iio: envelope-detector: ADC driver based on a DAC and a comparator

The DAC is used to find the peak level of an alternating voltage input
signal by a binary search using the output of a comparator wired to
an interrupt pin. Like so:
                          _
                         | \
    input +------>-------|+ \
                         |   \
           .-------.     |    }---.
           |       |     |   /    |
           |    dac|-->--|- /     |
           |       |     |_/      |
           |       |              |
           |       |              |
           |    irq|------<-------'
           |       |
           '-------'
Signed-off-by: Peter Rosin <peda@axentia.se>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio-adc-envelope-detector

+What:		/sys/bus/iio/devices/iio:deviceX/in_altvoltageY_invert
+Date:		October 2016
+KernelVersion:	4.9
+Contact:	Peter Rosin <peda@axentia.se>
+Description:
+		The DAC is used to find the peak level of an alternating
+		voltage input signal by a binary search using the output
+		of a comparator wired to an interrupt pin. Like so:
+		                           _
+		                          | \
+		     input +------>-------|+ \
+		                          |   \
+		            .-------.     |    }---.
+		            |       |     |   /    |
+		            |    dac|-->--|- /     |
+		            |       |     |_/      |
+		            |       |              |
+		            |       |              |
+		            |    irq|------<-------'
+		            |       |
+		            '-------'
+		The boolean invert attribute (0/1) should be set when the
+		input signal is centered around the maximum value of the
+		dac instead of zero. The envelope detector will search
+		from below in this case and will also invert the result.
+		The edge/level of the interrupt is also switched to its
+		opposite value.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_altvoltageY_compare_interval
+Date:		October 2016
+KernelVersion:	4.9
+Contact:	Peter Rosin <peda@axentia.se>
+Description:
+		Number of milliseconds to wait for the comparator in each
+		step of the binary search for the input peak level. Needs
+		to relate to the frequency of the input signal.

MAINTAINERS

@@ -6131,7 +6131,9 @@ IIO ENVELOPE DETECTOR
 M:	Peter Rosin <peda@axentia.se>
 L:	linux-iio@vger.kernel.org
 S:	Maintained
+F:	Documentation/ABI/testing/sysfs-bus-iio-adc-envelope-detector
 F:	Documentation/devicetree/bindings/iio/adc/envelope-detector.txt
+F:	drivers/iio/adc/envelope-detector.c
 
 IIO SUBSYSTEM AND DRIVERS
 M:	Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/Kconfig

@@ -207,6 +207,16 @@ config DA9150_GPADC
 	  To compile this driver as a module, choose M here: the module will be
 	  called berlin2-adc.
 
+config ENVELOPE_DETECTOR
+	tristate "Envelope detector using a DAC and a comparator"
+	depends on OF
+	help
+	  Say yes here to build support for an envelope detector using a DAC
+	  and a comparator.
+
+	  To compile this driver as a module, choose M here: the module will be
+	  called envelope-detector.
+
 config EXYNOS_ADC
 	tristate "Exynos ADC driver support"
 	depends on ARCH_EXYNOS || ARCH_S3C24XX || ARCH_S3C64XX || (OF && COMPILE_TEST)

drivers/iio/adc/Makefile

@@ -21,6 +21,7 @@ obj-$(CONFIG_BCM_IPROC_ADC) += bcm_iproc_adc.o
 obj-$(CONFIG_BERLIN2_ADC) += berlin2-adc.o
 obj-$(CONFIG_CC10001_ADC) += cc10001_adc.o
 obj-$(CONFIG_DA9150_GPADC) += da9150-gpadc.o
+obj-$(CONFIG_ENVELOPE_DETECTOR) += envelope-detector.o
 obj-$(CONFIG_EXYNOS_ADC) += exynos_adc.o
 obj-$(CONFIG_FSL_MX25_ADC) += fsl-imx25-gcq.o
 obj-$(CONFIG_HI8435) += hi8435.o