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

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>

0 files changed, 0 insertions, 0 deletions