Re: RT-friendly IRQ management in USB

Jiri Kosina wrote:
> On Tue, 3 Jul 2007, Felipe Balbi wrote:
> 
>> So looks like we don't have (yet) a way to make it work nicely in both 
>> cases... does anyone has a clue about how to implement this one??
> 
> You of course still can
> 
> 	local_irq_save(flags);
> 	spin_lock(&some_lock);
> 	...
> 	spin_unlock(&some_lock);
> 	usb_hcd_giveback_urb(hcd, urb);
> 	local_irq_restore(flags);
> 
> which is safe even against future removal of preempt inc/dec from 
> spin_lock_irq{save,restore}() functions.
> 

Unfortunately that would not work in the RT case either. Because in RT, 
that spin_lock can schedule, and we are not allowed to schedule with 
interrupts disabled.

What we could do that would work in all cases (but is butt-ugly) is the 
following:

	{
		DEFINE_SPINLOCK(local);
		unsigned long flags;

		spin_lock_irqsave(&local, flags);
		spin_lock(&some_lock);
		...
		spin_unlock(&some_lock);
		usb_hcb_giveback_urb(hcd, urb);
		spin_unlock_irqrestore(&local, flags);
	}


Because the "local" lock is on the stack, it will never be in 
contention. Which means that it basically acts like a local_irq_save. 
The benefit is that it also works in RT, because that case the 
interrupts would *not* be disabled in that entire path. For those that 
don't know, RT has interrupts running as threads, so the need here to 
disable interrupts is gone.

I can send a note to Ingo, and see if he has better ideas. Perhaps 
there's a way that we can do a local_irq_restore that matches spinlocks. 
Maybe we can add a primitive of

    local_irqs_restore_from_spinlock(flags);

This would document that the local_irq_save came from a spinlock as it 
will allow things like this to seamlessly change for RT.

(/me decides to just add Ingo on this thread)

-- Steve


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