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When transferring data from a USB gadget to a host, a transfer is considered complete when the host receives a short packet (a packet smaller than wMaxPacketSize). If the total transfer length is an exact multiple of wMaxPacketSize, a Zero-Length Packet (ZLP) must be appended to signal the end of the transfer. FunctionFS currently provides no mechanism for userspace to instruct the kernel to set the `req->zero` flag on transfers. Userspace workarounds, such as manually submitting separate 0-byte requests, may not be available for legacy protocols which must maintain write behavior compatibility when moved to functionfs implementations. To resolve this, introduce a new ioctl, FUNCTIONFS_ENDPOINT_ENABLE_ZLP, which takes a pointer to a __u32 flag. When enabled, all subsequent transfers on that endpoint will have the `req->zero` flag set, allowing the underlying USB Device Controller (UDC) hardware to automatically append a ZLP only when mathematically required. For logical transfers chunked across multiple requests, userspace can dynamically toggle this flag, enabling it only prior to submitting the final chunk. The flag defaults to false to maintain backward compatibility. Once enabled, the state is persistent for the lifetime of the endpoint and will not be reset by opening or closing the endpoint file descriptors. Assisted-by: Gemini-CLI:gemini-3.1-pro-preview Signed-off-by: Neill Kapron <nkapron@google.com> Link: https://patch.msgid.link/20260619040609.4010746-4-nkapron@google.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>