Re: [PATCH v3 3/6] iio: try searching for exact scan_mask

[Matti Vaittinen]

On 9/24/23 19:07, Jonathan Cameron wrote:
> On Fri, 22 Sep 2023 14:17:49 +0300
> Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:
>
> > When IIO goes through the available scan masks in order to select the
> > best suiting one, it will just accept the first listed subset of channels
> > which meets the user's requirements. This works great for most of the
> > drivers as they can sort the list of channels in the order where
> > the 'least costy' channel selections come first.
> >
> > It may be that in some cases the ordering of the list of available scan
> > masks is not thoroughly considered. We can't really try outsmarting the
> > drivers by selecting the smallest supported subset - as this might not
> > be the 'least costy one' - but we can at least try searching through the
> > list to see if we have an exactly matching mask. It should be sane
> > assumption that if the device can support reading only the exact
> > channels user is interested in, then this should be also the least costy
> > selection - and if it is not and optimization is important, then the
> > driver could consider omitting setting the 'available_scan_mask' and
> > doing demuxing - or just omitting the 'costy exact match' and providing
> > only the more efficient broader selection of channels.
> >
> > Signed-off-by: Matti Vaittinen <mazziesaccount@xxxxxxxxx>
>
> Whilst I fully agree with the reasoning behind this, I'd rather we
> did an audit of drivers to find any that have a non logical order
> (one came up today in review) and fix them up.
>
> A quick and dirty grep didn't find it to be a common problem, at least
> partly as most users of this feature only provide one valid mask.

It's always good to hear there is not many problems found :) This patch 
was not inspired by auditing the existing code - it was inspired by the 
fact that I would have wrongly ordered the available_scan_masks for 
bm1390 myself. I just happened to notice the oddity in active_scan_masks 
while I was trying to figure out if it was the driver, IIO or user-space 
code which messed my buffer when I disabled timestamps.

> The few complex corners I found appear to be fine with the expected
> shortest sequences first.
>
> Defending against driver bugs is losing game if it makes the core
> code more complex to follow by changing stuff in non debug paths.

I think I agree, although I could argue that it depends on the amount of 
added complexity. Still ...

> One option might be to add a trivial check at iio_device_register()

... this suggestion is superior to the check added in this patch.

> that we don't have scan modes that are subsets of modes earlier in the list.
> These lists are fairly short so should be cheap to run.

Yes. And running the check at the registration phase should not be a big 
problem. And, if it appears to be a problem, then we can add a 
registration variant which omits the checks for those rare drivers which 
would _really_ be hurt by the few extra cycles spent on registration.

> That would incorporate ensuring exact matches come earlier by default.

Yes. I like the idea, wish I had invented it myself ;)

> Jonathan
>
>
> > ---
> >   drivers/iio/industrialio-buffer.c | 25 +++++++++++++++++++------
> >   1 file changed, 19 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
> > index 176d31d9f9d8..e97396623373 100644
> > --- a/drivers/iio/industrialio-buffer.c
> > +++ b/drivers/iio/industrialio-buffer.c
> > @@ -411,19 +411,32 @@ static const unsigned long *iio_scan_mask_match(const unsigned long *av_masks,
> >   						const unsigned long *mask,
> >   						bool strict)
> >   {
> > +	const unsigned long *first_subset = NULL;
> > +
> >   	if (bitmap_empty(mask, masklength))
> >   		return NULL;
> > -	while (*av_masks) {
> > -		if (strict) {
> > +
> > +	if (strict) {
> > +		while (*av_masks) {
> >   			if (bitmap_equal(mask, av_masks, masklength))
> >   				return av_masks;
> > -		} else {
> > -			if (bitmap_subset(mask, av_masks, masklength))
> > -				return av_masks;
> > +
> > +			av_masks += BITS_TO_LONGS(masklength);
> >   		}
> > +
> > +		return NULL;
> > +	}
> > +	while (*av_masks) {
> > +		if (bitmap_equal(mask, av_masks, masklength))
> > +			return av_masks;
> > +
> > +		if (!first_subset && bitmap_subset(mask, av_masks, masklength))
> > +			first_subset = av_masks;
> > +
> >   		av_masks += BITS_TO_LONGS(masklength);
> >   	}
> > -	return NULL;
> > +
> > +	return first_subset;
> >   }
> >   
> >   static bool iio_validate_scan_mask(struct iio_dev *indio_dev,

--
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

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