regulator: core: don't fail regulator_register() with missing required supply

Since commit 98e48cd9283d ("regulator: core: resolve supply for
boot-on/always-on regulators"), the regulator core returns
-EPROBE_DEFER if a supply can not be resolved at regulator_register()
time due to set_machine_constraints() requiring that supply (e.g.
because of always-on or boot-on).

In some hardware designs, multiple PMICs are used where individual
rails of each act as supplies for rails of the other, and vice-versa.
In such a design no PMIC driver can probe when registering one top-
level regulator device (as is common practice for almost all regulator
drivers in Linux) since that commit. Supplies are only considered when
their driver has fully bound, but because in a design like the above
two drivers / devices depend on each other, neither will have fully
bound while the other probes. The Google Pixel 6 and 6 Pro (oriole and
raven) are examples of such a design.

One way to make this work would be to register each rail as an
individual device, rather than just one top-level regulator device.
Then, fw-devlink and Linux' driver core could do their usual handling
of deferred device probe as each rail would be probed individually.
This approach was dismissed in [1] as each regulator driver would have
to take care of this itself.

Alternatively, we can change the regulator core to not fail
regulator_register() if a rail's required supply can not be resolved
while keeping the intended change from above mentioned commit, and
instead retry whenever a new rail is registered. This commit implements
such an approach:

    If set_machine_constraints() requests probe deferral,
    regulator_register() still succeeds and we retry setting
    constraints as part of regulator_resolve_supply().
    We still do not enable the regulator or allow consumers to use it
    until constraints have been set (including resolution of the
    supply) to prevent enabling of a regulator before its supply.

With this change, we keep track of regulators with missing required
supplies and can therefore try to resolve them again and try to set
the constraints again once more regulators become available.

Care has to be taken to not allow consumers to use regulators that
haven't had their constraints set yet. regulator_get() ensures that
and now returns -EPROBE_DEFER in that case.

The implementation is straight-forward, thanks to our newly introduced
regulator-bus. Locking in regulator_resolve_supply() has to be done
carefully, as a combination of regulator_(un)lock_two() and
regulator_(un)lock_dependent() is needed. The reason is that
set_machine_constraints() might call regulator_enable() which needs
rdev and all its dependents locked, but everything else requires to
only have rdev and its supply locked.

Link: https://lore.kernel.org/all/aRn_-o-vie_QoDXD@sirena.co.uk/ [1]
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://patch.msgid.link/20260109-regulators-defer-v2-8-1a25dc968e60@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>

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