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Date:   Thu, 02 Nov 2017 11:15:43 +0200
From:   Felipe Balbi <felipe.balbi@...ux.intel.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     'Mathias Nyman' <mathias.nyman@...ux.intel.com>,
        "linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] usb: xhci: Add DbC support in xHCI driver


Hi,

Greg Kroah-Hartman <gregkh@...uxfoundation.org> writes:
>> > 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 <baolu.lu@...ux.intel.com>
>> > ---
>> >  .../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
>> >
>> 
>> [snip]
>> 
>> > +#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?
>
> No.  Why can't you use the same ids as Windows?  This is implementing
> the same protocol, right?

the protocol running on top is 100% vendor specific. More than likely,
we would just run kgdb on top of this, right? We really don't support
microsoft's debug architecture.

-- 
balbi

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