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

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 22 Sep 2023 14:17:49 +0300
Matti Vaittinen <mazziesaccount@gmail.com> 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@gmail.com>

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

That would incorporate ensuring exact matches come earlier by default.

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,