iio: adc: add MAX40080 current-sense amplifier driver

This series adds support for the Maxim MAX40080, a bidirectional
current-sense amplifier with an integrated 12-bit ADC and an I2C/SMBus
interface. It measures the voltage across an external shunt resistor and
the input bus voltage.

The driver operates in direct (INDIO_DIRECT_MODE) mode. Each raw read
triggers a single on-demand conversion (SMBus Quick Command) and reads
back the matched current/voltage pair, so results are always fresh. It
exposes the current and voltage channels with raw and scale attributes,
a configurable oversampling (digital averaging) ratio, and PEC-protected
register access. The two selectable current-sense ranges are exposed
through scale/scale_available; the current scale is derived from the
shunt-resistor-micro-ohms device-tree property.

Tested on hardware with four MAX40080 devices on an I2C bus.

Regarding Andy's question about *iv being updated even on error in
read_poll_timeout: yes, this is intentional. The poll loop must update
*iv on each iteration to check the valid bit. On timeout (conversion
never completes), *iv contains the last-read value with the valid bit
still clear, but the caller always checks the return value first —
"if (ret) return ret;" prevents any use of the stale data.

Changes in v3:
  - Remove mod_devicetable.h include (Andy, Uwe's rework)
  - Use lowercase _mV suffix for unit defines: MAX40080_INTER_VREF_mV,
    MAX40080_CSA_50mV_GAIN, MAX40080_CSA_10mV_GAIN (Andy)
  - Remove MAX40080_NUM_RANGES macro, use ARRAY_SIZE() directly (Andy)
  - Fix reversed xmas tree ordering in get_oversampling_ratio() (Andy)
  - Split semantically different variable declarations in
    set_oversampling_ratio() and max40080_init() (Andy)
  - Join *val, *val2, long mask on same line in read_raw signature (Andy)
  - Use const int * cast in read_avail (Andy)
  - Join FIFO_CFG write onto single line in max40080_init() (Andy)
  - Return i2c_smbus_write_word_data() directly at end of
    max40080_init() (Andy)
  - Use device_property_present() pattern for optional shunt-resistor
    property (Andy)
  - Make vdd-supply required in DT bindings (Conor)
  - Restore i2c_set_clientdata() needed for PM resume (Sashiko)
  - Add bounds check in get_oversampling_ratio() for hardware register
    values outside expected range (Sashiko)

Changes in v2:
  - Add vdd-supply and interrupts properties to the binding (David)
  - Add types.h and time.h includes (Andy)
  - Use USEC_PER_MSEC for poll timeout readability (Andy)
  - Use 1 * MICRO for default shunt resistor (Andy)
  - Rename field macros to include register name, e.g. MAX40080_CFG_MODE_MSK
    (Jonathan, David)
  - Add indexed defines for RANGE field values and use in gain array
    (Jonathan)
  - Use array lookup in get_oversampling_ratio instead of formula (Andy)
  - Use switch statement for chan->type in read_raw (David)
  - Simplify update_bits: one-liner RMW, return write directly (Andy)
  - Add local client variable in trigger_measurement (Andy)
  - Add braces to for loops and use C99 loop variables (Siratul, Andy)
  - Flip if/else in reg_access, separate declaration from assignment (Andy)
  - Add .name= in i2c_device_id (David, Siratul)
  - Reorder declarations to reverse christmas tree (Siratul)
  - Return directly from oversampling case in write_raw (Andy)
  - Add blank lines before return in read_avail (Siratul)
  - Add Co-developed-by tag for Ciprian (Andy)
  - Use Link: tag for datasheet URL in commit message

Stefan Popa (2):
  dt-bindings: iio: adc: add maxim,max40080
  iio: adc: add MAX40080 current-sense amplifier driver

 .../bindings/iio/adc/maxim,max40080.yaml      |  64 ++
 MAINTAINERS                                   |   9 +
 drivers/iio/adc/Kconfig                       |  11 +
 drivers/iio/adc/Makefile                      |   1 +
 drivers/iio/adc/max40080.c                    | 630 ++++++++++++++++++
 5 files changed, 715 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/iio/adc/maxim,max40080.yaml
 create mode 100644 drivers/iio/adc/max40080.c

--
2.53.0