[PATCH v6] tools: iio: iio_generic_buffer ensure alignment

[Matti Vaittinen]

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@xxxxxxxxx>
Fixes: e58537ccce73 ("staging: iio: update example application.")

---
Revision history
v5 => v6:
 - fix typo pointed by Andy (wan't => want).
v4 => v5:
 - drop unnecessary comment.
 - drop all other patches as those were already applied.
 - add Fixes-tag.
v3 => v4:
 - drop extra print and TODO coment
 - add comment clarifying alignment sizes
---
 tools/iio/iio_generic_buffer.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tools/iio/iio_generic_buffer.c b/tools/iio/iio_generic_buffer.c
index 44bbf80f0cfd..0d0a7a19d6f9 100644
--- a/tools/iio/iio_generic_buffer.c
+++ b/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;
 }

base-commit: 0bb80ecc33a8fb5a682236443c1e740d5c917d1d
-- 
2.41.0


-- 
Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

~~~ "I don't think so," said Rene Descartes. Just then he vanished ~~~
Simon says - in Latin please.
~~~ "non cogito me" dixit Rene Descarte, deinde evanescavit ~~~
Thanks to Simon Glass for the translation =]

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