Re: [PATCH v3 1/6] tools: iio: iio_generic_buffer ensure alignment

From: Matti Vaittinen
Date: Tue Sep 26 2023 - 06:29:14 EST

On 9/25/23 16:16, Jonathan Cameron wrote:
On Mon, 25 Sep 2023 10:01:09 +0300
Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:

On 9/24/23 18:57, Jonathan Cameron wrote:
On Fri, 22 Sep 2023 14:16:08 +0300
Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:
The iio_generic_buffer can return garbage values when the total size of
scan data is not a multiple of 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 elemnt is first in the buffer.

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

Add padding bytes required to align the next sample into the scan size.

Signed-off-by: Matti Vaittinen <mazziesaccount@xxxxxxxxx>
---
Please note, This one could have RFC in subject.:
I attempted to write the fix so that the alignment is done based on the
biggest channel data. This may be wrong. Maybe a fixed 8 byte alignment
should be used instead? This patch can be dropped from the series if the
fix is not correct / agreed.

tools/iio/iio_generic_buffer.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/tools/iio/iio_generic_buffer.c b/tools/iio/iio_generic_buffer.c
index 44bbf80f0cfd..fc562799a109 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 +69,16 @@ 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. TODO: should we use fixed
+ * 8 byte alignment instead of the size of the biggest samnple?
+ */

Should be aligned to max size seen in the scan.

Or, maybe it should be
min(max_size_in_scan, 8);
?

Definitely not. If you are grabbing just one channel of 8 bit data,
we want it to be tightly packed.

I think that in this case the max_size_in_scan would be 1, and min(1, 8) would be 1 as well, resulting a tightly packed data. I am just wondering if we should use 8 as maximum alignment - eg, if our scan has 16 bytes data + 1 byte data, we would add 7 bytes of padding, not 15 bytes of padding. I am not sure what is the right thing to do.

If we have a bug that already made that true then we might be stuck
with it, but I'm fairly sure we don't.

I think my suggestion above may yield undesirable effects should the
scan elements be greater than 8 bytes. (Don't know if this is supported
though)

It is supported in theory, in practice not seen one yet.

So, whether to unconditionally use largest scan element sized alignment - or largest scan element up to 8 bytes - is a question we haven't hit yet :)

Actually, more I stare at the alignment code here, less sure I am it is correct - but maybe I don't understand how the data should be aligned.

I think it works if allowed data sizes are 1, 2, 4, and 8. However, I suspect it breaks for other sizes.

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 :)

In any case, the patch here should fix things when allowed scan element sizes are 1, 2, 4 and 8 and we have to add padding after last scan element. It won't work for other sizes, but as I wrote, I suspect the whole alignment code here may be broken for other sizes so things shouldn't at least get worse with this patch... I think this should be revised if we see samples of other sizes - and in any case, this might at least warrant a comment here :) (I reserve a right to be wrong. Haven't been sleeping too well lately and my head is humming...)

+ misalignment = bytes % max;
+ if (misalignment) {
+ printf("Misalignment %u. Adding Padding %u\n", misalignment, max - misalignment);

No print statement as this is correct behaviour (well the tool is buggy but the kernel generates it
correctly I believe). Fine to add a comment though!

Oh, indeed. The print was forgotten from my test runs. Thanks for
pointing it out!

+ bytes += max - misalignment;
+ }
return bytes;
}

Yours,
-- Matti

--
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~