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SubjectRe: [PATCH 3/3] USB: Disable LPM on WD19's Realtek Hub during setting its ports to U0
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.

Alan Stern

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