On Thu, 21 Sep 2023 12:00:39 +0300
Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:
> On 9/21/23 11:17, Matti Vaittinen wrote:
>
> > Another thing to note is that, when we build the available_scan_mask
> > array - we should either pay attention to the order of masks - or change
> > the iio_scan_mask_match() to not accept first matching subset but to go
> > through all of the masks unless it finds and exactly matching one (and
> > in general prefer the smallest subset). Not sure this is worth the extra
> > cycles though.
>
> Replying to myself and to those who I perhaps managed to confuse :)
>
> As a result of above pondering I wrote this:
>
> @@ -411,6 +418,8 @@ static const unsigned long
> *iio_scan_mask_match(const unsigned long *av_masks,
> const unsigned long *mask,
> bool strict)
> {
> + const unsigned long *smallest = NULL;
> +
> if (bitmap_empty(mask, masklength))
> return NULL;
> while (*av_masks) {
> @@ -418,12 +427,16 @@ static const unsigned long
> *iio_scan_mask_match(const unsigned long *av_masks,
> if (bitmap_equal(mask, av_masks, masklength))
> return av_masks;
> } else {
> - if (bitmap_subset(mask, av_masks, masklength))
> - return av_masks;
> + if (bitmap_subset(mask, av_masks, masklength)) {
> + if (!smallest ||
> + bitmap_weight(av_masks, BITS_PER_LONG) <
> + bitmap_weight(smallest, BITS_PER_LONG))
> + smallest = av_masks;
> + }
> }
> av_masks += BITS_TO_LONGS(masklength);
> }
> - return NULL;
> + return smallest;
> }
>
> but ...
> ... I see a problem that some of the channels may be more costly to
> access than the other. It could be that reading some of the channels is
> just a matter of getting a cached value, while other could require a
> long measurement time and access to significant amount of registers. So,
> the knowledge of preferred scan masks should indeed be on the driver
> side. Hence, the ordering of the masks in the order of preference makes
> perfect sense. What we could do in the IIO core side is still go through
> all of the available masks to see if we find an exact match. I guess we
> could also document the fact that the order of masks matters.
I should have read on in the thread. Indeed - ordering of preferences needs
to be in driver control for exactly the reason you came up with!
Thanks,
Jonathan
>
> Thanks for listening - and sorry for the noise :)
>
> Yours,
> -- Matti
>
