linux-kernel - Re: [PATCH v5 1/4] usb: dbc: early driver for xhci debug capability

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Message-ID: <589C0525.3050006@linux.intel.com>
Date:   Thu, 9 Feb 2017 13:59:01 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, tglx@...utronix.de,
        linux-usb@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH v5 1/4] usb: dbc: early driver for xhci debug capability

Hi Ingo,

On 01/26/2017 03:19 PM, Ingo Molnar wrote:
> * Lu Baolu <baolu.lu@...ux.intel.com> wrote:
>
>> Fair enough.
>>
>> USB connection is stable enough, unless the user unplugs the
>> USB cable during debugging.
> What does the hardware do in this case? The XHCI registers are in the host 
> hardware, so they won't disappear, right? Is there some cable connection status 
> bit we can extract without interrupts?

The hardware has a register for the debug port status. Software is able to
check cable unplugging through this register.

>
> I.e. if there's any polling component then it would be reasonable to add an error 
> component: poll the status and if it goes 'disconnected' then disable early-printk 
> altogether in this case and trigger an emergency printk() so that there's chance 
> that the user notices [if the system does not misbehave otherwise].

Sure. I will add a kernel thread to polling the errors. I will show you the
code in the new upcoming version.

Best regards,
Lu Baolu

>
> I.e. try to be as robust and informative as lockdep - yet don't lock up the host 
> kernel: lockdep too is called from very deep internals, there are various 
> conditions where it sees corrupt data structures (i.e. a 'disconnect' - a system 
> environment outside the normal bounds of operation), yet of the kernel and over 
> the last 10+ years of lockdep's existence we had very, very few cases of lockdep 
> itself locking up and behaving unpredictably.
>
> Thanks,
>
> 	Ingo
>