iio: adc: add a driver for the SAR ADC found in Amlogic Meson SoCs

This adds support for the SAR (Successive Approximation Register) ADC
on the Amlogic Meson SoCs.

The code is based on the public S805 (Meson8b) and S905 (GXBB)
datasheets (see [0] and [1]), as well as by reading (various versions
of) the vendor driver and by inspecting the registers on the vendor
kernels of my testing-hardware.

Currently the GXBB, GXL and GXM SoCs are supported. GXBB hardware has
10-bit ADC resolution, while GXL and GXM have 12-bit ADC resolution.
The code was written to support older SoCs (Meson8 and Meson8b) as well,
but due to lack of actual testing-hardware no of_device_id was added for
these.

Two "features" from the vendor driver are currently missing:
- the vendor driver uses channel #7 for calibration (this improves the
  accuracy of the results - in my tests the results were less than 3%
  off without calibration compared to the vendor driver). Adding support
  for this should be easy, but is not required for most applications.
- channel #6 is connected to the SoCs internal temperature sensor.
  Adding support for this is probably not so easy since (based on the
  u-boot sources) most SoC versions are using different registers and
  algorithms for the conversion from "ADC value" to temperature.

Supported by the hardware but currently not supported by the driver:
- reading multiple channels at the same time (the hardware has a FIFO
  buffer which stores multiple results)
- continuous sampling (this would require a way to enable this
  individually because otherwise the ADC would be drawing power
  constantly)
- interrupt support (similar to the vendor driver this new driver is
  polling the results. It is unclear if the IRQ-mode is supported on
  older (Meson6 or Meson8) hardware as well or if there are any errata)

[0]
http://dn.odroid.com/S805/Datasheet/S805_Datasheet%20V0.8%2020150126.pdf
[1] http://dn.odroid.com/S905/DataSheet/S905_Public_Datasheet_V1.1.4.pdfSigned-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/Kconfig

@@ -399,6 +399,18 @@ config MEN_Z188_ADC
 	  This driver can also be built as a module. If so, the module will be
 	  called men_z188_adc.
 
+config MESON_SARADC
+	tristate "Amlogic Meson SAR ADC driver"
+	default ARCH_MESON
+	depends on OF && COMMON_CLK && (ARCH_MESON || COMPILE_TEST)
+	select REGMAP_MMIO
+	help
+	  Say yes here to build support for the SAR ADC found in Amlogic Meson
+	  SoCs.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called meson_saradc.
+
 config MXS_LRADC
         tristate "Freescale i.MX23/i.MX28 LRADC"
         depends on (ARCH_MXS || COMPILE_TEST) && HAS_IOMEM

drivers/iio/adc/Makefile

@@ -38,6 +38,7 @@ obj-$(CONFIG_MCP320X) += mcp320x.o
 obj-$(CONFIG_MCP3422) += mcp3422.o
 obj-$(CONFIG_MEDIATEK_MT6577_AUXADC) += mt6577_auxadc.o
 obj-$(CONFIG_MEN_Z188_ADC) += men_z188_adc.o
+obj-$(CONFIG_MESON_SARADC) += meson_saradc.o
 obj-$(CONFIG_MXS_LRADC) += mxs-lradc.o
 obj-$(CONFIG_NAU7802) += nau7802.o
 obj-$(CONFIG_PALMAS_GPADC) += palmas_gpadc.o