MdeMdeModulePkg/UsbBusDxe: Fixed a possible memory leak bug introduced at r14226

The r14226 check-in indeed has memory leak in allocated "Child" pointer. UsbBusDriverBindingStop() may dereference this pointer and may bring exception on invalid memory access

Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>



git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14251 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Bus/Usb/UsbBusDxe/UsbEnumer.c

@@ -814,11 +814,20 @@ UsbEnumerateNewDev (
   return EFI_SUCCESS;
 
 ON_ERROR:
-
+  //
-  if (Child != NULL) {
+  // If reach here, it means the enumeration process on a given port is interrupted due to error.
-    UsbFreeDevice (Child);
+  // The s/w resources, including the assigned address(Address) and the allocated usb device data
-  }
+  // structure(Bus->Devices[Address]), will NOT be freed here. These resources will be freed when
-
+  // the device is unplugged from the port or DriverBindingStop() is invoked.
+  //
+  // This way is used to co-work with the lower layer EDKII UHCI/EHCI/XHCI host controller driver.
+  // It's mainly because to keep UEFI spec unchanged EDKII XHCI driver have to maintain a state machine
+  // to keep track of the mapping between actual address and request address. If the request address
+  // (Address) is freed here, the Address value will be used by next enumerated device. Then EDKII XHCI
+  // host controller driver will have wrong information, which will cause further transaction error.
+  //
+  // EDKII UHCI/EHCI doesn't get impacted as it's make sense to reserve s/w resource till it gets unplugged.
+  //
   return Status;
 }