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

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Message-ID: <20201103200716.GA1538425@rowland.harvard.edu>
Date:   Tue, 3 Nov 2020 15:07:16 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Wesley Cheng <wcheng@...eaurora.org>
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 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).

Alan Stern