usb: gadget: Store endpoint pointer in usb_request

Gadget function drivers often have goto-based error handling in their
bind paths, which can be bug-prone. Refactoring these paths to use
__free() scope-based cleanup is desirable, but currently blocked.

The blocker is that usb_ep_free_request(ep, req) requires two
parameters, while the __free() mechanism can only pass a pointer to the
request itself.

Store an endpoint pointer in the struct usb_request. The pointer is
populated centrally in usb_ep_alloc_request() on every successful
allocation, making the request object self-contained.

Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://lore.kernel.org/r/20250916-ready-v1-1-4997bf277548@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250916-ready-v1-1-4997bf277548@google.com

Author: Kuen-Han Tsai

drivers/usb/gadget/udc/core.c

@@ -194,6 +194,9 @@ struct usb_request *usb_ep_alloc_request(struct usb_ep *ep,
 
 	req = ep->ops->alloc_request(ep, gfp_flags);
 
+	if (req)
+		req->ep = ep;
+
 	trace_usb_ep_alloc_request(ep, req, req ? 0 : -ENOMEM);
 
 	return req;

include/linux/usb/gadget.h

@@ -32,6 +32,7 @@ struct usb_ep;
 
 /**
  * struct usb_request - describes one i/o request
+ * @ep: The associated endpoint set by usb_ep_alloc_request().
  * @buf: Buffer used for data.  Always provide this; some controllers
  *	only use PIO, or don't use DMA for some endpoints.
  * @dma: DMA address corresponding to 'buf'.  If you don't set this
@@ -98,6 +99,7 @@ struct usb_ep;
  */
 
 struct usb_request {
+	struct usb_ep		*ep;
 	void			*buf;
 	unsigned		length;
 	dma_addr_t		dma;