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

[Kai-Heng Feng <kai.heng.feng@canonical.com>]

> On Jan 4, 2020, at 00:54, Alan Stern <stern@rowland.harvard.edu> wrote:
> 
> On Sat, 4 Jan 2020, Kai-Heng Feng wrote:
> 
>> Hi Alan,
>> 
>>> On Jan 3, 2020, at 23:21, Alan Stern <stern@rowland.harvard.edu> 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@canonical.com>
>>>> ---
>>>> 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?

There can be multiple usb_port_resume() run at the same time for different ports, so this is to prevent LPM enable/disable race.

Kai-Heng

> 
> Alan Stern