iio: adc: add ADC driver for the TI TSC2046 controller

Basically the TI TSC2046 touchscreen controller is 8 channel ADC optimized for
the touchscreen use case. By implementing it as an IIO ADC device, we can
make use of resistive-adc-touch and iio-hwmon drivers.

Polled readings are currently not implemented to keep this patch small, so
iio-hwmon will not work out of the box for now.

So far, this driver was tested with a custom version of resistive-adc-touch driver,
since it needs to be extended to make use of Z1 and Z2 channels. The X/Y
are working without additional changes.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20210428073208.19570-4-o.rempel@pengutronix.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

MAINTAINERS

@@ -18332,6 +18332,14 @@ S:	Supported
 F:	Documentation/devicetree/bindings/net/nfc/trf7970a.txt
 F:	drivers/nfc/trf7970a.c
 
+TI TSC2046 ADC DRIVER
+M:	Oleksij Rempel <o.rempel@pengutronix.de>
+R:	kernel@pengutronix.de
+L:	linux-iio@vger.kernel.org
+S:	Maintained
+F:	Documentation/devicetree/bindings/iio/adc/ti,tsc2046.yaml
+F:	drivers/iio/adc/ti-tsc2046.c
+
 TI TWL4030 SERIES SOC CODEC DRIVER
 M:	Peter Ujfalusi <peter.ujfalusi@gmail.com>
 L:	alsa-devel@alsa-project.org (moderated for non-subscribers)

drivers/iio/adc/Kconfig

@@ -1190,6 +1190,18 @@ config TI_TLC4541
 	  This driver can also be built as a module. If so, the module will be
 	  called ti-tlc4541.
 
+config TI_TSC2046
+	tristate "Texas Instruments TSC2046 ADC driver"
+	depends on SPI
+	select IIO_BUFFER
+	select IIO_TRIGGERED_BUFFER
+	help
+	  Say yes here to build support for ADC functionality of Texas
+	  Instruments TSC2046 touch screen controller.
+
+	  This driver can also be built as a module. If so, the module will be
+	  called ti-tsc2046.
+
 config TWL4030_MADC
 	tristate "TWL4030 MADC (Monitoring A/D Converter)"
 	depends on TWL4030_CORE

drivers/iio/adc/Makefile

@@ -106,6 +106,7 @@ obj-$(CONFIG_TI_ADS124S08) += ti-ads124s08.o
 obj-$(CONFIG_TI_ADS131E08) += ti-ads131e08.o
 obj-$(CONFIG_TI_AM335X_ADC) += ti_am335x_adc.o
 obj-$(CONFIG_TI_TLC4541) += ti-tlc4541.o
+obj-$(CONFIG_TI_TSC2046) += ti-tsc2046.o
 obj-$(CONFIG_TWL4030_MADC) += twl4030-madc.o
 obj-$(CONFIG_TWL6030_GPADC) += twl6030-gpadc.o
 obj-$(CONFIG_VF610_ADC) += vf610_adc.o