usb: gadget: uvc: Allow linking XUs to string descriptors

Add .allow_link() and .drop_link() callbacks to allow users to link
an extension unit descriptor to a string descriptor.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-9-dan.scally@ideasonboard.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 53 insertions, 0 deletions

diff --git a/drivers/usb/gadget/function/uvc_configfs.c b/drivers/usb/gadget/function/uvc_configfs.c
index c365f323af45..3ac27838514c 100644
--- a/drivers/usb/gadget/function/uvc_configfs.c
+++ b/drivers/usb/gadget/function/uvc_configfs.c
@@ -1064,8 +1064,60 @@ static void uvcg_extension_release(struct config_item *item)
 	kfree(xu);
 }
 
+static int uvcg_extension_allow_link(struct config_item *src, struct config_item *tgt)
+{
+	struct mutex *su_mutex = &src->ci_group->cg_subsys->su_mutex;
+	struct uvcg_extension *xu = to_uvcg_extension(src);
+	struct config_item *gadget_item;
+	struct gadget_string *string;
+	struct config_item *strings;
+	int ret = 0;
+
+	mutex_lock(su_mutex); /* for navigating configfs hierarchy */
+
+	/* Validate that the target of the link is an entry in strings/<langid> */
+	gadget_item = src->ci_parent->ci_parent->ci_parent->ci_parent->ci_parent;
+	strings = config_group_find_item(to_config_group(gadget_item), "strings");
+	if (!strings || tgt->ci_parent->ci_parent != strings) {
+		ret = -EINVAL;
+		goto put_strings;
+	}
+
+	string = to_gadget_string(tgt);
+	xu->string_descriptor_index = string->usb_string.id;
+
+put_strings:
+	config_item_put(strings);
+	mutex_unlock(su_mutex);
+
+	return ret;
+}
+
+static void uvcg_extension_drop_link(struct config_item *src, struct config_item *tgt)
+{
+	struct mutex *su_mutex = &src->ci_group->cg_subsys->su_mutex;
+	struct uvcg_extension *xu = to_uvcg_extension(src);
+	struct config_item *opts_item;
+	struct f_uvc_opts *opts;
+
+	mutex_lock(su_mutex); /* for navigating configfs hierarchy */
+
+	opts_item = src->ci_parent->ci_parent->ci_parent;
+	opts = to_f_uvc_opts(opts_item);
+
+	mutex_lock(&opts->lock);
+
+	xu->string_descriptor_index = 0;
+
+	mutex_unlock(&opts->lock);
+
+	mutex_unlock(su_mutex);
+}
+
 static struct configfs_item_operations uvcg_extension_item_ops = {
 	.release	= uvcg_extension_release,
+	.allow_link	= uvcg_extension_allow_link,
+	.drop_link	= uvcg_extension_drop_link,
 };
 
 static const struct config_item_type uvcg_extension_type = {
diff --git a/drivers/usb/gadget/function/uvc_configfs.h b/drivers/usb/gadget/function/uvc_configfs.h
index 5557813bcca9..c6a690158138 100644
--- a/drivers/usb/gadget/function/uvc_configfs.h
+++ b/drivers/usb/gadget/function/uvc_configfs.h
@@ -163,6 +163,7 @@ struct uvcg_extension_unit_descriptor {
 struct uvcg_extension {
 	struct config_item item;
 	struct list_head list;
+	u8 string_descriptor_index;
 	struct uvcg_extension_unit_descriptor desc;
 };