iio: adc: nau7802: Expose possible gains in sysfs

The Nuvoton NAU7802 ADC is able to adjust its gain but prior knowledge of
its possible values was required to adjust it. Users had to guess the
possible gain values based on the ADC datasheet or on this driver's code.

This exposes the possible values in the in_voltage_scale_available file of
each nau7802 ADC device. The gain is set for the whole ADC and is therefore
not configurable by channel. Thus, there exists only one
in_voltage_scale_available file for each nau7802 ADC device even if it has
two separate channels.

Signed-off-by: Quentin Schulz <[e-mail]>
Acked-by: Alexandre Belloni <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/nau7802.c

···
 static const u16 nau7802_sample_freq_avail[] = {10, 20, 40, 80,
 						10, 10, 10, 320};
 
+static ssize_t nau7802_show_scales(struct device *dev,
+				   struct device_attribute *attr, char *buf)
+{
+	struct nau7802_state *st = iio_priv(dev_to_iio_dev(dev));
+	int i, len = 0;
+
+	for (i = 0; i < ARRAY_SIZE(st->scale_avail); i++)
+		len += scnprintf(buf + len, PAGE_SIZE - len, "0.%09d ",
+				 st->scale_avail[i]);
+
+	buf[len-1] = '\n';
+
+	return len;
+}
+
 static IIO_CONST_ATTR_SAMP_FREQ_AVAIL("10 40 80 320");
+
+static IIO_DEVICE_ATTR(in_voltage_scale_available, S_IRUGO, nau7802_show_scales,
+		       NULL, 0);
 
 static struct attribute *nau7802_attributes[] = {
 	&iio_const_attr_sampling_frequency_available.dev_attr.attr,
+	&iio_dev_attr_in_voltage_scale_available.dev_attr.attr,
 	NULL
 };