usb: gadget: uvc: configfs: Sort frame intervals upon writing

There is an issue where the host is unable to tell the gadget what frame
rate it wants if the dwFrameIntervals in the interface descriptors are
not in ascending order. This means that when instantiating a uvc gadget
via configfs the user must make sure the dwFrameIntervals are in
ascending order.

Instead of silently failing the breaking of this rule, we sort the
dwFrameIntervals upon writing to configfs.
Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

drivers/usb/gadget/function/uvc_configfs.c

@@ -9,6 +9,9 @@
  *
  * Author: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
  */
+
+#include <linux/sort.h>
+
 #include "u_uvc.h"
 #include "uvc_configfs.h"
 
@@ -35,6 +38,14 @@ static struct configfs_attribute prefix##attr_##cname = { \
 	.show		= prefix##cname##_show,				\
 }
 
+static int uvcg_config_compare_u32(const void *l, const void *r)
+{
+	u32 li = *(const u32 *)l;
+	u32 ri = *(const u32 *)r;
+
+	return li < ri ? -1 : li == ri ? 0 : 1;
+}
+
 static inline struct f_uvc_opts *to_f_uvc_opts(struct config_item *item)
 {
 	return container_of(to_config_group(item), struct f_uvc_opts,
@@ -1325,6 +1336,8 @@ static ssize_t uvcg_frame_dw_frame_interval_store(struct config_item *item,
 	kfree(ch->dw_frame_interval);
 	ch->dw_frame_interval = frm_intrv;
 	ch->frame.b_frame_interval_type = n;
+	sort(ch->dw_frame_interval, n, sizeof(*ch->dw_frame_interval),
+	     uvcg_config_compare_u32, NULL);
 	ret = len;
 
 end: