usb: gadget: f_fs: Introduce rw_proxy file descriptors

Currently, FunctionFS exposes each USB endpoint as a separate,
unidirectional file descriptor (e.g., `ep1` for IN, `ep2` for OUT).
While this mirrors the underlying hardware structure, it forces
userspace daemons implementing bidirectional protocols to manage
multiple file descriptors. When dealing with legacy protocols which
require exposing a single, bi-directional fd to userspace, this becomes
problematic.

This patch introduces the `FUNCTIONFS_RW_PROXY_EPS` UAPI flag. When
passed in the descriptor header during initialization, FunctionFS
provisions a "rw_proxy" bidirectional file descriptor (e.g., `ep1_rw`)
alongside every pair of IN/OUT endpoints.

Implementation details:
- RW proxy files act as a pure VFS alias, proxying operations
  directly to the base ffs_epfile instances. A `read()` proxies to
  the OUT endpoint's file, and a `write()` proxies to the IN file.
- Because operations are proxied natively, they reuse the underlying
  base endpoint's lock (`epfile->mutex`) and tracking state. This
  serializes concurrent I/O, preventing buffer corruption or races
  even if userspace mixes transfers across both the rw_proxy and base files
  while allowing full-duplex synchronous operations to occur concurrently
  without serializing on a single lock.
- Control operations (like IOCTLs) and intentional stalls (via
  reverse-direction I/O) must still be issued on the base endpoints, as the
  rw_proxy returns `-ENOTTY` for IOCTLs and cannot trigger stalls.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Neill Kapron <nkapron@google.com>
Link: https://patch.msgid.link/20260619040609.4010746-5-nkapron@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/uapi/linux/usb/functionfs.h b/include/uapi/linux/usb/functionfs.h
index 06134dca4e8a..290308c9dd92 100644
--- a/include/uapi/linux/usb/functionfs.h
+++ b/include/uapi/linux/usb/functionfs.h
@@ -25,6 +25,7 @@ enum functionfs_flags {
 	FUNCTIONFS_EVENTFD = 32,
 	FUNCTIONFS_ALL_CTRL_RECIP = 64,
 	FUNCTIONFS_CONFIG0_SETUP = 128,
+	FUNCTIONFS_RW_PROXY_EPS = 256,
 };
 
 /* Descriptor of an non-audio endpoint */