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Date:   Thu, 2 Nov 2017 09:36:42 +0800
From:   Lu Baolu <>
To:     Greg Kroah-Hartman <>
Cc:     'Mathias Nyman' <>,
        Felipe Balbi <>,
        "" <>,
        "" <>
Subject: Re: [PATCH v3 2/3] usb: xhci: Add DbC support in xHCI driver

Hi Greg,

On 09/05/2017 09:58 AM, Lu Baolu wrote:
> 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/ttyDBC0 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.
> Signed-off-by: Lu Baolu <>
> ---
>  .../ABI/testing/sysfs-bus-pci-drivers-xhci_hcd     |   25 +
>  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-trace.h                      |   60 ++
>  drivers/usb/host/xhci.c                            |   10 +
>  drivers/usb/host/xhci.h                            |    1 +
>  9 files changed, 1959 insertions(+)
>  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


> +#define DBC_VENDOR_ID			0x1d6b	/* Linux Foundation 0x1d6b */
> +#define DBC_PRODUCT_ID			0x0004	/* device 0004 */

The DbC (xHCI DeBug Capability) is an optional functionality in
some xHCI host controllers. It will present a super-speed debug
device through the debug port after it is enabled.

The DbC register set defines an interface for system software
to specify the vendor id and product id of the debug device.
These two values will be presented by the debug device in its
device descriptor idVendor and idProduct fields.

Microsoft Windows have a well established protocol for
debugging over DbC. And it assigns below values for its use.

USB\VID_045E&PID_062D.DeviceDesc="Microsoft USB Debug Target"

I'm going to use 0x1d6b/0x0004 value pair for DbC use in
Linux. Do you approve me to do so?

Best regards,
Lu Baolu

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