Re: [PATCH 3/3] USB: Disable LPM on WD19's Realtek Hub during setting its ports to U0 - Kai-Heng Feng

From: Kai-Heng Feng <kai.heng.feng@canonical.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Mathias Nyman <mathias.nyman@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	AceLan Kao <acelan.kao@canonical.com>,
	USB list <linux-usb@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/3] USB: Disable LPM on WD19's Realtek Hub during setting its ports to U0
Date: Fri, 10 Jan 2020 15:35:39 +0800	[thread overview]
Message-ID: <CAAd53p7NhhQL=5teoaLQ4r5Y-QDGysGoXS-wwGc1fh59rfYu2g@mail.gmail.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.2001061007070.1514-100000@iolanthe.rowland.org>

On Mon, Jan 6, 2020 at 11:08 PM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Mon, 6 Jan 2020, Kai-Heng Feng wrote:
>
> > > On Jan 5, 2020, at 00:20, Alan Stern <stern@rowland.harvard.edu> wrote:
> > >
> > > 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.
> >
> > For enable/disable LPM itself, there's no race.
> > But the lock here is to protect hub_set_port_link_state().
> > If we don't lock the hub, other instances of usb_port_resume()
> > routine can enable LPM and we want the LPM stays disabled until
> > hub_set_port_link_state() is done.
>
> That's what I was trying to explain above.  Other instances of
> usb_port_resume() _can't_ enable LPM while this instance is running,
> because the lpm_disable_counter value will be > 0.

Yes you are right, there's actually no race.
The hub is still a bit flaky with this approach, so I'll resend a v2
to simply disable LPM for this hub.

Kai-Heng

>
> Alan Stern
>