From 0f0ea552cd215a01baca1e99eec02e2f8df0c897 Mon Sep 17 00:00:00 2001 From: Neill Kapron Date: Fri, 19 Jun 2026 04:06:06 +0000 Subject: 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 Link: https://patch.msgid.link/20260619040609.4010746-5-nkapron@google.com Signed-off-by: Greg Kroah-Hartman --- include/uapi/linux/usb/functionfs.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/uapi/linux') 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 */ -- cgit v1.2.3