iio: magn: ak8975: deploy runtime and system PM

This adds runtime PM support to the AK8975 driver. It solves two
problems:

- After reading the first value the chip was left in MODE_ONCE,
  meaning (presumably) it may be consuming more power. Now the
  runtime PM hooks kick in and set it to POWER_DOWN.

- Regulators were simply enabled and left on, making it
  impossible to turn the power consuming regulators off because
  of the increased refcount. We now disable the regulators at
  autosuspend.

- We also handle system suspend: by using pm_runtime_force_suspend()
  and pm_runtime_force_resume() from the system PM sleep hooks,
  the runtime PM code is managing the power also for this case.
  It is currently not completely optimal: when the system resumes
  the AK8975 goes into active mode even if noone is going to use
  it: currently the force calls need to be paired, but the runtime
  PM people are working on making it possible to leave devices
  runtime suspended when coming back from sleep.

Inspired by my work on the BH1780 light sensor driver.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/magnetometer/ak8975.c

@@ -33,6 +33,7 @@
 #include <linux/of_gpio.h>
 #include <linux/acpi.h>
 #include <linux/regulator/consumer.h>
+#include <linux/pm_runtime.h>
 
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>
@@ -692,6 +693,8 @@ static int ak8975_read_axis(struct iio_dev *indio_dev, int index, int *val)
 	u16 buff;
 	int ret;
 
+	pm_runtime_get_sync(&data->client->dev);
+
 	mutex_lock(&data->lock);
 
 	ret = ak8975_start_read_axis(data, client);
@@ -706,6 +709,9 @@ static int ak8975_read_axis(struct iio_dev *indio_dev, int index, int *val)
 
 	mutex_unlock(&data->lock);
 
+	pm_runtime_mark_last_busy(&data->client->dev);
+	pm_runtime_put_autosuspend(&data->client->dev);
+
 	/* Swap bytes and convert to valid range. */
 	buff = le16_to_cpu(buff);
 	*val = clamp_t(s16, buff, -def->range, def->range);
@@ -975,6 +981,18 @@ static int ak8975_probe(struct i2c_client *client,
 		goto cleanup_buffer;
 	}
 
+	/* Enable runtime PM */
+	pm_runtime_get_noresume(&client->dev);
+	pm_runtime_set_active(&client->dev);
+	pm_runtime_enable(&client->dev);
+	/*
+	 * The device comes online in 500us, so add two orders of magnitude
+	 * of delay before autosuspending: 50 ms.
+	 */
+	pm_runtime_set_autosuspend_delay(&client->dev, 50);
+	pm_runtime_use_autosuspend(&client->dev);
+	pm_runtime_put(&client->dev);
+
 	return 0;
 
 cleanup_buffer:
@@ -989,6 +1007,9 @@ static int ak8975_remove(struct i2c_client *client)
 	struct iio_dev *indio_dev = i2c_get_clientdata(client);
 	struct ak8975_data *data = iio_priv(indio_dev);
 
+	pm_runtime_get_sync(&client->dev);
+	pm_runtime_put_noidle(&client->dev);
+	pm_runtime_disable(&client->dev);
 	iio_device_unregister(indio_dev);
 	iio_triggered_buffer_cleanup(indio_dev);
 	ak8975_set_mode(data, POWER_DOWN);
@@ -997,6 +1018,56 @@ static int ak8975_remove(struct i2c_client *client)
 	return 0;
 }
 
+#ifdef CONFIG_PM
+static int ak8975_runtime_suspend(struct device *dev)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	struct iio_dev *indio_dev = i2c_get_clientdata(client);
+	struct ak8975_data *data = iio_priv(indio_dev);
+	int ret;
+
+	/* Set the device in power down if it wasn't already */
+	ret = ak8975_set_mode(data, POWER_DOWN);
+	if (ret < 0) {
+		dev_err(&client->dev, "Error in setting power-down mode\n");
+		return ret;
+	}
+	/* Next cut the regulators */
+	ak8975_power_off(data);
+
+	return 0;
+}
+
+static int ak8975_runtime_resume(struct device *dev)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	struct iio_dev *indio_dev = i2c_get_clientdata(client);
+	struct ak8975_data *data = iio_priv(indio_dev);
+	int ret;
+
+	/* Take up the regulators */
+	ak8975_power_on(data);
+	/*
+	 * We come up in powered down mode, the reading routines will
+	 * put us in the mode to read values later.
+	 */
+	ret = ak8975_set_mode(data, POWER_DOWN);
+	if (ret < 0) {
+		dev_err(&client->dev, "Error in setting power-down mode\n");
+		return ret;
+	}
+
+	return 0;
+}
+#endif /* CONFIG_PM */
+
+static const struct dev_pm_ops ak8975_dev_pm_ops = {
+	SET_SYSTEM_SLEEP_PM_OPS(pm_runtime_force_suspend,
+				pm_runtime_force_resume)
+	SET_RUNTIME_PM_OPS(ak8975_runtime_suspend,
+			   ak8975_runtime_resume, NULL)
+};
+
 static const struct i2c_device_id ak8975_id[] = {
 	{"ak8975", AK8975},
 	{"ak8963", AK8963},
@@ -1024,6 +1095,7 @@ MODULE_DEVICE_TABLE(of, ak8975_of_match);
 static struct i2c_driver ak8975_driver = {
 	.driver = {
 		.name	= "ak8975",
+		.pm = &ak8975_dev_pm_ops,
 		.of_match_table = of_match_ptr(ak8975_of_match),
 		.acpi_match_table = ACPI_PTR(ak_acpi_match),
 	},