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

[Matti Vaittinen]

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);
?

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)

> > +	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

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