[PATCH v5 0/4] usb: early: add support for early printk through USB3 debug port

[Lu Baolu]

xHCI debug capability (DbC) is an optional but standalone
functionality provided by an xHCI host controller. With DbC
hardware initialized, the system will present a debug device
through the USB3 debug port (normally the first USB3 port).
The debug device is fully compliant with the USB framework
and provides the equivalent of a very high performance (USB3)
full-duplex serial link between the debug host and target.
The DbC functionality is independent of xHCI host. There
isn't any precondition from xHCI host side for DbC to work.

This patch set adds support for early printk functionality
through a USB3 debug port by 1) initializing and enabling
the DbC hardware during early boot; 2) registering a boot
console to the system so that early printk messages can go
through the USB3 debug port. It also includes some lines
of changes in usb_debug driver so that it can be bound when
a USB3 debug device is enumerated.

This code is designed to be used only for kernel debugging
when machine crashes very early before the console code is
initialized. It makes the life of kernel debugging easier
when people work with a modern machine without any legacy
serial ports.

---
Change log:
v4->v5:
  - add raw_spin_lock to make xdbc_bulk_write() reentrant. 

v3->v4:
  - Rename the document with .dst suffix.
  - Add the list of hardware that has been succesfuly
    tested on in the document.

v2->v3:
  - Removed spinlock usage.
  - Removed work queue usage.
  - Refined the user guide document.

v1->v2:
  - Refactor the duplicate code in xdbc_early_start() and
    xdbc_handle_external_reset().
  - Free resources when hardware not used any more.
  - Refine the user guide document.

Lu Baolu (4):
  usb: dbc: early driver for xhci debug capability
  x86: add support for earlyprintk via USB3 debug port
  usb: serial: usb_debug: add support for dbc debug device
  usb: doc: add document for USB3 debug port usage

 Documentation/kernel-parameters.txt   |    1 +
 Documentation/usb/usb3-debug-port.rst |   95 +++
 arch/x86/Kconfig.debug                |   14 +
 arch/x86/kernel/early_printk.c        |    5 +
 arch/x86/kernel/setup.c               |    7 +
 drivers/usb/Kconfig                   |    3 +
 drivers/usb/Makefile                  |    2 +-
 drivers/usb/early/Makefile            |    1 +
 drivers/usb/early/xhci-dbc.c          | 1068 +++++++++++++++++++++++++++++++++
 drivers/usb/early/xhci-dbc.h          |  205 +++++++
 drivers/usb/serial/usb_debug.c        |   28 +-
 include/linux/usb/xhci-dbgp.h         |   22 +
 12 files changed, 1447 insertions(+), 4 deletions(-)
 create mode 100644 Documentation/usb/usb3-debug-port.rst
 create mode 100644 drivers/usb/early/xhci-dbc.c
 create mode 100644 drivers/usb/early/xhci-dbc.h
 create mode 100644 include/linux/usb/xhci-dbgp.h

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2.1.4