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
--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project
