iio: Handle enumerated properties with gaps

Some enums might have gaps or reserved values in the middle of their value
range. E.g. consider a 2-bit enum where the values 0, 1 and 3 have a
meaning, but 2 is a reserved value and can not be used.

Add support for such enums to the IIO enum helper functions. A reserved
values is marked by setting its entry in the items array to NULL rather
than the normal descriptive string value.

Signed-off-by: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Alexandru Ardelean <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/industrialio-core.c

···
 	[IIO_CHAN_INFO_CALIBAMBIENT] = "calibambient",
 };
 
+/**
+ * iio_sysfs_match_string_with_gaps - matches given string in an array with gaps
+ * @array: array of strings
+ * @n: number of strings in the array
+ * @str: string to match with
+ *
+ * Returns index of @str in the @array or -EINVAL, similar to match_string().
+ * Uses sysfs_streq instead of strcmp for matching.
+ *
+ * This routine will look for a string in an array of strings.
+ * The search will continue until the element is found or the n-th element
+ * is reached, regardless of any NULL elements in the array.
+ */
+static int iio_sysfs_match_string_with_gaps(const char * const *array, size_t n,
+					    const char *str)
+{
+	const char *item;
+	int index;
+
+	for (index = 0; index < n; index++) {
+		item = array[index];
+		if (!item)
+			continue;
+		if (sysfs_streq(item, str))
+			return index;
+	}
+
+	return -EINVAL;
+}
+
 #if defined(CONFIG_DEBUG_FS)
 /*
  * There's also a CONFIG_DEBUG_FS guard in include/linux/iio/iio.h for
···
 	if (!e->num_items)
 		return 0;
 
-	for (i = 0; i < e->num_items; ++i)
+	for (i = 0; i < e->num_items; ++i) {
+		if (!e->items[i])
+			continue;
 		len += scnprintf(buf + len, PAGE_SIZE - len, "%s ", e->items[i]);
+	}
 
 	/* replace last space with a newline */
 	buf[len - 1] = '\n';
···
 	i = e->get(indio_dev, chan);
 	if (i < 0)
 		return i;
-	else if (i >= e->num_items)
+	else if (i >= e->num_items || !e->items[i])
 		return -EINVAL;
 
 	return snprintf(buf, PAGE_SIZE, "%s\n", e->items[i]);
···
 	if (!e->set)
 		return -EINVAL;
 
-	ret = __sysfs_match_string(e->items, e->num_items, buf);
+	ret = iio_sysfs_match_string_with_gaps(e->items, e->num_items, buf);
 	if (ret < 0)
 		return ret;