On Sat, 4 Jan 2020, Kai-Heng Feng wrote:
> Hi Alan,
>
> > On Jan 3, 2020, at 23:21, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Fri, 3 Jan 2020, Kai-Heng Feng wrote:
> >
> >> Realtek Hub (0bda:0x0487) used in Dell Dock WD19 sometimes drops off the
> >> bus when bringing underlying ports from U3 to U0.
> >>
> >> After some expirements and guessworks, the hub itself needs to be U0
> >> during setting its port's link state back to U0.
> >>
> >> So add a new quirk to let the hub disables LPM on setting U0 for its
> >> downstream facing ports.
> >>
> >> Signed-off-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>
> >> ---
> >> drivers/usb/core/hub.c | 12 ++++++++++--
> >> drivers/usb/core/quirks.c | 7 +++++++
> >> include/linux/usb/quirks.h | 3 +++
> >> 3 files changed, 20 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> >> index f229ad6952c0..35a035781c5a 100644
> >> --- a/drivers/usb/core/hub.c
> >> +++ b/drivers/usb/core/hub.c
> >> @@ -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?
Alan Stern
