pre/post
config hooks change the power state of the PCS block (for errata
handling), and the only way that gets undone is via a call to
pcs_disable() which explicitly disables IRQs and power for the PCS. Its
pcs_disable() isn't a strict reversal of pcs_enable(), it does more.

We already declare the interface to be dead on pcs_post_config()
failure, but we don't do that for pcs_enable() failure.

Maybe I need to explicitly state that pcs_disable() does not directly
balance pcs_enable(), but that _and_ the effects of pcs_pre_config()
and pcs_post_config(). However, that itself will add to the problems.
What if pcs_pre_config() and pcs_post_config() succeed but not
pcs_enable()? pcs-639x needs pcs_disable() to be called, but if we
require pcs_disable() to be balanced with a successful call to
pcs_enable(), that messes up that driver, and pretty much makes it
impossible to work around the errata.

If you feel strongly about this, then the only thing I can think of
doing is to move this SerDes support out of stmmac and into phylink
(which is a point I already raised in the cover message) so that
its failure can be dealt with at the higher level, where we can
ensure that phy_power_off() is balaced with phy_power_on(). However,
that means pushing even more of the stmmac specific "we need the
clocks running to access registers XYZ or reset" weirdness into
phylink.

-- 
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