iio: adc: ad799x: modernize resource management

This series modernizes the ad799x driver by converting resource 
management to the devm_ infrastructure. This results in cleaner 
probe error handling, the removal of the ad799x_remove() function, 
and improved code maintainability.

Key changes in this series include:
- Static allocation of the rx_buf using IIO_DECLARE_BUFFER_WITH_TS()
  to avoid dynamic memory leaks during scan mode updates.
- Caching of VCC and VREF regulator voltages during probe() to 
  optimize read_raw() performance and simplify power management logic.
- Migration of regulators and mutex initialization to device-managed 
  resources (devm).
- Removal of the manual ad799x_remove() function and all goto-based 
  error cleanup labels in ad799x_probe().

These changes ensure the teardown sequence is always correct, 
following the reverse order of allocation, and remove unnecessary 
boilerplate code from the driver.

Changes in v6:
- Addressed style feedback from Andy Shevchenko: used '(MICRO / MILLI)'
  for clearer unit conversion and refactored the optional VREF error
  path for better readability.
- Removed the phrase "Reading the regulator voltage via
  regulator_get_voltage() can be a slow operation" from the commit
  message of patch 3/4 as requested by Jonathan Cameron.
- Confirmed with maintainers that the devm_add_action_or_reset() pattern
  must remain to preserve PM functionality.

Changes in v5:
- Used devm_add_action_or_reset() for regulator disabling instead of
  devm_regulator_get_enable_read_voltage() as suggested by Jonathan 
  Cameron. This was necessary to preserve the regulator pointers (st->reg 
  and st->vref) required by the driver's suspend/resume power management 
  callbacks.
- Simplified voltage caching to use a single 'vref_uV' variable.
- Converted mutex_init() to devm_mutex_init().
- Resolved error path ordering issues by using devm_* exclusively, 
  allowing for the complete removal of the ad799x_remove() function.

Changes in v4:
- Simplified voltage caching to use a single 'vref_uV' variable as
suggested by David Lechner.
- Capitalized 'uV' in variable names as suggested by Andy Shevchenko.
- Removed redundant 'if (ret < 0)' check in read_raw() (dead check).
- Used devm_mutex_init() to replace manual mutex handling.
- Introduced a local 'dev' pointer in probe() to simplify function calls.
- Switched to IIO_DECLARE_BUFFER_WITH_TS() for the buffer declaration.
- Moved goto removals into the respective patches where devm conversion
occurs.

Changes in v3:
- Split the monolithic v2 patch into 4 logical commits as requested by
  Andy Shevchenko.

Changes in v2:
- Eliminated dynamic buffer allocation (feedback from David Lechner).
- Cached voltages to prevent devm/manual ordering hazards (feedback 
  from Jonathan Cameron).

Archit Anant (4):
  iio: adc: ad799x: use local device pointer in probe
  iio: adc: ad799x: use a static buffer for scan data
  iio: adc: ad799x: cache regulator voltages during probe
  iio: adc: ad799x: use devm_iio_device_register and drop remove()

 drivers/iio/adc/ad799x.c | 110 ++++++++++++++++++---------------------
 1 file changed, 51 insertions(+), 59 deletions(-)

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2.39.5