linux-kernel - Re: [PATCH 09/10] usb: Add USB subsystem notifications [ver #3]

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Date:   Tue, 11 Jun 2019 09:53:03 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Felipe Balbi <felipe.balbi@...ux.intel.com>
cc:     Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Howells <dhowells@...hat.com>, <viro@...iv.linux.org.uk>,
        <linux-usb@...r.kernel.org>, <raven@...maw.net>,
        <linux-fsdevel@...r.kernel.org>, <linux-api@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <keyrings@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/10] usb: Add USB subsystem notifications [ver #3]

On Tue, 11 Jun 2019, Felipe Balbi wrote:

> >> >> > So for "severe" issues, yes, we should do this, but perhaps not for all
> >> >> > of the "normal" things we see when a device is yanked out of the system
> >> >> > and the like.
> >> >> 
> >> >> Then what counts as a "severe" issue?  Anything besides enumeration 
> >> >> failure?
> >> >
> >> > Not that I can think of at the moment, other than the other recently
> >> > added KOBJ_CHANGE issue.  I'm sure we have other "hard failure" issues
> >> > in the USB stack that people will want exposed over time.
> >> 
> >> From an XHCI standpoint, Transaction Errors might be one thing. They
> >> happen rarely and are a strong indication that the bus itself is
> >> bad. Either bad cable, misbehaving PHYs, improper power management, etc.
> >
> > Don't you also get transaction errors if the user unplugs a device in 
> > the middle of a transfer?  That's not the sort of thing we want to sent 
> > notifications about.
> 
> Mathias, do we get Transaction Error if user removes cable during a
> transfer? I thought we would just get Port Status Change with CC bit
> cleared, no?

Even if xHCI doesn't give Transaction Errors when a cable is unplugged 
during a transfer, other host controllers do.  Sometimes quite a lot -- 
they continue to occur until the kernel polls the parent hub's 
interrupt ep and learns that the port is disconnected, which can take 
up to 250 ms.

Alan Stern