[PATCH 1/2] iio: Fix scan mask subset check logic

[Fabio Estevam <festevam@gmail.com>]

From: Fabio Estevam <festevam@denx.de>

Since commit 2718f15403fb ("iio: sanity check available_scan_masks array"),
verbose and misleading warnings are printed for devices like the MAX11601:

max1363 1-0064: available_scan_mask 8 subset of 0. Never used
max1363 1-0064: available_scan_mask 9 subset of 0. Never used
max1363 1-0064: available_scan_mask 10 subset of 0. Never used
max1363 1-0064: available_scan_mask 11 subset of 0. Never used
max1363 1-0064: available_scan_mask 12 subset of 0. Never used
max1363 1-0064: available_scan_mask 13 subset of 0. Never used
...
[warnings continue] 

Fix the available_scan_masks sanity check logic so that it
only prints the warning when an element of available_scan_mask
is in fact a subset of a previous one.

These warnings incorrectly report that later scan masks are subsets of
the first one, even when they are not. The issue lies in the logic that
checks for subset relationships between scan masks.

Fix the subset detection to correctly compare each mask only
against previous masks, and only warn when a true subset is found.

With this fix, the warning output becomes both correct and more
informative:

max1363 1-0064: Mask 7 (0xc) is a subset of mask 6 (0xf) and will be ignored

Cc: stable@vger.kernel.org
Fixes: 2718f15403fb ("iio: sanity check available_scan_masks array")
Signed-off-by: Fabio Estevam <festevam@denx.de>
---
 drivers/iio/industrialio-core.c | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index 6a6568d4a2cb..855d5fd3e6b2 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -1904,6 +1904,11 @@ static int iio_check_extended_name(const struct iio_dev *indio_dev)
 
 static const struct iio_buffer_setup_ops noop_ring_setup_ops;
 
+static int is_subset(unsigned long a, unsigned long b)
+{
+	return (a & ~b) == 0;
+}
+
 static void iio_sanity_check_avail_scan_masks(struct iio_dev *indio_dev)
 {
 	unsigned int num_masks, masklength, longs_per_mask;
@@ -1947,21 +1952,13 @@ static void iio_sanity_check_avail_scan_masks(struct iio_dev *indio_dev)
 	 * available masks in the order of preference (presumably the least
 	 * costy to access masks first).
 	 */
-	for (i = 0; i < num_masks - 1; i++) {
-		const unsigned long *mask1;
-		int j;
 
-		mask1 = av_masks + i * longs_per_mask;
-		for (j = i + 1; j < num_masks; j++) {
-			const unsigned long *mask2;
-
-			mask2 = av_masks + j * longs_per_mask;
-			if (bitmap_subset(mask2, mask1, masklength))
+	for (i = 1; i < num_masks; ++i)
+		for (int j = 0; j < i; ++j)
+			if (is_subset(av_masks[i], av_masks[j]))
 				dev_warn(indio_dev->dev.parent,
-					 "available_scan_mask %d subset of %d. Never used\n",
-					 j, i);
-		}
-	}
+					 "Mask %d (0x%lx) is a subset of mask %d (0x%lx) and will be ignored\n",
+					 i, av_masks[i], j, av_masks[j]);
 }
 
 /**
-- 
2.34.1