Re: [PATCH 0/3] usb: gadget: f_fs: Add R/W proxy EPs and ZLP support

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Jun 14, 2026 at 06:09:59PM +0000, Neill Kapron wrote:
> We are working to deprecate a widely used, out of tree gadget driver by
> moving the functionality to userspace via functionfs. To do so, we have to
> maintain strict compatibility with the legacy driver, as there are many
> third party applications which can’t be modified and are reliant on this
> interface. Specifically, the following requirements must be met:
> 
> - The function must expose a single file descriptor to userspace for both
>   reads and writes,
> - It must block on writes when it can not handle more data, 
> - It must handle arbitrary write transaction sizes,
> - It must automatically append a zero length packet (ZLP) when the write
>   transaction ends on a boundary of a multiple of the max packet size.
> 
> Initially, we pursued a compatibility layer in userspace which implemented
> a socket pair to combine the OUT and IN endpoint files. This approach
> proved problematic for several reasons.
> 
> To preserve the write transaction boundary for ZLP calculation, we
> attempted to use SOCK_SEQPACKET. This created problems as larger
> transactions required contiguous buffers to be allocated, and, even if we
> ignore the constraint to the arbitrary write size and limited it to 1MB,
> the socket would occasionally return -ENOBUFS to the end user if a write
> operation was attempted when other sockets on the system were consuming
> more than 7MB of the 8MB wmem_max limit.
> 
> After significant investigation including switching to SOCK_STREAM and
> attempting a heuristic timeout approach, we decided the best path forward
> was to pursue a native proxy endpoint approach in the functionfs driver
> itself.
> 
> This patchset introduces the `FUNCTIONFS_RW_PROXY_EPS` flag to functionfs
> which, when set, creates an additional proxy file for reading or writing
> to a pair of endpoints. In an attempt to limit the change to the UAPI
> surface, we added several constraints to this proxy file. We chose not to
> handle ioctls on this proxy file, as the current ioctls do not have a
> directionality associated with them, and would require essentially
> creating duplicate ioctls with a direction argument. To use this flag, an
> even number of in/out endpoints must be created, and a proxy ep is created
> for each pair of endpoints in the descriptors.
> 
> With this new r/w proxy ep, we are able to transparently hand it to the
> end application. However, to match the legacy driver’s ZLP functionality,
> a new ioctl is added, `FUNCTIONFS_ENDPOINT_ENABLE_ZLP`. This allows the
> userspace functionfs daemon to write the necessary descriptors, configure
> the auto ZLP functionality on the IN EP, then handoff the proxy ep to the
> application. When enabled, functionfs sets the req->zero flag. The UDC
> driver then automatically adds the ZLP if the transaction length % max
> packet size is 0.
> 
> An addition, a patch is included which fixes an issue if certain ioctls
> (like the new `FUNCTIONFS_ENDPOINT_ENABLE_ZLP` or
> `FUNCTIONFS_DMABUF_ATTACH`) are called prior to the host being connected.
> 
> This patchset has been tested on a kernel based on 7.1-rc7, as well as a
> backported version tested on 6.1. Existing functionfs implementations
> continue to work without modification, and the new functionality passes
> tests designed for our legacy kernel driver implementation.

Very cool, thanks for these changes.  I'll look at them after -rc1 is
out!

greg k-h