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

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Message-ID: <20170126102848.GZ6515@twins.programming.kicks-ass.net>
Date:   Thu, 26 Jan 2017 11:28:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        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

On Thu, Jan 26, 2017 at 08:19:37AM +0100, 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?
> 
> 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].

That'll be fun when printk() == early_printk() :-)

I myself wouldn't mind the system getting stuck until the link is
re-established. My own damn fault for taking that cable out etc.