Re: [PATCH v6] tools: iio: iio_generic_buffer ensure alignment

[Jonathan Cameron]

On Tue, 3 Oct 2023 12:57:47 +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>
> Fixes: e58537ccce73 ("staging: iio: update example application.")

Applied to the togreg branch of iio.git and pushed out as testing for
all the normal reasons.

Thanks,

Jonathan
> 
> ---
> Revision history
> v5 => v6:
>  - fix typo pointed by Andy (wan't => want).
> v4 => v5:
>  - drop unnecessary comment.
>  - drop all other patches as those were already applied.
>  - add Fixes-tag.
> v3 => v4:
>  - drop extra print and TODO coment
>  - add comment clarifying alignment sizes
> ---
>  tools/iio/iio_generic_buffer.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/iio/iio_generic_buffer.c b/tools/iio/iio_generic_buffer.c
> index 44bbf80f0cfd..0d0a7a19d6f9 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 +69,14 @@ static unsigned int size_from_channelarray(struct iio_channel_info *channels, in
>  		bytes = channels[i].location + channels[i].bytes;
>  		i++;
>  	}
> +	/*
> +	 * We want the data in next sample to also be properly aligned so
> +	 * we'll add padding at the end if needed. Adding padding only
> +	 * works for channel data which size is 2^n bytes.
> +	 */
> +	misalignment = bytes % max;
> +	if (misalignment)
> +		bytes += max - misalignment;
>  
>  	return bytes;
>  }
> 
> base-commit: 0bb80ecc33a8fb5a682236443c1e740d5c917d1d