Re: [PATCH v3 2/3] usb: xhci: Add DbC support in xHCI driver

From: Felipe Balbi
Date: Thu Nov 02 2017 - 05:16:13 EST



Hi,

Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
>> > ---
>> > .../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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