Question/proposal for DPLL NCO (DCO) mode

[Ivan Vecera <ivecera@redhat.com>]

Hi all,

I am currently working on adding PTP clock support (PHC) to the ZL3073x
driver, and I would like to discuss an architectural addition to the
DPLL subsystem enum dpll_mode before sending the formal patch series.

To support IEEE 1588 PTP, the hardware DPLL must be decoupled from
tracking physical reference pins. Instead, its output frequency needs to
be continuously steered by a software PTP stack (e.g., linuxptp) using
the .adjfine() callback. In the industry, this specific hardware state
is widely known as NCO (Numerically Controlled Oscillator) or DCO
(Digitally Controlled Oscillator) mode.

Currently, the DPLL subsystem defines two modes in enum dpll_mode:

MANUAL: The user explicitly selects a physical input pin to track.

AUTOMATIC: The hardware automatically selects the best physical input 
pin based on priorities.

Neither of these accurately represents the NCO/DCO state, where physical
pins are ignored, and the loop is fully software-steered.

Without a dedicated mode, drivers are forced into "automagic"
workarounds. A driver might implicitly switch the hardware to NCO mode
in the background as soon as .adjfine() is called, while misreporting
its mode as MANUAL to the DPLL subsystem.

I strongly believe this hidden "automagic" behavior is a bad design. The
user-space should have full visibility and explicit control over what
the DPLL state machine is actually doing.

The Proposal:
I would like to propose adding a new mode: DPLL_MODE_NCO (or
DPLL_MODE_DCO, I am open to suggestions regarding the terminology).

The expected workflow would be explicit:
If a user wants to actively steer the DPLL frequency via software (for
PTP or other Time-over-Packet applications), they must explicitly set
the DPLL device to this mode via Netlink first. Only then will the
hardware accept frequency adjustments via .adjfine().

I would appreciate any feedback from the DPLL subsystem maintainers and
the community on this approach.

Thanks,
Ivan