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

From: Ingo Molnar
Date: Thu Jan 26 2017 - 02:19:53 EST

* Lu Baolu <> 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].

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.