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Date: Sat, 4 Jan 2020 11:20:57 -0500 (EST) From: Alan Stern <stern@...land.harvard.edu> To: Kai-Heng Feng <kai.heng.feng@...onical.com> cc: Mathias Nyman <mathias.nyman@...el.com>, <gregkh@...uxfoundation.org>, <acelan.kao@...onical.com>, <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org> Subject: Re: [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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