[PATCH v4 1/5] tools: iio: iio_generic_buffer ensure alignment

[Matti Vaittinen <mazziesaccount@gmail.com>]

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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>

---
I think the whole alignment code could be revised here, but I am unsure
what kind of alignment is expected, and if it actually depends on the
architecture. Anyways, I'll quote myself from another mail to explain
how this patch handles things:

> For non power of2 sizes, the alignment code will result strange alignments.
> For example, scan consisting of two 6-byte elements would be packed -
> meaning the second element would probably break the alignment rules by
> starting from address '6'. I think that on most architectures the proper
> access would require 2 padding bytes to be added at the end of the first
> sample. Current code wouldn't do that.

> If we allow only power of 2 sizes - I would expect a scan consisting of a
> 8 byte element followed by a 16 byte element to be tightly packed. I'd
> assume that for the 16 byte data, it'd be enough to ensure 8 byte alignment.
> Current code would however add 8 bytes of padding at the end of the first
> 8 byte element to make the 16 byte scan element to be aligned at 16 byte
> address. To my uneducated mind this is not needed - but maybe I just don't
> know what I am writing about :)

Revision history
v3 => v4:
 - drop extra print and TODO coment
 - add comment clarifying alignment sizes
---
 tools/iio/iio_generic_buffer.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/tools/iio/iio_generic_buffer.c b/tools/iio/iio_generic_buffer.c
index 44bbf80f0cfd..c07c49397b19 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 +69,19 @@ static unsigned int size_from_channelarray(struct iio_channel_info *channels, in
 		bytes = channels[i].location + channels[i].bytes;
 		i++;
 	}
+	/*
+	 * We wan't the data in next sample to also be properly aligned so
+	 * we'll add padding at the end if needed.
+	 *
+	 * Please note, this code does ensure alignment to maximum channel
+	 * size. It works only as long as the channel sizes are 1, 2, 4 or 8
+	 * bytes. Also, on 32 bit platforms it might be enough to align also
+	 * the 8 byte elements to 4 byte boundary - which this code is not
+	 * doing.
+	 */
+	misalignment = bytes % max;
+	if (misalignment)
+		bytes += max - misalignment;
 
 	return bytes;
 }
-- 
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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