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Date:   Wed, 2 Oct 2019 11:47:39 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Kai-Heng Feng <kai.heng.feng@canonical.com>
cc:     mathias.nyman@intel.com, <gregkh@linuxfoundation.org>,
        <linux-usb@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Revert "usb: Avoid unnecessary LPM enabling and disabling
 during suspend and resume"
In-Reply-To: <20191002151512.28517-1-kai.heng.feng@canonical.com>
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On Wed, 2 Oct 2019, Kai-Heng Feng wrote:

> This reverts commit d590c23111505635e1beb01006612971e5ede8aa.
> 
> Dell WD15 dock has a topology like this:
> /:  Bus 04.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/2p, 10000M
>     |__ Port 1: Dev 2, If 0, Class=Hub, Driver=hub/7p, 5000M
>             |__ Port 2: Dev 3, If 0, Class=Vendor Specific Class, Driver=r8152, 5000M
> 
> Their IDs:
> Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
> Bus 004 Device 002: ID 0424:5537 Standard Microsystems Corp.
> Bus 004 Device 004: ID 0bda:8153 Realtek Semiconductor Corp.
> 
> Ethernet cannot be detected after plugging ethernet cable to the dock,
> the hub and roothub get runtime resumed and runtime suspended
> immediately:
> ...

> After some trial and errors, the issue goes away if LPM on the SMSC hub
> is disabled. Digging further, enabling and disabling LPM during runtime
> resume and runtime suspend respectively can solve the issue.
> 
> So bring back the old LPM behavior, which the SMSC hub inside Dell WD15
> depends on.
> 
> Fixes: d590c2311150 ("usb: Avoid unnecessary LPM enabling and disabling during suspend and resume")
> Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>

Maybe it would be better to have a VID/PID-specific quirk for this?

Alan Stern