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    SubjectRe: [PATCH v7 5/5] usb: doc: add document for USB3 debug port usage
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    Hi,

    On 02/14/2017 11:45 AM, Peter Chen wrote:
    > On Tue, Feb 14, 2017 at 10:27 AM, Lu Baolu <baolu.lu@linux.intel.com> wrote:
    >
    >> Add Documentation/usb/usb3-debug-port.rst. This document includes
    >> the user guide for USB3 debug port.
    >>
    >> Cc: linux-doc@vger.kernel.org
    >> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
    >> ---
    >> Documentation/usb/usb3-debug-port.rst | 98 ++++++++++++++++++++++++++++++
    >> +++++
    >> 1 file changed, 98 insertions(+)
    >> create mode 100644 Documentation/usb/usb3-debug-port.rst
    >>
    >> diff --git a/Documentation/usb/usb3-debug-port.rst
    >> b/Documentation/usb/usb3-debug-port.rst
    >> new file mode 100644
    >> index 0000000..9eddb3a
    >> --- /dev/null
    >> +++ b/Documentation/usb/usb3-debug-port.rst
    >> @@ -0,0 +1,98 @@
    >> +===============
    >> +USB3 debug port
    >> +===============
    >> +
    >> +:Author: Lu Baolu <baolu.lu@linux.intel.com>
    >> +:Date: January 2017
    >> +
    >> +GENERAL
    >> +=======
    >> +
    >> +This is a HOWTO for using USB3 debug port on x86 systems.
    >> +
    >> +Before using any kernel debugging functionality based on USB3
    >> +debug port, you need to check 1) whether debug port is supported
    >> +by the xHCI host; 2) which port is used for debugging purposes
    >> +(normally the first USB3 root port). You must have a USB 3.0
    >> +super-speed A-to-A debugging cable to connect the debug target
    >> +with a debug host. In this document, "debug target" stands for
    >> +the system under debugging, and "debug host" stands for a
    >> +stand-alone system that is able to talk to the debugging target
    >> +through the USB3 debug port.
    >> +
    >> +EARLY PRINTK
    >> +============
    >> +
    >> +On the debug target system, you need to customize a debugging
    >> +kernel with CONFIG_EARLY_PRINTK_USB_XDBC enabled. And, add
    >> +below kernel boot parameter::
    >> +
    >> + "earlyprintk=xdbc"
    >> +
    >> +If there are multiple xHCI controllers in the system, you can
    >> +append a host contoller index to this kernel parameter. This
    >> +index starts from 0.
    >> +
    >> +If you are going to use the "keep" option defined by the
    >> +early printk framework to keep the boot console alive after
    >> +early boot, you'd better add below kernel boot parameter::
    >> +
    >> + "usbcore.autosuspend=-1"
    >> +
    >> +On the debug host side, you don't need to customize the kernel,
    >> +but you'd better disable usb subsystem runtime power management
    >> +by adding below kernel boot parameter::
    >> +
    >> + "usbcore.autosuspend=-1"
    >> +
    >>
    > Just curious, why autosuspend needs to be disabled for this function?

    This implementation doesn't support suspend/resume yet.

    Best regards,
    Lu Baolu

    >
    > BR,
    > Peter Chen
    >

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