linux-kernel - Re: [PATCH 1/2] usb: dwc3: gadget: Allow runtime suspend if UDC unbinded

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Date:   Tue, 3 Nov 2020 13:53:01 -0800
From:   Wesley Cheng <wcheng@...eaurora.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     balbi@...nel.org, gregkh@...uxfoundation.org,
        Thinh.Nguyen@...opsys.com, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org, jackp@...eaurora.org
Subject: Re: [PATCH 1/2] usb: dwc3: gadget: Allow runtime suspend if UDC
 unbinded



On 11/3/2020 12:07 PM, Alan Stern wrote:
> On Tue, Nov 03, 2020 at 11:02:25AM -0800, Wesley Cheng wrote:
>>
>>
>> On 10/28/2020 6:07 PM, Alan Stern wrote:
>>>> --- a/drivers/usb/dwc3/gadget.c
>>>> +++ b/drivers/usb/dwc3/gadget.c
>>>> @@ -1995,6 +1995,11 @@ static int dwc3_gadget_pullup(struct usb_gadget *g, int is_on)
>>>>  	unsigned long		flags;
>>>>  	int			ret;
>>>>  
>>>> +	if (pm_runtime_suspended(dwc->dev)) {
>>>> +		pm_request_resume(dwc->dev);
>>>> +		return 0;
>>>> +	}
>>>
>>> Isn't this racy?  What happens if the controller was active but a 
>>> runtime suspend occurs right here?
>>>
>>> Alan Stern
>>>
>>
>> Hi Alan,
>>
>> Ah, yes you're right.  I was hoping that the PM runtime layer would be
>> utilizing the spinlock when reading out the runtime status, but even
>> then, we wouldn't be able to catch intermediate states with this API
>> (i.e. RPM_RESUMING or RPM_SUSPENDING)
>>
>> Tried a few different approaches, and came up with something like the
>> following:
>>
>> static int dwc3_gadget_pullup(struct usb_gadget *g, int is_on)
>> {
>> ...
>> 	ret = pm_runtime_get_sync(dwc->dev);
>> 	if (!ret) {
>> 		pm_runtime_put(dwc->dev);
>> 		return 0;
>> 	}
>> 	...
>> 	pm_runtime_put(dwc->dev);
>>
>> 	return 0;
>> }
>>
>> I think this should be good to address your concern.  The only way we
>> would be able to ensure that the runtime PM state doesn't enter
>> idle/suspend is if we increment the usage count for the duration we're
>> accessing the DWC3 registers.  With the synchronous PM runtime resume
>> call, we can also ensure that no pending runtime suspends are executing
>> in parallel while running this code.
> 
> That's correct.
> 
>> The check for the zero return value would be for avoiding running the
>> DWC3 run stop sequence for the case where we executed the runtime PM
>> resume, as the DWC3 runtime PM resume routine will set the run stop bit
>> in there.
> 
> If you need to add an explanation of this subtle point in your email 
> message, then you should add a similar explanation as a comment in the 
> code.  And don't forget to check for ret < 0 (i.e., a resume error).
> 

Hi Alan,

Got it, will do.  Yes, I'll include the error conditions as well in the
actual change.  Thanks again!

Thanks

Regards,
Wesley Cheng

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