iio: adc: ad4691: add initial driver for AD4691 family

Add support for the Analog Devices AD4691 family of high-speed,
low-power multichannel SAR ADCs: AD4691 (16-ch, 500 kSPS),
AD4692 (16-ch, 1 MSPS), AD4693 (8-ch, 500 kSPS) and
AD4694 (8-ch, 1 MSPS).

The driver implements a custom regmap layer over raw SPI to handle the
device's mixed 1/2/3/4-byte register widths and uses the standard IIO
read_raw/write_raw interface for single-channel reads.

The chip idles in Autonomous Mode so that single-shot read_raw can use
the internal oscillator without disturbing the hardware configuration.

Three voltage supply domains are managed: avdd (required), vio, and a
reference supply on either the REF pin (ref-supply, external buffer)
or the REFIN pin (refin-supply, uses the on-chip reference buffer;
REFBUF_EN is set accordingly). Hardware reset is performed by asserting
then deasserting the reset-gpios GPIO line (tRESETL minimum pulse width
is 10 ns, satisfied by function-call overhead); the driver then waits
300 µs for the chip to complete its internal reset sequence. A software
reset via SPI_CONFIG_A is used as fallback when no reset GPIO is
provided.

Accumulator channel masking for single-shot reads uses ACC_MASK_REG via
an ADDR_DESCENDING SPI write, which covers both mask bytes in a single
16-bit transfer.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Radu Sabau <radu.sabau@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Author: RaduSabau1

MAINTAINERS

@@ -1482,6 +1482,7 @@ L:	linux-iio@vger.kernel.org
 S:	Supported
 W:	https://ez.analog.com/linux-software-drivers
 F:	Documentation/devicetree/bindings/iio/adc/adi,ad4691.yaml
+F:	drivers/iio/adc/ad4691.c
 
 ANALOG DEVICES INC AD4695 DRIVER
 M:	Michael Hennerich <michael.hennerich@analog.com>

drivers/iio/adc/Kconfig

@@ -144,6 +144,18 @@ config AD4170_4
 	  To compile this driver as a module, choose M here: the module will be
 	  called ad4170-4.
 
+config AD4691
+	tristate "Analog Devices AD4691 Family ADC Driver"
+	depends on SPI
+	depends on REGULATOR || COMPILE_TEST
+	select REGMAP
+	help
+	  Say yes here to build support for Analog Devices AD4691 Family MuxSAR
+	  SPI analog to digital converters (ADC).
+
+	  To compile this driver as a module, choose M here: the module will be
+	  called ad4691.
+
 config AD4695
 	tristate "Analog Device AD4695 ADC Driver"
 	depends on SPI

drivers/iio/adc/Makefile

@@ -16,6 +16,7 @@ obj-$(CONFIG_AD4080) += ad4080.o
 obj-$(CONFIG_AD4130) += ad4130.o
 obj-$(CONFIG_AD4134) += ad4134.o
 obj-$(CONFIG_AD4170_4) += ad4170-4.o
+obj-$(CONFIG_AD4691) += ad4691.o
 obj-$(CONFIG_AD4695) += ad4695.o
 obj-$(CONFIG_AD4851) += ad4851.o
 obj-$(CONFIG_AD7091R) += ad7091r-base.o