iio: buffer: document known issue

Add documentation explaining why the code which scans all available scan
masks is checking only a single long worth of bits even though the code
was intended to be supporting masks wider than single long.
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/ef61c2c1e9a1c5e9f713f656871fdcb1652afdc2.1697452986.git.mazziesaccount@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-buffer.c

@@ -413,6 +413,22 @@ static const unsigned long *iio_scan_mask_match(const unsigned long *av_masks,
 {
 	if (bitmap_empty(mask, masklength))
 		return NULL;
+	/*
+	 * The condition here do not handle multi-long masks correctly.
+	 * It only checks the first long to be zero, and will use such mask
+	 * as a terminator even if there was bits set after the first long.
+	 *
+	 * Correct check would require using:
+	 * while (!bitmap_empty(av_masks, masklength))
+	 * instead. This is potentially hazardous because the
+	 * avaliable_scan_masks is a zero terminated array of longs - and
+	 * using the proper bitmap_empty() check for multi-long wide masks
+	 * would require the array to be terminated with multiple zero longs -
+	 * which is not such an usual pattern.
+	 *
+	 * As writing of this no multi-long wide masks were found in-tree, so
+	 * the simple while (*av_masks) check is working.
+	 */
 	while (*av_masks) {
 		if (strict) {
 			if (bitmap_equal(mask, av_masks, masklength))