tools: iio: iio_generic_buffer ensure alignment

The iio_generic_buffer can return garbage values when the total size of
scan data is not a multiple of the largest element in the scan. This can be
demonstrated by reading a scan, consisting, for example of one 4-byte and
one 2-byte element, where the 4-byte element is first in the buffer.

The IIO generic buffer code does not take into account the last two
padding bytes that are needed to ensure that the 4-byte data for next
scan is correctly aligned.

Add the padding bytes required to align the next sample with the scan size.
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Fixes: e58537cc ("staging: iio: update example application.")
Link: https://lore.kernel.org/r/ZRvlm4ktNLu+qmlf@dc78bmyyyyyyyyyyyyydt-3.rev.dnainternet.fiSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

tools/iio/iio_generic_buffer.c

@@ -54,9 +54,12 @@ enum autochan {
 static unsigned int size_from_channelarray(struct iio_channel_info *channels, int num_channels)
 {
 	unsigned int bytes = 0;
-	int i = 0;
+	int i = 0, max = 0;
+	unsigned int misalignment;
 
 	while (i < num_channels) {
+		if (channels[i].bytes > max)
+			max = channels[i].bytes;
 		if (bytes % channels[i].bytes == 0)
 			channels[i].location = bytes;
 		else
@@ -66,6 +69,14 @@ static unsigned int size_from_channelarray(struct iio_channel_info *channels, in
 		bytes = channels[i].location + channels[i].bytes;
 		i++;
 	}
+	/*
+	 * We want the data in next sample to also be properly aligned so
+	 * we'll add padding at the end if needed. Adding padding only
+	 * works for channel data which size is 2^n bytes.
+	 */
+	misalignment = bytes % max;
+	if (misalignment)
+		bytes += max - misalignment;
 
 	return bytes;
 }