Re: [PATCH 00/12] usb: early: add support for early printk through USB3 debug port

From: Lu, Baolu
Date: Tue Nov 03 2015 - 19:42:46 EST

On 11/03/2015 05:42 PM, Dave Young wrote:
On 10/28/15 at 04:00pm, Lu Baolu wrote:
This patch series adds support for early printk through USB3 debug port.
USB3 debug port is described in xHCI specification as an optional extended

The first patch adds a file in debugfs, through which users can check
whether the debug capability is supported by a specific host controller.

Patch 2 to 10 add the driver for xHCI debug capability. It interfaces with
the register set and provides the required ops (read/write/control) to upper
layers. Early printk is one consumer of these ops. The hooks for early printk
are introduced in patch 9. This design is similar to what we have done in

Patch 11 is a minor change to usb_debug module. This change is required to
bind usb_debug with the USB3 debug device.

Patch 12 is the design document and user guide.

Nice work, I want to try your patches. But I have only one machine with
debug capability, I think I can use it as debug target. Can I use another
machine with usb 3.0 but no debug capability as the debug host?

Yes, you can. The debug host doesn't require debug capability.

Please follow guide in patch 12 when you try it. It has been verified
on Intel Skylake machine.

BTW, I hacked a cable according to usb 3 spec, cut vbus/d+/d-, cross wired
ss pins.

Good luck. I just bought one from


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