linux-kernel - Re: [PATCH v2] usb: dwc3: gadget: Wait for end transfer complete before free irq

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Date:   Fri, 14 Oct 2016 10:36:25 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Felipe Balbi <balbi@...nel.org>
cc:     Baolin Wang <baolin.wang@...aro.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Mark Brown <broonie@...nel.org>,
        USB <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] usb: dwc3: gadget: Wait for end transfer complete
 before free irq

On Fri, 14 Oct 2016, Felipe Balbi wrote:

> argh, we have nested spinlocks :-( Well, we shouldn't call
> usb_ep_disable() with locks held AFAICR. So the following should be
> enough:
> 
> diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
> index 919d7d1b611c..2e9359c58eb9 100644
> --- a/drivers/usb/gadget/composite.c
> +++ b/drivers/usb/gadget/composite.c
> @@ -732,8 +732,10 @@ static void reset_config(struct usb_composite_dev *cdev)
>         DBG(cdev, "reset config\n");
>  
>         list_for_each_entry(f, &cdev->config->functions, list) {
> +               spin_unlock_irq(&cdev->lock);
>                 if (f->disable)
>                         f->disable(f);
> +               spin_lock_irq(&cdev->lock);
>  
>                 bitmap_zero(f->endpoints, 32);
>         }
> 
> Alan, do you remember if we have a requirement for not holding locks
> when calling usb_ep_disable()? I can't find Documentation about it, but
> history shows me that usb_ep_disable() was called without locks and IRQs
> enabled. Do you think we should update documentation about this?

I don't think there is any requirement for interrupts to be enabled 
when usb_ep_disable() runs.  At least, a quick check shows that both 
net2280 and dummy-hcd use spin_lock_irqsave() rather than spin_lock() 
in their disable routines.

Holding locks is a different story.  It should be okay for a gadget 
driver to hold one of its own locks when disabling an endpoint (which 
means that the gadget's disable routine shouldn't wait for a concurrent 
giveback to finish), but we might want to avoid holding a lock in the 
composite core.  Although even that might be okay -- I can't think of 
any reason why a udc driver would need to call back into the composite 
core while disabling an endpoint.  It should be a pretty self-contained 
operation.

Alan Stern