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

From: Kai-Heng Feng <kai.heng.feng@canonical.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Mathias Nyman <mathias.nyman@intel.com>,
	gregkh@linuxfoundation.org, acelan.kao@canonical.com,
	linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] USB: Disable LPM on WD19's Realtek Hub during setting its ports to U0
Date: Mon, 6 Jan 2020 14:19:07 +0800	[thread overview]
Message-ID: <90B37743-30D1-41BB-8272-D5FBDC89C88F@canonical.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.2001041117130.7125-100000@netrider.rowland.org>

> 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.

Kai-Heng

> 
> So I think you don't need to lock the hub here.
> 
> Alan Stern
> 