Date:   Thu, 2 Nov 2017 17:51:13 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Felipe Balbi <felipe.balbi@...ux.intel.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        'Mathias Nyman' <mathias.nyman@...ux.intel.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] usb: xhci: Add DbC support in xHCI driver

On Thu, Nov 02, 2017 at 12:38:57PM +0200, Felipe Balbi wrote:
> 
> 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?

> > As the above text said "well established protocol", I assumed we
> > implemented the same thing :)
> 
> It's "well established" from Microsoft's point of view :-) They have
> that same protocol running over USB, TCP, UDP...
> 
> It would be nice if we could ditch TTY and teach gdb about other
> transports, but then again, why bother if we can reuse code that already
> works? :-)

gdb knows about other transports, but I don't know about the kernel
interface to it :)

thanks,

greg k-h

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