tools: usb: p9_fwd: add usb gadget packet forwarder script

This patch is adding an small python tool to forward 9pfs requests
from the USB gadget to an existing 9pfs TCP server. Since currently all
9pfs servers lack support for the usb transport this tool is an useful
helper to get started.

Refer the Documentation section "USBG Example" in
Documentation/filesystems/9p.rst on how to use it.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Tested-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Link: https://lore.kernel.org/r/20240116-ml-topic-u9p-v12-3-9a27de5160e0@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 41 insertions, 0 deletions

diff --git a/Documentation/filesystems/9p.rst b/Documentation/filesystems/9p.rst
index 10cf79dc287f..2cc85f3e8659 100644
--- a/Documentation/filesystems/9p.rst
+++ b/Documentation/filesystems/9p.rst
@@ -67,6 +67,47 @@ To mount a 9p FS on a USB Host accessible via the gadget as root filesystem::
 where <device> is the tag associated by the usb gadget transport.
 It is defined by the configfs instance name.
 
+USBG Example
+============
+
+The USB host exports a filesystem, while the gadget on the USB device
+side makes it mountable.
+
+Diod (9pfs server) and the forwarder are on the development host, where
+the root filesystem is actually stored. The gadget is initialized during
+boot (or later) on the embedded board. Then the forwarder will find it
+on the USB bus and start forwarding requests.
+
+In this case the 9p requests come from the device and are handled by the
+host. The reason is that USB device ports are normally not available on
+PCs, so a connection in the other direction would not work.
+
+When using the usbg transport, for now there is no native usb host
+service capable to handle the requests from the gadget driver. For
+this we have to use the extra python tool p9_fwd.py from tools/usb.
+
+Just start the 9pfs capable network server like diod/nfs-ganesha e.g.:
+
+        $ diod -f -n -d 0 -S -l 0.0.0.0:9999 -e $PWD
+
+Optionaly scan your bus if there are more then one usbg gadgets to find their path:
+
+        $ python $kernel_dir/tools/usb/p9_fwd.py list
+
+        Bus | Addr | Manufacturer     | Product          | ID        | Path
+        --- | ---- | ---------------- | ---------------- | --------- | ----
+          2 |   67 | unknown          | unknown          | 1d6b:0109 | 2-1.1.2
+          2 |   68 | unknown          | unknown          | 1d6b:0109 | 2-1.1.3
+
+Then start the python transport:
+
+        $ python $kernel_dir/tools/usb/p9_fwd.py --path 2-1.1.2 connect -p 9999
+
+After that the gadget driver can be used as described above.
+
+One use-case is to use it as an alternative to NFS root booting during
+the development of embedded Linux devices.
+
 Options
 =======