linux-kernel - Re: [PATCH] usb: hub: Resume hubs to find newly connected device

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Message-ID: <CAAd53p5HfGz-D-QvYvPuDY4qLe0nYncY077=n-gvnYym4A8E0w@mail.gmail.com>
Date:   Fri, 10 Dec 2021 14:06:10 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     gregkh@...uxfoundation.org, mathias.nyman@...ux.intel.com,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Bixuan Cui <cuibixuan@...wei.com>,
        Andrew Lunn <andrew@...n.ch>,
        Chris Chiu <chris.chiu@...onical.com>,
        Rajat Jain <rajatja@...gle.com>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: hub: Resume hubs to find newly connected device

On Fri, Dec 10, 2021 at 12:04 AM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Thu, Dec 09, 2021 at 09:19:24AM +0800, Kai-Heng Feng wrote:
> > On Thu, Dec 9, 2021 at 5:45 AM Alan Stern <stern@...land.harvard.edu> wrote:
> > >
> > > On Wed, Dec 08, 2021 at 03:08:33PM +0800, Kai-Heng Feng wrote:
> > > > When a new USB device gets plugged to nested hubs, the affected hub,
> > > > which connects to usb 2-1.4-port2, doesn't report there's any change,
> > > > hence the nested hubs go back to runtime suspend like nothing happened:
> > >
> > > That's a bug in the hub.  When there's a change in the connection status
> > > of one of its ports, it should report this change to the kernel.
> >
> > I think it should, but when I searched through the USB spec and I
> > can't find anywhere specify hub requires to report it in change
> > status.
>
> USB-2.0 spec, section 11.24.2.7.2.1 (C_PORT_CONNECTION):
>
>         This bit is set when the PORT_CONNECTION bit changes because of an
>         attach or detach detect event (see Section 7.1.7.3). This bit will be
>         cleared to zero by a ClearPortFeature(C_PORT_CONNECTION) request or
>         while the port is in the Powered-off state.

It's indeed set for the hub's downstream facing port, and that's why
wake up the hub and check its ports can still find connect event.
But I can't find anywhere stats how hub's upstream facing port should be set.

>
> > > So because of this buggy hub, you now want to wake up _every_ hub in the
> > > system whenever any wakeup event occurs?  Is this really a good idea?
> > > Is there a better way to solve the problem, such as a special quirk
> > > flag?
> >
> > If there's no other activities, the USB hub should go back to suspend
> > immediately, so the impact is minimal.
>
> Not immediately, but after a few seconds.  However your patch will affect every
> hub, not just the one that the new device was plugged into.

Yes, that's the case here.

>
> > I've seen several similar bug reports so I think this solution should
> > be applied for all hubs.
>
> Maybe those bug reports all had something in common, such as the type of hub or
> the bus speed they were running at.  Did you check?

The hub in question is a SuperSpeed hub. Let me scan bug reports on
Launchpad and see what I can find.

Kai-Heng

>
> Alan Stern