[PATCH v6 0/7] Add support for AD4000 series of ADCs

[Marcelo Schmitt <marcelo.schmitt@xxxxxxxxxx>]

This patch series extends the SPI bitbang, gpio, and spi-engine controllers to
support configurable MOSI line idle states.
It then introduces the ad4000 driver which uses the MOSI idle configuration to
provide improved support for the AD4000 series of ADCs.
Documentation is added describing the new extension to the SPI protocol.
The currently supported wiring modes for AD4000 devices were documented under
IIO documentation directory.

To apply this series, it requires the patches for SPI-Engine SPI_CS_HIGH feature
and the patches for devm_spi_optimize_message() helper.

89c2657429c4822a2697077bbb3a8d126d826ced "spi: axi-spi-engine: remove platform_set_drvdata()"
https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git/commit/?h=for-6.11&id=89c2657429c4822a2697077bbb3a8d126d826ced

7e74a45c7afdd8a9f82d14fd79ae0383bbaaed1e "spi: add EXPORT_SYMBOL_GPL(devm_spi_optimize_message)"
https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git/commit/?h=for-6.11&id=7e74a45c7afdd8a9f82d14fd79ae0383bbaaed1e

d4a0055fdc22381fa256e345095e88d134e354c5 "spi: add devm_spi_optimize_message() helper"
https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git/commit/?h=for-6.11&id=d4a0055fdc22381fa256e345095e88d134e354c5

6ecdb0aa4dca62d236a659426e11e6cf302e8f18 "spi: axi-spi-engine: Add SPI_CS_HIGH support"
https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git/commit/?h=for-6.11&id=6ecdb0aa4dca62d236a659426e11e6cf302e8f18

Change log v5 -> v6:
[SPI]
spi.h: Removed unused SPI_CONTROLLER_MOSI_IDLE_LOW and SPI_CONTROLLER_MOSI_IDLE_HIGH
spi-summay: Minor nit: inactive -> not asserted
spi-engine: Moved MOSI idle support check to IP core version check section
[IIO]
ad4000: Fixed ad4000_read_reg(). *val = st->tx_buf[1]; -> *val = st->rx_buf[1];
ad4000: Use devm_regulator_bulk_get_enable()
ad4000: Use iio_device_claim_direct_scoped() and guard() to protect scale update
ad4000: Fail probe if ad4000_config() fail
ad4000: Moved ad4000_prepare_..._message() closer to probe to reduce scope
ad4000: Added AD4000_SDI_GND and switch case for more accurate error msg
ad4000: Removed unused st->turbo_mode
ad4000: Removed old misleading comments in enum ad4000_sdi
ad4000: A few minor readability and code style nits

Link to v5: https://lore.kernel.org/linux-iio/cover.1719351923.git.marcelo.schmitt@xxxxxxxxxx/
Link to v4: https://lore.kernel.org/linux-iio/cover.1718749981.git.marcelo.schmitt@xxxxxxxxxx/
Link to v3: https://lore.kernel.org/linux-iio/cover.1717539384.git.marcelo.schmitt@xxxxxxxxxx/
Link to v2: https://lore.kernel.org/linux-iio/cover.1712585500.git.marcelo.schmitt@xxxxxxxxxx/
Link to v1: https://lore.kernel.org/linux-iio/cover.1711131830.git.marcelo.schmitt@xxxxxxxxxx/

Regard using spi_w8r8(), I tried it again and it doesn't work for ad4000.
Looks like the smallest transfer size for these devices is 16-bit.