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.
> + 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!
> + bytes += max - misalignment;
> + }
>
> return bytes;
> }
