iio: frequency: adf4371: Add support for ADF4371 PLL

The ADF4371 is a frequency synthesizer with an integrated voltage
controlled oscillator (VCO) for phase-locked loops (PLLs). The ADF4371
has an integrated VCO with a fundamental output frequency ranging from
4000 MHz to 8000 MHz. In addition, the VCO frequency is connected to
divide by 1, 2, 4, 8, 16, 32, or 64 circuits that allows the user to
generate radio frequency (RF) output frequencies as low as 62.5 MHz at
RF8x. A frequency multiplier at RF16x generates from 8 GHz to 16 GHz. A
frequency quadrupler generates frequencies from 16 GHz to 32 GHz at RF32x.
RFAUX8x duplicates the frequency range of RF8x or permits direct access to
the VCO output.

The driver takes the reference input frequency from the device tree and
uses it to calculate and maximize the PFD frequency (frequency of the phase
frequency detector). The PFD frequency is further used to calculate the
timeouts: synthesizer lock, VCO band selection, automatic level
calibration (ALC) and PLL settling time.

This initial driver exposes the attributes for setting the frequency and
enabling/disabling the different adf4371 channels.

Datasheet:
Link: https://www.analog.com/media/en/technical-documentation/data-sheets/adf4371.pdf

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio-frequency-adf4371

@@ -0,0 +1,44 @@
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_frequency
+KernelVersion:
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Stores the PLL frequency in Hz for channel Y.
+		Reading returns the actual frequency in Hz.
+		The ADF4371 has an integrated VCO with fundamendal output
+		frequency ranging from 4000000000 Hz 8000000000 Hz.
+
+		out_altvoltage0_frequency:
+			A divide by 1, 2, 4, 8, 16, 32 or circuit generates
+			frequencies from 62500000 Hz to 8000000000 Hz.
+		out_altvoltage1_frequency:
+			This channel duplicates the channel 0 frequency
+		out_altvoltage2_frequency:
+			A frequency doubler generates frequencies from
+			8000000000 Hz to 16000000000 Hz.
+		out_altvoltage3_frequency:
+			A frequency quadrupler generates frequencies from
+			16000000000 Hz to 32000000000 Hz.
+
+		Note: writes to one of the channels will affect the frequency of
+		all the other channels, since it involves changing the VCO
+		fundamental output frequency.
+
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_name
+KernelVersion:
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Reading returns the datasheet name for channel Y:
+
+		out_altvoltage0_name: RF8x
+		out_altvoltage1_name: RFAUX8x
+		out_altvoltage2_name: RF16x
+		out_altvoltage3_name: RF32x
+
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_powerdown
+KernelVersion:
+Contact:	linux-iio@vger.kernel.org
+Description:
+		This attribute allows the user to power down the PLL and it's
+		RFOut buffers.
+		Writing 1 causes the specified channel to power down.
+		Clearing returns to normal operation.

drivers/iio/frequency/Kconfig

@@ -38,5 +38,15 @@ config ADF4350
 	  To compile this driver as a module, choose M here: the
 	  module will be called adf4350.
 
+config ADF4371
+	tristate "Analog Devices ADF4371 Wideband Synthesizer"
+	depends on SPI
+	select REGMAP_SPI
+	help
+	  Say yes here to build support for Analog Devices  ADF4371
+	  Wideband Synthesizer. The driver provides direct access via sysfs.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called adf4371.
 endmenu
 endmenu

drivers/iio/frequency/Makefile

@@ -5,3 +5,4 @@
 # When adding new entries keep the list in alphabetical order
 obj-$(CONFIG_AD9523) += ad9523.o
 obj-$(CONFIG_ADF4350) += adf4350.o
+obj-$(CONFIG_ADF4371) += adf4371.o