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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>
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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>
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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 <david.sands@biamp.com>
Co-developed-by: Chris Wulff <crwulff@gmail.com>
Signed-off-by: Chris Wulff <crwulff@gmail.com>
Link: https://lore.kernel.org/r/20240811000004.1395888-2-crwulff@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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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 <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20240130122340.54813-5-paul@crapouillou.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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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 <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Link: https://lore.kernel.org/r/20221118231422.14076-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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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 <mchehab+samsung@kernel.org>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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