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

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

On Wed, 2020-03-25 at 08:17 +0000, Ardelean, Alexandru wrote:
> On Sat, 2020-03-21 at 18:21 +0000, Jonathan Cameron wrote:
> > On Fri, 20 Mar 2020 11:16:12 +0000
> > "Ardelean, Alexandru" <alexandru.Ardelean@analog.com> wrote:
> > 
> > > 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 can't immediately think of anything that would stop this case.
> > 
> > However, good if you could confirm it.  (I don't have a setup running
> > right now to test against)
> 
> I've instrumented the code a bit.
> So, with this change [moved in iio_verify_update() as Lars suggested]:
> 
> root@analog:~# cd /sys/bus/iio/devices/iio\:device3/buffer
> root@analog:/sys/bus/iio/devices/iio:device3/buffer# echo 1 > enable 
> 000000000 iio_verify_update 748
>           indio_dev->masklength 2
>           *insert_buffer->scan_mask 00000000
> 11111111 iio_verify_update 753
> 222222 __iio_update_buffers 1115 ret -22
> 333333333 iio_buffer_store_enable 1241 ret -22
> -bash: echo: write error: Invalid argument
> 
> so, 11111111, 222222 & 333333333 are all error paths.
> 
> Without the patch:
> 
> root@analog:~# cd /sys/bus/iio/devices/iio\:device3/buffer
> root@analog:/sys/bus/iio/devices/iio:device3/buffer# echo 1 > enable
> 000000000 iio_verify_update 748 
>           indio_dev->masklength 2 
>           *insert_buffer->scan_mask 00000000
> root@analog:/sys/bus/iio/devices/iio:device3/buffer# 
> 
> no error path is hit;
> error paths are still there, but the bitmap_empty() check removed;
> 
> So, buffer gets enabled, but scan_mask is empty.
> 
> Will follow-up a V2 on this and attach this information.

Right.
As a quick follow-up here: if the scan_mask is not empty [i.e. at least one
scan_element is enabled] all works fine.

> 
> 
> > 
> > > 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;
> > > > >