Device Tree channel nodes are associated with the adapters created by
i2c-mux, but equivalent firmware-node descriptions are not. Convert the
existing lookup to generic firmware-node operations so software-node and
ACPI descriptions can use the same property traversal.

This is the remaining I2C prerequisite split out of the Time Card series.
The channel-node leak fix from v7 was applied to i2c-next as commit
385c7af4e3b9 ("i2c: mux: Fix channel node leak on adapter add failure"), so
it is no longer included here. The first patch factors the Device Tree
lookup into a helper without changing behavior. The second converts the
helper and node lifetime handling to generic firmware-node operations.

The existing acpi_preset_companion() call remains in place after the generic
lookup. A separate reference to the node returned by that lookup is retained
because ACPI companion setup may replace the adapter's primary firmware
node. The dependent ptp_ocp board profiles will be resent to net-next after
the LED and I2C prerequisites land.

Changes since v7:
  - Drop the channel-node leak fix now present in i2c-next.
  - Rebase onto the current i2c-next branch.
  - Remove the firmware-node type restriction so ACPI firmware nodes also
    use the generic lookup, as discussed with Peter Rosin and Andy
    Shevchenko.
  - Retain the existing acpi_preset_companion() path.
  - Keep a separate reference to the generic lookup result in case ACPI
    companion setup replaces the adapter's primary firmware node.
  - Return arbitrator and gate nodes directly from the lookup helper, as
    suggested by Peter.
  - Describe the conversion as firmware-node propagation.

Validation:
  - git diff --check
  - strict checkpatch with an 80-column limit
  - x86-64 GCC W=1 object build of drivers/i2c/i2c-mux.o
  - mailbox replay onto the declared base with an identical resulting tree

v7: https://lore.kernel.org/r/cover.1787502619.git.ahmadexp@gmail.com/

Ahmad Byagowi (2):
  i2c: mux: Factor out channel node lookup
  i2c: mux: Propagate firmware nodes to channel adapters

 drivers/i2c/i2c-mux.c | 96 ++++++++++++++++++++++---------------------
 1 file changed, 50 insertions(+), 46 deletions(-)


base-commit: 1f3e66348d2527c31b9bedb0b4d9e28bdad9bc83
-- 
2.50.1 (Apple Git-155)