Add support for the AD4880, a dual-channel 20-bit 40MSPS SAR ADC with
integrated fully differential amplifiers (FDA).

Architecture notes:

The AD4880 is modeled as a single IIO device rather than two independent
devices because the channels share power supplies, a voltage reference,
the CNV conversion clock, and a single interleaved data output stream.
Splitting them into separate IIO devices would make synchronized
dual-channel capture impossible from userspace.

An MFD approach does not apply here either - the channels are not
functionally distinct sub-devices but identical ADC paths sharing a
common data interface.

Each channel has fully independent configuration registers accessible
through separate SPI chip selects, so per-channel regmaps are used with
no locking between them. The data path has no software involvement at
runtime: the CNV clock triggers simultaneous conversions and the device
outputs an interleaved bitstream captured directly by the IIO backend
(FPGA). spi_new_ancillary_device() handles the configuration path;
the IIO backend handles the data path.

The debugfs_reg_access callback is not exposed for the dual-channel
variant since the IIO framework provides a single (reg, val) interface
with no channel parameter, and exposing only one channel would be
misleading.

The AD4880 is a fairly unique part - having separate SPI config
interfaces per channel with a shared interleaved data output is not
a common pattern.

NOTE: The AD4880 driver has a cross-tree dependency on two SPI patches
that are queued in spi/for-7.1:

- ffef4123043c ("spi: allow ancillary devices to share parent's chip selects")
- 463279e58811 ("spi: add devm_spi_new_ancillary_device()")

Changes in v9:
  - Rebase on jic23/togreg
  - Add Conor's ack on dt-bindings patch

Antoniu Miclaus (3):
  iio: backend: add devm_iio_backend_get_by_index()
  dt-bindings: iio: adc: ad4080: add AD4880 support
  iio: adc: ad4080: add support for AD4880 dual-channel ADC

 .../bindings/iio/adc/adi,ad4080.yaml          |  53 +++-
 drivers/iio/adc/ad4080.c                      | 257 +++++++++++++-----
 drivers/iio/industrialio-backend.c            |  53 +++-
 include/linux/iio/backend.h                   |   1 +
 4 files changed, 285 insertions(+), 79 deletions(-)


base-commit: d2a4ec19d2a2e54c23b5180e939994d3da4a6b91
-- 
2.43.0