usb: xhci: Add debug capability support in xhci

Hi,

This series is for xHCI debug capability (spec section 7.6.8) support in the xHCI driver.

xHCI compatible USB host controllers(i.e. super-speed USB3 controllers) can be implemented with the Debug Capability(DbC). It presents a debug device which is fully compliant with the USB framework and provides the equivalent of a very high performance full-duplex serial link. The debug capability operation model and registers interface are defined in 7.6.8 of the xHCI specification, revision 1.1.

The DbC debug device shares a root port with the xHCI host. By default, the debug capability is disabled and the root port is assigned to xHCI. When the DbC is enabled, the root port will be assigned to the DbC debug device, and the xHCI sees nothing on this port. This implementation uses a sysfs node named <dbc> under the xHCI device to manage the enabling and disabling of the debug capability.

When the debug capability is enabled, it will present a debug device through the debug port. This debug device is fully compliant with the USB3 framework, and it can be enumerated by a debug host on the other end of the USB link. As soon as the debug device is configured, a TTY serial device named /dev/ttyGSn will be created.

One use of this link is running a login service on the debug target. Hence it can be remote accessed by a debug host. Another use case can probably be found in servers. It provides a peer-to-peer USB link between two host-only machines. This provides a reasonable out-of-band communication method between two servers.

Best regards,
Lu Baolu

---
Change log:
v1->v2:
- Add a new patch to move u_serial.c from drivers/usb/gadget/function to drivers/usb/common/ and move u_serial.h to include/linux/usb/.
v2->v3:
- Remove the use of u_serial and add a new tty glue for debug capability.

Lu Baolu (3):
usb: xhci: Make some static functions global
usb: xhci: Add DbC support in xHCI driver
usb: doc: Update document for USB3 debug port usage

 .../ABI/testing/sysfs-bus-pci-drivers-xhci_hcd     |   25 +
 Documentation/driver-api/usb/usb3-debug-port.rst   |   68 ++
 drivers/usb/host/Kconfig                           |    9 +
 drivers/usb/host/Makefile                          |    5 +
 drivers/usb/host/xhci-dbgcap.c                     | 1016 ++++++++++++++++++++
 drivers/usb/host/xhci-dbgcap.h                     |  247 +++++
 drivers/usb/host/xhci-dbgtty.c                     |  586 +++++++++++
 drivers/usb/host/xhci-mem.c                        |   94 +-
 drivers/usb/host/xhci-ring.c                       |    4 +-
 drivers/usb/host/xhci-trace.h                      |   60 ++
 drivers/usb/host/xhci.c                            |   10 +
 drivers/usb/host/xhci.h                            |   17 +-
 12 files changed, 2099 insertions(+), 42 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-bus-pci-drivers-xhci_hcd
 create mode 100644 drivers/usb/host/xhci-dbgcap.c
 create mode 100644 drivers/usb/host/xhci-dbgcap.h
 create mode 100644 drivers/usb/host/xhci-dbgtty.c

2.7.4