iio: adc: ad7173: document sampling frequency behaviour

The ADCs supported by this driver feature a sequencer that read in a
loop all the enabled chanels. When setting the individual sampling
frequency for each channel and enabling multiple channels, the effective
of each channel will be lower than the actual set value. Document this
behaviour in a comment.

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Dumitru Ceclan <dumitru.ceclan@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240607-ad4111-v7-8-97e3855900a0@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: dumitruceclan

drivers/iio/adc/ad7173.c

@@ -732,6 +732,21 @@ static int ad7173_write_raw(struct iio_dev *indio_dev,
 		return ret;
 
 	switch (info) {
+	/*
+	 * This attribute sets the sampling frequency for each channel individually.
+	 * There are no issues for raw or buffered reads of an individual channel.
+	 *
+	 * When multiple channels are enabled in buffered mode, the effective
+	 * sampling rate of a channel is lowered in correlation to the number
+	 * of channels enabled and the sampling rate of the other channels.
+	 *
+	 * Example: 3 channels enabled with rates CH1:6211sps CH2,CH3:10sps
+	 * While the reading of CH1 takes only 0.16ms, the reading of CH2 and CH3
+	 * will take 100ms each.
+	 *
+	 * This will cause the reading of CH1 to be actually done once every
+	 * 200.16ms, an effective rate of 4.99sps.
+	 */
 	case IIO_CHAN_INFO_SAMP_FREQ:
 		freq = val * MILLI + val2 / MILLI;
 		for (i = st->info->odr_start_value; i < st->info->num_sinc5_data_rates - 1; i++)