Add driver logic and documentation for the oversampling feature of the
AD469x parts from Analog Devices. For now, this only works with offload
support, and takes advantage of that mode's higher performance to make
oversampling possible on multiple channels with varying sampling
frequencies. Some significant rework of the driver had to be done in
order to conditionally support this feature, including use of
iio_scan_types to help determine the appropriate spi message
configurations depending on oversampling ratio.

This series depends on David's recent SPI engine changes for adding
offload support:

https://lore.kernel.org/all/20241211-dlech-mainline-spi-engine-offload-2-v6-0-88ee574d5d03@baylibre.com/

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
---
Changes in v2:
- Removed section in cover letter about correct implementation
- Simplify math of ad4695_get_calibbias() in ad4695.c, based on
  Jonathan and David's suggestions
- Link to v1: https://lore.kernel.org/r/20241217-ad4695-oversampling-v1-0-0b045d835dac@baylibre.com

---
Trevor Gamblin (2):
      iio: adc: ad4695: add offload-based oversampling support
      doc: iio: ad4695: describe oversampling support

 Documentation/iio/ad4695.rst |  36 ++++-
 drivers/iio/adc/ad4695.c     | 333 +++++++++++++++++++++++++++++++++++++++----
 2 files changed, 338 insertions(+), 31 deletions(-)
---
base-commit: 0c6c3bf84f541fb4ec7097baf9eac10136f98c62
change-id: 20241217-ad4695-oversampling-2946fbe3aff3

Best regards,
-- 
Trevor Gamblin <tgamblin@baylibre.com>

On 1/9/25 12:47 PM, Trevor Gamblin wrote:
> Add driver logic and documentation for the oversampling feature of the
> AD469x parts from Analog Devices. For now, this only works with offload
> support, and takes advantage of that mode's higher performance to make
> oversampling possible on multiple channels with varying sampling
> frequencies. Some significant rework of the driver had to be done in
> order to conditionally support this feature, including use of
> iio_scan_types to help determine the appropriate spi message
> configurations depending on oversampling ratio.
> 
> This series depends on David's recent SPI engine changes for adding
> offload support:
> 
> https://lore.kernel.org/all/20241211-dlech-mainline-spi-engine-offload-2-v6-0-88ee574d5d03@baylibre.com/
> 
> Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
> ---

Tested-by: David Lechner <dlechner@baylibre.com>