On Wed, 27 Sep 2023 11:26:07 +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 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.
Very much not! We need to present same data alignment to userspace
indpendent of what architecture is running.
It's annoyingly inconsistent how 8 byte elements are handled on 32 bit
architectures as some have optimized aligned access routines and others
will read as 2 32 bit fields. Hence we just stick to 8 byte value is
8 byte aligned which is always fine but wastes a bit of space on x86 32
bit - which I don't care about ;)
Please drop this last bit of the comment as we should just say what it
does, not conjecture what it might do!
> + */
> + misalignment = bytes % max;
> + if (misalignment)
> + bytes += max - misalignment;
>
> return bytes;
> }
