usb: gadget: uvc: Make bSourceID read/write

At the moment, the UVC function graph is hardcoded IT -> PU -> OT.
To add XU support we need the ability to insert the XU descriptors
into the chain. To facilitate that, make the output terminal's
bSourceID attribute writeable so that we can configure its source.
Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-2-dan.scally@ideasonboard.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/configfs-usb-gadget-uvc

@@ -54,7 +54,7 @@ Date:		Dec 2014
 KernelVersion:	4.0
 Description:	Default output terminal descriptors
 
-		All attributes read only:
+		All attributes read only except bSourceID:
 
 		==============	=============================================
 		iTerminal	index of string descriptor

drivers/usb/gadget/function/uvc_configfs.c

@@ -484,11 +484,68 @@ UVC_ATTR_RO(uvcg_default_output_, cname, aname)
 UVCG_DEFAULT_OUTPUT_ATTR(b_terminal_id, bTerminalID, 8);
 UVCG_DEFAULT_OUTPUT_ATTR(w_terminal_type, wTerminalType, 16);
 UVCG_DEFAULT_OUTPUT_ATTR(b_assoc_terminal, bAssocTerminal, 8);
-UVCG_DEFAULT_OUTPUT_ATTR(b_source_id, bSourceID, 8);
 UVCG_DEFAULT_OUTPUT_ATTR(i_terminal, iTerminal, 8);
 
 #undef UVCG_DEFAULT_OUTPUT_ATTR
 
+static ssize_t uvcg_default_output_b_source_id_show(struct config_item *item,
+						    char *page)
+{
+	struct config_group *group = to_config_group(item);
+	struct f_uvc_opts *opts;
+	struct config_item *opts_item;
+	struct mutex *su_mutex = &group->cg_subsys->su_mutex;
+	struct uvc_output_terminal_descriptor *cd;
+	int result;
+
+	mutex_lock(su_mutex); /* for navigating configfs hierarchy */
+
+	opts_item = group->cg_item.ci_parent->ci_parent->
+			ci_parent->ci_parent;
+	opts = to_f_uvc_opts(opts_item);
+	cd = &opts->uvc_output_terminal;
+
+	mutex_lock(&opts->lock);
+	result = sprintf(page, "%u\n", le8_to_cpu(cd->bSourceID));
+	mutex_unlock(&opts->lock);
+
+	mutex_unlock(su_mutex);
+
+	return result;
+}
+
+static ssize_t uvcg_default_output_b_source_id_store(struct config_item *item,
+						     const char *page, size_t len)
+{
+	struct config_group *group = to_config_group(item);
+	struct f_uvc_opts *opts;
+	struct config_item *opts_item;
+	struct mutex *su_mutex = &group->cg_subsys->su_mutex;
+	struct uvc_output_terminal_descriptor *cd;
+	int result;
+	u8 num;
+
+	mutex_lock(su_mutex); /* for navigating configfs hierarchy */
+
+	opts_item = group->cg_item.ci_parent->ci_parent->
+			ci_parent->ci_parent;
+	opts = to_f_uvc_opts(opts_item);
+	cd = &opts->uvc_output_terminal;
+
+	result = kstrtou8(page, 0, &num);
+	if (result)
+		return result;
+
+	mutex_lock(&opts->lock);
+	cd->bSourceID = num;
+	mutex_unlock(&opts->lock);
+
+	mutex_unlock(su_mutex);
+
+	return len;
+}
+UVC_ATTR(uvcg_default_output_, b_source_id, bSourceID);
+
 static struct configfs_attribute *uvcg_default_output_attrs[] = {
 	&uvcg_default_output_attr_b_terminal_id,
 	&uvcg_default_output_attr_w_terminal_type,