linux-kernel - Re: [PATCH v5 0/4] usb: early: add support for early printk through USB3 debug port

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Message-ID: <58817C51.4080408@linux.intel.com>
Date:   Fri, 20 Jan 2017 10:56:17 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, tglx@...utronix.de,
        peterz@...radead.org, linux-usb@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v5 0/4] usb: early: add support for early printk through
 USB3 debug port

Hi Ingo,

On 01/19/2017 05:12 PM, Ingo Molnar wrote:
> * Lu Baolu <baolu.lu@...ux.intel.com> wrote:
>
>> xHCI debug capability (DbC) is an optional but standalone
>> functionality provided by an xHCI host controller. With DbC
>> hardware initialized, the system will present a debug device
>> through the USB3 debug port (normally the first USB3 port).
>> The debug device is fully compliant with the USB framework
>> and provides the equivalent of a very high performance (USB3)
>> full-duplex serial link between the debug host and target.
>> The DbC functionality is independent of xHCI host. There
>> isn't any precondition from xHCI host side for DbC to work.
>>
>> This patch set adds support for early printk functionality
>> through a USB3 debug port by 1) initializing and enabling
>> the DbC hardware during early boot; 2) registering a boot
>> console to the system so that early printk messages can go
>> through the USB3 debug port. It also includes some lines
>> of changes in usb_debug driver so that it can be bound when
>> a USB3 debug device is enumerated.
>>
>> This code is designed to be used only for kernel debugging
>> when machine crashes very early before the console code is
>> initialized. It makes the life of kernel debugging easier
>> when people work with a modern machine without any legacy
>> serial ports.
> BTW., just a side note, some kernel developers (like PeterZ - and I do it 
> sometimes too) remap early_printk to printk permanently and use it as their main 
> printk facility - because printk() reliability has suffered over the last couple 
> of years.
>
> So it's more than just early boot debugging - it's a very simple state-less 
> logging facility to an external computer.

Thanks for the information. I will rework this message.

Best regards,
Lu Baolu