> -----Original Message-----
> From: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> Sent: Thursday, November 12, 2020 6:28 PM
> To: Sa, Nuno <Nuno.Sa@xxxxxxxxxx>
> Cc: linux-iio@xxxxxxxxxxxxxxx; Jonathan Cameron <jic23@xxxxxxxxxx>;
> Lars-Peter Clausen <lars@xxxxxxxxxx>; Peter Meerwald-Stadler
> <pmeerw@xxxxxxxxxx>
> Subject: Re: [PATCH] iio: buffer: Fix demux update
>
>
> On Thu, 12 Nov 2020 15:43:22 +0100
> Nuno Sá <nuno.sa@xxxxxxxxxx> wrote:
>
> > When updating the buffer demux, we will skip a scan element from
> the
> > device in the case `in_ind != out_ind` and we enter the while loop.
> > in_ind should only be refreshed with `find_next_bit()` in the end of
> the
> > loop.
> >
> > Fixes: 5ada4ea9be16 ("staging:iio: add demux optionally to path from
> device to buffer")
> > Signed-off-by: Nuno Sá <nuno.sa@xxxxxxxxxx>
>
> Yikes that's been there a long time.
>
> Could you provide an example of a particular layout and the result of
> this being wrong?
>
Hi Jonathan,
Let's say:
iio_dev_mask: 0x0111
buffer_mask: 0x0100
We would get out_ind = 2 and in_ind = 0 and enter the loop. In the first
iteration we call find_next_bit() before doing the in_ind=0 computation which means we
will skip it and go directly to bit 1... And if we continue the path flow, we see that bit 2 will
be computed two times, so if we are lucky and scan_index0_len == scan_index2_len this
will go unnoticed...
Honestly, I didn't test this but it looks one of those things more or less clear by reading
the code or am I missing something here?
- Nuno Sá
> Thanks,
>
> Jonathan
>
