usb: gadget: webcam: Make g_webcam loadable again

commit 588b9e85 ("usb: gadget: uvc: add v4l2 enumeration api calls")
has rendered the precomposed (aka legacy) webcam gadget unloadable.

uvc_alloc() since then has depended on certain config groups being
available in configfs tree related to the UVC function. However, legacy
gadgets do not create anything in configfs, so uvc_alloc() must fail
with -ENOENT no matter what.

This patch mimics the required configfs hierarchy to satisfy the code which
inspects formats and frames found in uvcg_streaming_header.

This has been tested with guvcview on the host side, using vivid as a
source of video stream on the device side and using the userspace program
found at https://gitlab.freedesktop.org/camera/uvc-gadget.git.
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Fixes: 588b9e85 ("usb: gadget: uvc: add v4l2 enumeration api calls")
Link: https://lore.kernel.org/r/20231215131614.29132-1-andrzej.p@collabora.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_uvc.c

@@ -959,7 +959,8 @@ static void uvc_free(struct usb_function *f)
 	struct uvc_device *uvc = to_uvc(f);
 	struct f_uvc_opts *opts = container_of(f->fi, struct f_uvc_opts,
 					       func_inst);
-	config_item_put(&uvc->header->item);
+	if (!opts->header)
+		config_item_put(&uvc->header->item);
 	--opts->refcnt;
 	kfree(uvc);
 }
@@ -1051,25 +1052,29 @@ static struct usb_function *uvc_alloc(struct usb_function_instance *fi)
 	uvc->desc.hs_streaming = opts->hs_streaming;
 	uvc->desc.ss_streaming = opts->ss_streaming;
 
-	streaming = config_group_find_item(&opts->func_inst.group, "streaming");
-	if (!streaming)
-		goto err_config;
-
-	header = config_group_find_item(to_config_group(streaming), "header");
-	config_item_put(streaming);
-	if (!header)
-		goto err_config;
-
-	h = config_group_find_item(to_config_group(header), "h");
-	config_item_put(header);
-	if (!h)
-		goto err_config;
-
-	uvc->header = to_uvcg_streaming_header(h);
-	if (!uvc->header->linked) {
-		mutex_unlock(&opts->lock);
-		kfree(uvc);
-		return ERR_PTR(-EBUSY);
+	if (opts->header) {
+		uvc->header = opts->header;
+	} else {
+		streaming = config_group_find_item(&opts->func_inst.group, "streaming");
+		if (!streaming)
+			goto err_config;
+
+		header = config_group_find_item(to_config_group(streaming), "header");
+		config_item_put(streaming);
+		if (!header)
+			goto err_config;
+
+		h = config_group_find_item(to_config_group(header), "h");
+		config_item_put(header);
+		if (!h)
+			goto err_config;
+
+		uvc->header = to_uvcg_streaming_header(h);
+		if (!uvc->header->linked) {
+			mutex_unlock(&opts->lock);
+			kfree(uvc);
+			return ERR_PTR(-EBUSY);
+		}
 	}
 
 	uvc->desc.extension_units = &opts->extension_units;

drivers/usb/gadget/function/u_uvc.h

@@ -98,6 +98,12 @@ struct f_uvc_opts {
 	 */
 	struct mutex			lock;
 	int				refcnt;
+
+	/*
+	 * Only for legacy gadget. Shall be NULL for configfs-composed gadgets,
+	 * which is guaranteed by alloc_inst implementation of f_uvc doing kzalloc.
+	 */
+	struct uvcg_streaming_header	*header;
 };
 
 #endif /* U_UVC_H */