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

[Matti Vaittinen <mazziesaccount@gmail.com>]

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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>
---
 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,
-- 
2.41.0


-- 
Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

~~~ "I don't think so," said Rene Descartes. Just then he vanished ~~~
Simon says - in Latin please.
~~~ "non cogito me" dixit Rene Descarte, deinde evanescavit ~~~
Thanks to Simon Glass for the translation =] 

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