Rework how the fractional frequency offset (FFO) is reported in
the DPLL subsystem.

Both fractional-frequency-offset (PPM) and
fractional-frequency-offset-ppt (PPT) attributes are now present at
the top level of a pin and inside each pin-parent-device nest. They
carry the same measurement at different precisions.

The ffo_get callback distinguishes the two contexts: dpll=NULL for
the top-level call (RX vs TX symbol rate offset) and a valid dpll
pointer for the nested per-parent call (pin vs DPLL offset). This
allows drivers to report a different value per parent DPLL if needed.

Patch 1 adds both attributes to the pin-parent-device subset, updates
the DPLL netlink handling to emit both at each level, updates the YAML
spec and driver-api documentation, and adds NULL guards to mlx5 and
zl3073x drivers.

Patch 2 implements the nested FFO for zl3073x using the
dpll_df_offset_x register with ref_ofst=1, providing 2^-48
resolution. The old per-reference frequency measurement is removed
as it was redundant with measured-frequency.

Changes v2 -> v3:
- Keep both FFO attributes (PPM and PPT) at both levels instead of
  moving PPT under pin-parent-device only (Jiri Pirko)
- Unify attribute documentation to describe semantics at each level

Changes v1 -> v2:
- Minor commit message fixes

Ivan Vecera (2):
  dpll: add fractional frequency offset to pin-parent-device
  dpll: zl3073x: report FFO as DPLL vs input reference offset

 Documentation/driver-api/dpll.rst             | 15 +++++++
 Documentation/netlink/specs/dpll.yaml         | 28 +++++++-----
 drivers/dpll/dpll_netlink.c                   | 23 +++++-----
 drivers/dpll/dpll_nl.c                        |  2 +
 drivers/dpll/zl3073x/chan.c                   | 31 ++++++++++++-
 drivers/dpll/zl3073x/chan.h                   | 14 ++++++
 drivers/dpll/zl3073x/core.c                   | 45 -------------------
 drivers/dpll/zl3073x/dpll.c                   | 34 +++++++-------
 drivers/dpll/zl3073x/ref.h                    | 14 ------
 drivers/dpll/zl3073x/regs.h                   | 15 +++++++
 .../net/ethernet/mellanox/mlx5/core/dpll.c    |  4 ++
 11 files changed, 125 insertions(+), 100 deletions(-)

-- 
2.53.0