[PATCH v7 0/4] iio: adc: ad4080: add support for AD4880 dual-channel ADC

[Antoniu Miclaus]

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.

Changes in v7:
  - Drop debugfs_reg_access for dual-channel AD4880 variant
  - Pass struct device * to ad4080_properties_parse() instead of
    using regmap_get_device(st->regmap[0])
  - Use 100-column limit consistently for function signatures
  - Add architecture summary to cover letter (per Andy's request)

Antoniu Miclaus (4):
  iio: backend: use __free(fwnode_handle) for automatic cleanup
  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                      | 231 ++++++++++++++----
 drivers/iio/industrialio-backend.c            |  59 +++--
 include/linux/iio/backend.h                   |   1 +
 4 files changed, 273 insertions(+), 71 deletions(-)

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2.43.0