[PATCH 4/4] iio: envelope-detector: ADC driver based on a DAC and a comparator

[Peter Rosin]

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@xxxxxxxxxx>
---
 MAINTAINERS                         |   1 +
 drivers/iio/adc/Kconfig             |  10 ++
 drivers/iio/adc/Makefile            |   1 +
 drivers/iio/adc/envelope-detector.c | 305 ++++++++++++++++++++++++++++++++++++
 4 files changed, 317 insertions(+)
 create mode 100644 drivers/iio/adc/envelope-detector.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 4b6f6ec1b703..d9a58b5f2b6f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -6123,6 +6123,7 @@ M:	Peter Rosin <peda@xxxxxxxxxx>
 L:	linux-iio@xxxxxxxxxxxxxxx
 S:	Maintained
 F:	Documentation/devicetree/bindings/iio/adc/envelope-detector.txt
+F:	drivers/iio/adc/envelope-detector.c
 
 IIO SUBSYSTEM AND DRIVERS
 M:	Jonathan Cameron <jic23@xxxxxxxxxx>
diff --git a/drivers/iio/adc/Kconfig b/drivers/iio/adc/Kconfig
index 7edcf3238620..d5c4a95855c2 100644
--- a/drivers/iio/adc/Kconfig
+++ b/drivers/iio/adc/Kconfig
@@ -195,6 +195,16 @@ 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 iio-envelope-detector.
+
 config EXYNOS_ADC
 	tristate "Exynos ADC driver support"
 	depends on ARCH_EXYNOS || ARCH_S3C24XX || ARCH_S3C64XX || (OF && COMPILE_TEST)
diff --git a/drivers/iio/adc/Makefile b/drivers/iio/adc/Makefile
index 7a40c04c311f..0d773c6a0578 100644
--- a/drivers/iio/adc/Makefile
+++ b/drivers/iio/adc/Makefile
@@ -20,6 +20,7 @@ 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
diff --git a/drivers/iio/adc/envelope-detector.c b/drivers/iio/adc/envelope-detector.c
new file mode 100644
index 000000000000..837646107beb
--- /dev/null
+++ b/drivers/iio/adc/envelope-detector.c
@@ -0,0 +1,305 @@
+/*
+ * Driver for an envelope detector using a DAC and a comparator
+ *
+ * Copyright (C) 2016 Axentia Technologies AB
+ *
+ * Author: Peter Rosin <peda@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+/*
+ * 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|------<-------'
+ *            |       |
+ *            '-------'
+ */
+
+#include <linux/completion.h>
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/iio/consumer.h>
+#include <linux/iio/iio.h>
+#include <linux/interrupt.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/spinlock.h>
+#include <linux/workqueue.h>
+
+struct envelope {
+	struct iio_channel *dac;
+	struct delayed_work comp_timeout;
+	int comp_irq;
+
+	spinlock_t comp_lock; /* protects comp */
+	int comp;
+
+	struct mutex read_lock; /* protects everything below */
+
+	u32 dac_max;
+	u32 comp_interval;
+	bool invert;
+
+	int high;
+	int level;
+	int low;
+
+	struct completion done;
+};
+
+static int envelope_detector_latch(struct envelope *env)
+{
+	int comp;
+
+	spin_lock_irq(&env->comp_lock);
+	comp = env->comp;
+	env->comp = 0;
+	spin_unlock_irq(&env->comp_lock);
+
+	if (comp)
+		enable_irq(env->comp_irq);
+
+	return comp;
+}
+
+static irqreturn_t envelope_detector_isr(int irq, void *ctx)
+{
+	struct envelope *env = ctx;
+
+	spin_lock(&env->comp_lock);
+	env->comp = 1;
+	disable_irq_nosync(env->comp_irq);
+	spin_unlock(&env->comp_lock);
+
+	return IRQ_HANDLED;
+}
+
+static void envelope_detector_setup_compare(struct envelope *env)
+{
+	int ret;
+
+	/*
+	 * Do a binary search for the peak input level, and stop
+	 * when that level is "trapped" between two adjacent DAC
+	 * values.
+	 * When invert is active, use the midpoint floor so that
+	 * env->level ends up as env->low when the termination
+	 * criteria below is fulfilled, and use the midpoint
+	 * ceiling when invert is not active so that env->level
+	 * ends up as env->high in that case.
+	 */
+	env->level = (env->high + env->low + !env->invert) / 2;
+
+	if (env->high == env->low + 1) {
+		complete(&env->done);
+		return;
+	}
+
+	/* Set a "safe" DAC level (if there is such a thing)... */
+	ret = iio_write_channel_raw(env->dac, env->invert ? 0 : env->dac_max);
+	if (ret < 0)
+		goto err;
+
+	/* ...clear the comparison result... */
+	envelope_detector_latch(env);
+
+	/* ...set the real DAC level... */
+	ret = iio_write_channel_raw(env->dac, env->level);
+	if (ret < 0)
+		goto err;
+
+	/* ...and wait for a bit to see if the latch catches anything. */
+	schedule_delayed_work(&env->comp_timeout,
+			      msecs_to_jiffies(env->comp_interval));
+	return;
+
+err:
+	env->level = ret;
+	complete(&env->done);
+}
+
+static void envelope_detector_timeout(struct work_struct *work)
+{
+	struct envelope *env = container_of(work, struct envelope,
+					    comp_timeout.work);
+
+	/* Adjust low/high depending on the latch content... */
+	if (!envelope_detector_latch(env) ^ !env->invert)
+		env->low = env->level;
+	else
+		env->high = env->level;
+
+	/* ...and continue the search. */
+	envelope_detector_setup_compare(env);
+}
+
+static int envelope_detector_read_raw(struct iio_dev *indio_dev,
+				      struct iio_chan_spec const *chan,
+				      int *val, int *val2, long mask)
+{
+	struct envelope *env = iio_priv(indio_dev);
+	int ret;
+
+	switch (mask) {
+	case IIO_CHAN_INFO_RAW:
+		/*
+		 * When invert is active, start with high=max+1 and low=0
+		 * since we will end up with the low value when the
+		 * termination criteria is fulfilled (rounding down). And
+		 * start with high=max and low=-1 when invert is not active
+		 * since we will end up with the high value in that case.
+		 * This ensures that we in both cases return a value in the
+		 * same range as the DAC and that as not triggered the
+		 * comparator.
+		 */
+		mutex_lock(&env->read_lock);
+		env->high = env->dac_max + 1 - !env->invert;
+		env->low = 0 - !env->invert;
+		envelope_detector_setup_compare(env);
+		wait_for_completion(&env->done);
+		if (env->level < 0) {
+			ret = env->level;
+			goto err_unlock;
+		}
+		*val = env->invert ? env->dac_max - env->level : env->level;
+		mutex_unlock(&env->read_lock);
+
+		return IIO_VAL_INT;
+
+	case IIO_CHAN_INFO_SCALE:
+		return iio_read_channel_scale(env->dac, val, val2);
+	}
+
+	return -EINVAL;
+
+err_unlock:
+	mutex_unlock(&env->read_lock);
+	return ret;
+}
+
+static const struct iio_chan_spec envelope_detector_iio_channel = {
+	.type = IIO_ALTVOLTAGE,
+	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW)
+			    | BIT(IIO_CHAN_INFO_SCALE),
+	.output = 1,
+};
+
+static const struct iio_info envelope_detector_info = {
+	.read_raw = &envelope_detector_read_raw,
+	.driver_module = THIS_MODULE,
+};
+
+static int envelope_detector_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct iio_dev *indio_dev;
+	struct envelope *env;
+	enum iio_chan_type type;
+	int ret;
+
+	indio_dev = devm_iio_device_alloc(dev, sizeof(*env));
+	if (!indio_dev)
+		return -ENOMEM;
+
+	platform_set_drvdata(pdev, indio_dev);
+	env = iio_priv(indio_dev);
+
+	spin_lock_init(&env->comp_lock);
+	mutex_init(&env->read_lock);
+	init_completion(&env->done);
+	INIT_DELAYED_WORK(&env->comp_timeout, envelope_detector_timeout);
+
+	indio_dev->name = dev_name(dev);
+	indio_dev->dev.parent = dev;
+	indio_dev->dev.of_node = dev->of_node;
+	indio_dev->info = &envelope_detector_info;
+	indio_dev->channels = &envelope_detector_iio_channel;
+	indio_dev->num_channels = 1;
+
+	env->dac = devm_iio_channel_get(dev, "dac");
+	if (IS_ERR(env->dac)) {
+		if (PTR_ERR(env->dac) != -EPROBE_DEFER)
+			dev_err(dev, "failed to get dac input channel\n");
+		return PTR_ERR(env->dac);
+	}
+
+	env->comp_irq = platform_get_irq_byname(pdev, "comp");
+	if (env->comp_irq < 0) {
+		if (env->comp_irq != -EPROBE_DEFER)
+			dev_err(dev, "failed to get compare interrupt\n");
+		return env->comp_irq;
+	}
+
+	ret = devm_request_irq(dev, env->comp_irq, envelope_detector_isr, 0,
+			       "env-env-dac-comp", env);
+	if (ret) {
+		if (ret != -EPROBE_DEFER)
+			dev_err(dev, "failed to request interrupt\n");
+		return ret;
+	}
+
+	ret = iio_get_channel_type(env->dac, &type);
+	if (ret < 0)
+		return ret;
+
+	if (type != IIO_VOLTAGE) {
+		dev_err(dev, "dac is of the wrong type\n");
+		return -EINVAL;
+	}
+
+	ret = of_property_read_u32(dev->of_node, "envelope-detector,dac-max",
+				   &env->dac_max);
+	if (ret) {
+		dev_err(dev, "the dac-max property is missing\n");
+		return ret;
+	}
+
+	ret = of_property_read_u32(dev->of_node,
+				   "envelope-detector,comp-interval-ms",
+				   &env->comp_interval);
+	if (ret) {
+		dev_err(dev, "the comp-interval-ms property is missing\n");
+		return ret;
+	}
+
+	env->invert = of_property_read_bool(dev->of_node,
+					    "envelope-detector,inverted");
+
+	return devm_iio_device_register(dev, indio_dev);
+}
+
+static const struct of_device_id envelope_detector_match[] = {
+	{ .compatible = "envelope-detector" },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, envelope_detector_match);
+
+static struct platform_driver envelope_detector_driver = {
+	.probe = envelope_detector_probe,
+	.driver = {
+		.name = "iio-envelope-detector",
+		.of_match_table = envelope_detector_match,
+	},
+};
+module_platform_driver(envelope_detector_driver);
+
+MODULE_DESCRIPTION("Envelope detector using a DAC and a comparator");
+MODULE_AUTHOR("Peter Rosin <peda@xxxxxxxxxx>");
+MODULE_LICENSE("GPL v2");
-- 
2.1.4