iio: buffer: align the size of scan bytes to size of the largest element

BugLink: https://bugs.launchpad.net/bugs/1864773

commit 883f6165 upstream.

Previous versions of `iio_compute_scan_bytes` only aligned each element
to its own length (i.e. its own natural alignment). Because multiple
consecutive sets of scan elements are buffered this does not work in
case the computed scan bytes do not align with the natural alignment of
the first scan element in the set.

This commit fixes this by aligning the scan bytes to the natural
alignment of the largest scan element in the set.

Fixes: 959d2952 ("staging:iio: make iio_sw_buffer_preenable much more general.")
Signed-off-by: Lars Möllendorf <lars.moellendorf@plating.de>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/iio/industrialio-buffer.c

@@ -527,7 +527,7 @@ static int iio_compute_scan_bytes(struct iio_dev *indio_dev,
 {
 	const struct iio_chan_spec *ch;
 	unsigned bytes = 0;
-	int length, i;
+	int length, i, largest = 0;
 
 	/* How much space will the demuxed element take? */
 	for_each_set_bit(i, mask,
@@ -540,6 +540,7 @@ static int iio_compute_scan_bytes(struct iio_dev *indio_dev,
 			length = ch->scan_type.storagebits / 8;
 		bytes = ALIGN(bytes, length);
 		bytes += length;
+		largest = max(largest, length);
 	}
 	if (timestamp) {
 		ch = iio_find_channel_from_si(indio_dev,
@@ -551,7 +552,10 @@ static int iio_compute_scan_bytes(struct iio_dev *indio_dev,
 			length = ch->scan_type.storagebits / 8;
 		bytes = ALIGN(bytes, length);
 		bytes += length;
+		largest = max(largest, length);
 	}
+
+	bytes = ALIGN(bytes, largest);
 	return bytes;
 }