linux-kernel - Re: [PATCH v3] usb: dwc3: Stop active transfers before halting the controller

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Date:   Fri, 25 Sep 2020 09:06:47 +0300
From:   Felipe Balbi <balbi@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Wesley Cheng <wcheng@...eaurora.org>, gregkh@...uxfoundation.org,
        Thinh.Nguyen@...opsys.com, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org, jackp@...eaurora.org
Subject: Re: [PATCH v3] usb: dwc3: Stop active transfers before halting the
 controller


Hi,

Alan Stern <stern@...land.harvard.edu> writes:
>> > Hence, the reason if there was already a pending IRQ triggered, the
>> > dwc3_gadget_disable_irq() won't ensure the IRQ is handled.  We can do
>> > something like:
>> > if (!is_on)
>> > 	dwc3_gadget_disable_irq()
>> > synchronize_irq()
>> > spin_lock_irqsave()
>> > if(!is_on) {
>> > ...
>> >
>> > But the logic to only apply this on the pullup removal case is a little
>> > messy.  Also, from my understanding, the spin_lock_irqsave() will only
>> > disable the local CPU IRQs, but not the interrupt line on the GIC, which
>> > means other CPUs can handle it, unless we explicitly set the IRQ
>> > affinity to CPUX.
>> 
>> Yeah, the way I understand this can't really happen. But I'm open to
>> being educated. Maybe Alan can explain if this is really possibility?
>
> It depends on the details of the hardware, but yes, it is possible in
> general for an interrupt handler to run after you have turned off the
> device's interrupt-request line.  For example:
>
> 	CPU A				CPU B
> 	---------------------------	----------------------
> 	Gets an IRQ from the device
> 	Calls handler routine		spin_lock_irq
> 	  spin_lock_irq			Turns off the IRQ line
> 	  ...spins...			spin_unlock_irq
> 	  Rest of handler runs
> 	  spin_unlock_irq
>
> That's why we have synchronize_irq().  The usual pattern is something
> like this:
>
> 	spin_lock_irq(&priv->lock);
> 	priv->disconnected = true;
> 	my_disable_irq(priv);
> 	spin_unlock_irq(&priv->lock);
> 	synchronize_irq(priv->irq);
>
> And of course this has to be done in a context that can sleep.
>
> Does this answer your question?

It does, thank you Alan. It seems like we don't need a call to
disable_irq(), only synchronize_irq() is enough, however it should be
called with spinlocks released, not held.

Thanks

-- 
balbi

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