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

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 22 Sep 2023 14:16:08 +0300
Matti Vaittinen <mazziesaccount@gmail.com> 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@gmail.com>
> ---
> 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;
>  }