[v8,0/3] usb: gadget: 9pfs transport

This series is adding support to mount usb hostside exported 9pfs
filesystems via the usb gadget interface. It also includes a simple tool
(p9_fwd.py) to translate an tcp 9pfs transport and reuse it via the usb
interface.

    +--------------------------+    |    +--------------------------+
    |  9PFS mounting client    |    |    |  9PFS exporting server   |
 SW |                          |    |    |                          |
    |   (this:trans_usbg)      |    |    |(e.g. diod or nfs-ganesha)|
    +-------------^------------+    |    +-------------^------------+
                  |                 |                  |
                  |                 |           +------v------+
                  |                 |           |  p9_fwd.py  |
                  |                 |           +------^------+
                  |                 |                  |
------------------|------------------------------------|-------------
                  |                 |                  |
    +-------------v------------+    |    +-------------v------------+
    |                          |    |    |                          |
 HW |   USB Device Controller  <--------->   USB Host Controller    |
    |                          |    |    |                          |
    +--------------------------+    |    +--------------------------+

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.

One use-case is to use it as an alternative to NFS root booting during
the development of embedded Linux devices.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
---
Changes in v8:
- this time really added the req_lock spinlock
- Link to v7: https://lore.kernel.org/r/20240116-ml-topic-u9p-v7-0-3a1eeef77fbe@pengutronix.de

Changes in v7:
- added back the req_lock spinlock
- Link to v6: https://lore.kernel.org/r/20240116-ml-topic-u9p-v6-0-695977d76dff@pengutronix.de

Changes in v6:
- fixed the python script not to have path set by default
- improved the lock init
- fixed usb9pfs status change to connected
- Link to v5: https://lore.kernel.org/r/20240116-ml-topic-u9p-v5-0-5ed0abd53ef5@pengutronix.de

Changes in v5:
- fixed lockup in mount -> remount -> monut scenario
- improved p9_fwd transport script with more options
- Link to v4: https://lore.kernel.org/r/20240116-ml-topic-u9p-v4-0-722ed28b0ade@pengutronix.de

Changes in v4:
- reworked the naming scheme to be set by the configfs instance
- added conn_cancel function to properly stop the transfers
- ensured that umount -f will work even when the host side has crahed
- added all the review feedback from Andrzej Pietrasiewicz
- Link to v3: https://lore.kernel.org/r/20240116-ml-topic-u9p-v3-0-c62a36eccda1@pengutronix.de

Changes in v3:
- dropped patch "usb: gadget: legacy: add 9pfs multi gadget" as discussed with gregkh
- Link to v2: https://lore.kernel.org/r/20240116-ml-topic-u9p-v2-0-b46cbf592962@pengutronix.de

Changes in v2:
- improved the commit messages
- introduced an patch to move the header u_f.h to include/linux/usb to compile usb gadget functions treewide
- moved usbg gadget function to net/9p/
- adderessed several comments in function driver, like the cleanup path and kbuild errors
- improved the documentation in Documentation/filesystems/9p.rst
- Link to v1: https://lore.kernel.org/r/20240116-ml-topic-u9p-v1-0-ad8c306f9a4e@pengutronix.de

---
Michael Grzeschik (3):
      usb: gadget: function: move u_f.h to include/linux/usb/func_utils.h
      net/9p/usbg: Add new usb gadget function transport
      tools: usb: p9_fwd: add usb gadget packet forwarder script

 Documentation/filesystems/9p.rst                   |   58 +-
 drivers/usb/gadget/configfs.c                      |    2 +-
 drivers/usb/gadget/function/f_fs.c                 |    2 +-
 drivers/usb/gadget/function/f_hid.c                |    2 +-
 drivers/usb/gadget/function/f_loopback.c           |    2 +-
 drivers/usb/gadget/function/f_midi.c               |    2 +-
 drivers/usb/gadget/function/f_midi2.c              |    2 +-
 drivers/usb/gadget/function/f_sourcesink.c         |    2 +-
 drivers/usb/gadget/u_f.c                           |    2 +-
 .../gadget/u_f.h => include/linux/usb/func_utils.h |    2 +-
 net/9p/Kconfig                                     |    6 +
 net/9p/Makefile                                    |    4 +
 net/9p/trans_usbg.c                                | 1006 ++++++++++++++++++++
 tools/usb/p9_fwd.py                                |  243 +++++
 14 files changed, 1325 insertions(+), 10 deletions(-)
---
base-commit: 2c9b3512402ed192d1f43f4531fb5da947e72bd0
change-id: 20240116-ml-topic-u9p-895274530eb1

Best regards,