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Date:   Sat, 4 Jan 2020 11:20:57 -0500 (EST)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Kai-Heng Feng <kai.heng.feng@canonical.com>
cc:     Mathias Nyman <mathias.nyman@intel.com>,
        <gregkh@linuxfoundation.org>, <acelan.kao@canonical.com>,
        <linux-usb@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/3] USB: Disable LPM on WD19's Realtek Hub during setting
 its ports to U0
In-Reply-To: <97F72C66-8D9B-4316-B096-1993FD18CF56@canonical.com>
Message-ID: <Pine.LNX.4.44L0.2001041117130.7125-100000@netrider.rowland.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Sat, 4 Jan 2020, Kai-Heng Feng wrote:

> >>>> @@ -3533,9 +3533,17 @@ int usb_port_resume(struct usb_device *udev, pm_message_t msg)
> >>>> 	}
> >>>> 
> >>>> 	/* see 7.1.7.7; affects power usage, but not budgeting */
> >>>> -	if (hub_is_superspeed(hub->hdev))
> >>>> +	if (hub_is_superspeed(hub->hdev)) {
> >>>> +		if (hub->hdev->quirks & USB_QUIRK_DISABLE_LPM_ON_U0) {
> >>>> +			usb_lock_device(hub->hdev);
> >>>> +			usb_unlocked_disable_lpm(hub->hdev);
> >>>> +		}
> >>>> 		status = hub_set_port_link_state(hub, port1, USB_SS_PORT_LS_U0);
> >>>> -	else
> >>>> +		if (hub->hdev->quirks & USB_QUIRK_DISABLE_LPM_ON_U0) {
> >>>> +			usb_unlocked_enable_lpm(hub->hdev);
> >>>> +			usb_unlock_device(hub->hdev);
> >>> 
> >>> The locking here seems questionable.  Doesn't this code sometimes get
> >>> called with the hub already locked?  Or with the child device locked
> >>> (in which case locking the hub would violate the normal locking order:  
> >>> parent first, child second)?
> > 
> > I did a little checking.  In many cases the child device _will_ be 
> > locked at this point.
> > 
> >> Maybe introduce a new lock? The lock however will only be used by this specific hub.
> >> But I still want the LPM can be enabled for this hub.
> > 
> > Do you really need to lock the hub at all?  What would the lock protect 
> > against?
> 
> There can be multiple usb_port_resume() run at the same time for different ports, so this is to prevent LPM enable/disable race.

But there can't really be an LPM enable/disable race, can there?  The 
individual function calls are protected by the bandwidth mutex taken by 
the usb_unlocked_{en|dis}able_lpm routines, and the overall LPM setting 
is controlled by the hub device's lpm_disable_counter.

So I think you don't need to lock the hub here.

Alan Stern