net/9p/usbg: Add new usb gadget function transport

Add the new gadget function for 9pfs transport. This function is
defining an simple 9pfs transport interface that consists of one in and
one out endpoint. The endpoints transmit and receive the 9pfs protocol
payload when mounting a 9p filesystem over usb.

Tested-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20240116-ml-topic-u9p-v12-2-9a27de5160e0@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/filesystems/9p.rst

@@ -48,11 +48,25 @@ For server running on QEMU host with virtio transport::
 
 	mount -t 9p -o trans=virtio <mount_tag> /mnt/9
 
-where mount_tag is the tag associated by the server to each of the exported
+where mount_tag is the tag generated by the server to each of the exported
 mount points. Each 9P export is seen by the client as a virtio device with an
 associated "mount_tag" property. Available mount tags can be
 seen by reading /sys/bus/virtio/drivers/9pnet_virtio/virtio<n>/mount_tag files.
 
+USBG Usage
+==========
+
+To mount a 9p FS on a USB Host accessible via the gadget at runtime::
+
+	mount -t 9p -o trans=usbg,aname=/path/to/fs <device> /mnt/9
+
+To mount a 9p FS on a USB Host accessible via the gadget as root filesystem::
+
+	root=<device> rootfstype=9p rootflags=trans=usbg,cache=loose,uname=root,access=0,dfltuid=0,dfltgid=0,aname=/path/to/rootfs
+
+where <device> is the tag associated by the usb gadget transport.
+It is defined by the configfs instance name.
+
 Options
 =======
 
@@ -68,6 +82,7 @@ Options
 			virtio	  connect to the next virtio channel available
 				  (from QEMU with trans_virtio module)
 			rdma	  connect to a specified RDMA channel
+			usbg	  connect to a specified usb gadget channel
 			========  ============================================
 
   uname=name	user name to attempt mount as on the remote server.  The

net/9p/Kconfig

@@ -40,6 +40,12 @@ config NET_9P_XEN
 	  This builds support for a transport for 9pfs between
 	  two Xen domains.
 
+config NET_9P_USBG
+	bool "9P USB Gadget Transport"
+	depends on USB_GADGET
+	help
+	  This builds support for a transport for 9pfs over
+	  usb gadget.
 
 config NET_9P_RDMA
 	depends on INET && INFINIBAND && INFINIBAND_ADDR_TRANS

net/9p/Makefile

@@ -4,6 +4,7 @@ obj-$(CONFIG_NET_9P_FD) += 9pnet_fd.o
 obj-$(CONFIG_NET_9P_XEN) += 9pnet_xen.o
 obj-$(CONFIG_NET_9P_VIRTIO) += 9pnet_virtio.o
 obj-$(CONFIG_NET_9P_RDMA) += 9pnet_rdma.o
+obj-$(CONFIG_NET_9P_USBG) += 9pnet_usbg.o
 
 9pnet-objs := \
 	mod.o \
@@ -23,3 +24,6 @@ obj-$(CONFIG_NET_9P_RDMA) += 9pnet_rdma.o
 
 9pnet_rdma-objs := \
 	trans_rdma.o \
+
+9pnet_usbg-objs := \
+	trans_usbg.o \