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Date:   Fri, 03 Nov 2017 12:52:43 +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:
>> >>>> > >> 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.
>> >>> > >
>> >>> > > Ah, I didn't know about the protocol specifics here, if it is
>> >>> > > vendor-specific, then yes, we need our own id.
>> >> > 
>> >> > Great, thanks :-)
>> >> > 
>> >> > Let us know which one we're allowed to use and I'm sure Baolu can respin
>> >> > the patch in no time.
>> > Can I get a "full" description of what string this device id will
>> > reference?  Is it "Linux USB Debug Target" or something else?
>> >
>> 
>> Current manufacturer and product strings are set like this.
>> 
>> +#define DBC_STRING_MANUFACTURER	"Linux"
>> +#define DBC_STRING_PRODUCT		"Remote	GDB"
>> 
>> These are also place holders. We can change them to more meaningful strings.
>
> As the vendor id is assigned to "Linux Foundation" can we change that
> string please?
>
> And why not match what Microsoft does here, "USB Debug Target" makes
> more sense than "Remote GDB", right?

I agree. It just happens to talk remote GDB protocol through KGDB, but
we could run KDB directly or, even, come up with a new protocol.

> If so, please use device id 0x0010, I've reserved that for the driver
> now.

thanks Greg

-- 
balbi

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