kernel/linux.git/Documentation/usb/functionfs.rst, branch v7.3-rc1

usb: gadget: f_fs: Introduce rw_proxy file descriptors

2026-07-10T13:00:47+00:00

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

usb: gadget: f_fs: Add zero-length packet ioctl

2026-07-10T13:00:45+00:00

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 Link: https://patch.msgid.link/20260619040609.4010746-4-nkapron@google.com Signed-off-by: Greg Kroah-Hartman

usb: gadget: f_fs: add capability for dfu functional descriptor

2024-08-13T08:38:46+00:00

Add the ability for the USB FunctionFS (FFS) gadget driver to be able to create Device Firmware Upgrade (DFU) functional descriptors. [1] This patch allows implementation of DFU in userspace using the FFS gadget. The DFU protocol uses the control pipe (ep0) for all messaging so only the addition of the DFU functional descriptor is needed in the kernel driver. The DFU functional descriptor is written to the ep0 file along with any other descriptors during FFS setup. DFU requires an interface descriptor followed by the DFU functional descriptor. This patch includes documentation of the added descriptor for DFU and conversion of some existing documentation to kernel-doc format so that it can be included in the generated docs. An implementation of DFU 1.1 that implements just the runtime descriptor using the FunctionFS gadget (with rebooting into u-boot for DFU mode) has been tested on an i.MX8 Nano. An implementation of DFU 1.1 that implements both runtime and DFU mode using the FunctionFS gadget has been tested on Xilinx Zynq UltraScale+. Note that for the best performance of firmware update file transfers, the userspace program should respond as quick as possible to the setup packets. [1] https://www.usb.org/sites/default/files/DFU_1.1.pdf Signed-off-by: David Sands Co-developed-by: Chris Wulff Signed-off-by: Chris Wulff Link: https://lore.kernel.org/r/20240811000004.1395888-2-crwulff@gmail.com Signed-off-by: Greg Kroah-Hartman

Documentation: usb: Document FunctionFS DMABUF API

2024-02-17T16:00:09+00:00

Add documentation for the three ioctls used to attach or detach externally-created DMABUFs, and to request transfers from/to previously attached DMABUFs. Signed-off-by: Paul Cercueil Link: https://lore.kernel.org/r/20240130122340.54813-5-paul@crapouillou.net Signed-off-by: Greg Kroah-Hartman

Documentation: USB: correct possessive "its" usage

2022-11-21T21:33:23+00:00

Correct uses of "it's" to possessive "its" or "its" to "it's" as needed. Correct associated grammar in one location. Signed-off-by: Randy Dunlap Cc: Jonathan Corbet Cc: Greg Kroah-Hartman Cc: linux-usb@vger.kernel.org Link: https://lore.kernel.org/r/20221118231422.14076-1-rdunlap@infradead.org Signed-off-by: Jonathan Corbet

docs: usb: rename files to .rst and add them to drivers-api

2019-06-20T12:28:36+00:00

While there are a mix of things here, most of the stuff were written from Kernel developer's PoV. So, add them to the driver-api book. A follow up for this patch would be to move documents from there that are specific to sysadmins, adding them to the admin-guide. Signed-off-by: Mauro Carvalho Chehab Acked-by: Johan Hovold Acked-by: Felipe Balbi Signed-off-by: Greg Kroah-Hartman