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>
---
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.
