Re: [RFC][PATCH] iio: buffer: Don't allow buffers without any channels enabled to be activated

["Ardelean, Alexandru" <alexandru.Ardelean@analog.com>]

On Fri, 2020-03-20 at 11:55 +0100, Lars-Peter Clausen wrote:
> On 3/20/20 11:40 AM, Alexandru Ardelean wrote:
> > From: Lars-Peter Clausen <lars@metafoo.de>
> > 
> > Before activating a buffer make sure that at least one channel is enabled.
> > Activating a buffer with 0 channels enabled doesn't make too much sense and
> > disallowing this case makes sure that individual driver don't have to add
> > special case code to handle it.
> > 
> > Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> > ---
> > 
> > * Found this relic-patch in our tree, from 6 years ago:
> >    https://github.com/analogdevicesinc/linux/commit/6d680e49d459c
> >    It got moved around a bit, and this is the current form in the ADI tree.
> >    So, this is also a bit of an RFC, but if the idea is valid, maybe it's
> >    worth considering upstream. I don't know of any arguments against it,
> >    but I could be surprised.
> 
> Hm, a bit weird that this one never made it upstream considering how 
> simple it is.
> 
> Did you check that the issue still occurs? I can't see anything in the 
> code that prevents it, but who knows, maybe it was fixed by something else.

i did not think to check behavior/issues;
i'll try to make some time for that;
i caught this one while diff-ing the upstream & ADI trees, and i needed to dig a
bit more into the ADI git history on it;

i was a bit puzzled for a while, because some rework patches were upstreamed
without this patch:

https://lore.kernel.org/linux-iio/55585CAA.6000506@kernel.org/
https://lore.kernel.org/linux-iio/5560685A.5060504@kernel.org/

i also did not find any discussions/upstream attempt for this patch particularly

so, it was easier for me just to RFC this

> 
> >   drivers/iio/industrialio-buffer.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-
> > buffer.c
> > index 4ada5592aa2b..f222a118d0d3 100644
> > --- a/drivers/iio/industrialio-buffer.c
> > +++ b/drivers/iio/industrialio-buffer.c
> > @@ -1031,6 +1031,12 @@ static int __iio_update_buffers(struct iio_dev
> > *indio_dev,
> >   		return ret;
> >   
> >   	if (insert_buffer) {
> > +		if (bitmap_empty(insert_buffer->scan_mask,
> > +			indio_dev->masklength)) {
> > +			ret = -EINVAL;
> > +			goto err_free_config;
> > +		}
> 
> Since the check is so simple it might make sense to do it as the very 
> first thing before iio_verify_update().

works for me;

> 
> > +
> >   		ret = iio_buffer_request_update(indio_dev, insert_buffer);
> >   		if (ret)
> >   			goto err_free_config;
> >