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

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