[PATCH v2 1/2] iio: sanity check available_scan_masks array

[Matti Vaittinen]

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. If driver lists a mask which is a
subset of some of the masks previously in the array of
avaliable_scan_masks, then the latter one will never be selected.

Add a warning if driver registers masks which can't be used due to the
available_scan_masks-array ordering.

Suggested-by: Jonathan Cameron <jic23@xxxxxxxxxx>
Signed-off-by: Matti Vaittinen <mazziesaccount@xxxxxxxxx>

---
Revision History:
v1 => v2:
 - warn if masklength of available_scan_masks is wider than a long
 - drop unnecessary comment and extra blank line

NOTE: the v2 was compile-tested only.

The change was suggested by Jonathan here:
https://lore.kernel.org/lkml/20230924170726.41443502@jic23-huawei/
---
 drivers/iio/industrialio-core.c | 63 +++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index c77745b594bd..34e1f8d0071c 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -1896,6 +1896,66 @@ static int iio_check_extended_name(const struct iio_dev *indio_dev)
 
 static const struct iio_buffer_setup_ops noop_ring_setup_ops;
 
+static void iio_sanity_check_avail_scan_masks(struct iio_dev *indio_dev)
+{
+	unsigned int num_masks, masklength, longs_per_mask;
+	const unsigned long *av_masks;
+	int i;
+
+	av_masks = indio_dev->available_scan_masks;
+	masklength = indio_dev->masklength;
+	longs_per_mask = BITS_TO_LONGS(masklength);
+
+	/*
+	 * The code determining how many available_scan_masks is in the array
+	 * will be assuming the end of masks when first long with all bits
+	 * zeroed is encountered. This is incorrect for masks where mask
+	 * consists of more than one long, and where some of the available masks
+	 * has long worth of bits zeroed (but has subsequent bit(s) set). This
+	 * is a safety measure against bug where array of masks is terminated by
+	 * a single zero while mask width is greater than width of a long.
+	 */
+	if (longs_per_mask > 1)
+		dev_warn(indio_dev->dev.parent,
+			 "multi long available scan masks not fully supported\n");
+
+	if (bitmap_empty(av_masks, masklength))
+		dev_warn(indio_dev->dev.parent, "empty scan mask\n");
+
+	for (num_masks = 0; *av_masks; num_masks++)
+		av_masks += longs_per_mask;
+
+	if (num_masks < 2)
+		return;
+
+	av_masks = indio_dev->available_scan_masks;
+
+	/*
+	 * Go through all the masks from first to one before the last, and see
+	 * that no mask found later from the available_scan_masks array is a
+	 * subset of mask found earlier. If this happens, then the mask found
+	 * later will never get used because scanning the array is stopped when
+	 * the first suitable mask is found. Drivers should order the array of
+	 * 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))
+				dev_warn(indio_dev->dev.parent,
+					 "available_scan_mask %d subset of %d. Never used\n",
+					 j, i);
+		}
+	}
+}
+
 int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
 {
 	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
@@ -1934,6 +1994,9 @@ int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
 		goto error_unreg_debugfs;
 	}
 
+	if (indio_dev->available_scan_masks)
+		iio_sanity_check_avail_scan_masks(indio_dev);
+
 	ret = iio_device_register_sysfs(indio_dev);
 	if (ret) {
 		dev_err(indio_dev->dev.parent,
-- 
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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