Hi Henk,
> From: Henk Medenblik <iio-developer@xxxxxxxxxx>
> Sent: Wednesday, June 9, 2021 9:04 AM
> To: Alexandru Ardelean <ardeleanalex@xxxxxxxxx>; Henk Medenblik
> <iio-developer@xxxxxxxxxx>
> Cc: linux-iio <linux-iio@xxxxxxxxxxxxxxx>
> Subject: Re: active_scan_mask
>
> [External]
>
> Dear Alexandru,
>
> > Hmm, so the "zerocopy" branch isn't at v0.21.
> > It's something much older [ ~6 years ? ].
> > Though the kernel active scan mask is unrelated to libiio, since
> > iio_readdev is being used in different sessions.
> >
> > The 4.14 version of the kernel, isn't quite new.
> > There was some recent fix with regards to active scan-mask
> computation.
> > But I am not sure if it's related to this.
> >
> > Will try to make some time to do some testing on this.
> >
> I believe it is a bug which still exists in the adi linux kernel sources.
> In industrialio-buffer.c :
>
> static int iio_channel_mask_clear(struct iio_dev *indio_dev,
> struct iio_buffer *buffer, int bit)
> {
> unsigned int ch;
>
> clear_bit(bit, buffer->channel_mask);
>
> memset(buffer->scan_mask, 0,
> BITS_TO_LONGS(indio_dev->masklength)); <---------
> for_each_set_bit(ch, buffer->channel_mask, indio_dev-
> >num_channels)
> set_bit(indio_dev->channels[ch].scan_index, buffer-
> >scan_mask);
> return 0;
> }
>
Take a look at [1] for more details. Long story short, I think this is here because we have
some devices where we have bit granularity. Hence, we map all channels to the same
scan_index and to distinguish which channels are enabled, we could not use the current
upstream implementation. I remember some talks about this but not actual patches
came from it (I will have to go over those emails to see if I can come up with some
patches).
Regarding the memset, that looks like a bug. I guess we want:
memset(buffer->scan_mask, 0, BITS_TO_LONGS(indio_dev->masklength) * sizeof(*buffer->scan_mask));
Ccing Lars as he implemented this patches and might have something to add to
your questions.
ps: feel free to push a PR to ADI tree if you fix this :)
- Nuno Sá
[1]: https://github.com/analogdevicesinc/linux/commit/81d00795b1537a8cfd3eb5152b30ae407c0e16e7
