netdev - Re: [PATCH 2/2] at86rf230: change irq handling to prevent lockups with edge type irq

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Date:	Sun, 07 Apr 2013 22:52:38 +0200
From:	Sascha Herrmann <sascha@...nvbi.de>
To:	Werner Almesberger <werner@...esberger.net>
CC:	netdev@...r.kernel.org, linux-zigbee-devel@...ts.sourceforge.net
Subject: Re: [PATCH 2/2] at86rf230: change irq handling to prevent lockups
 with edge type irq

On 06.04.2013 16:20, Werner Almesberger wrote:

> Sascha Herrmann wrote:
>> The trigger level isn't configurable in the rf230,
> 
> Right, I should have mentioned that the polarity can only be set on
> the AT86RF231. The 230 has to make do with rising edge or high level.


Oh, I didn't realized, that this is possible at all with the rf231...

 
>> I fear the sollution to read the interrupt status register in a loop
>> (as suggested in your earlier message) would leave chances for race
>> conditions or spurious interrupts, depending on wheter interrupts are
>> enabled before or after reading the status register. 
> 
> I don't think the analysis is worse than for any other solution.
> There are also tools and methods that can help if it becomes too
> much of a headache.
> 
> If you don't like the loop, a double read without loop would work in
> this case as well:
> 
> 	irq = read_and_clear_interrupt();
> 	enable_irq();
> 	irq |= read_and_clear_interrupt();
> 	...


Maybe one way to eliminate the extra latency of the second register read
would be to split the interrupt handling function into a generic part
and two different functions to handle the different types of interrupts:

	static void at86rf230_irqwork_level(void) {
		__at86rf230_irqwork();

		spin_lock_irqsave(&lp->lock, flags);
		lp->irq_busy = 0;
		enable_irq()
		spin_unlock_irqrestore(&lp->lock, flags);
	}

For edge type configuration the call to enable_irq() must be removed.
The same would be required for the isr function. The probe function
could than decide, which isr function should be registered for the
interrupt.

> By the way, once we switch to early RX_ON, I think we'll have the
> problem that two TRX_END interrupts may be generated without any
> host action between them, in which case the first will be
> interpreted as the end of sending and the second will be ignored,
> leaving a received frame in the buffer, which in turn leaves
> dynamic buffer protection on and thus prevents any further
> reception.


I think this is right, we should have an eye on this when working on the
early RX_ON...

> Not sure yet how to solve this. I also don't know how bad our
> latencies are, but I fear that they can at times be substantial.
> Already a single register access with spi-gpio takes some 80 us
> (measured on an otherwise idle Ben, 336 MHz MIPS).
> 
>> Surely for this option, the assumption that (at least nearly) every
>> platform supports edge type interrupts should hold.
> 
> I think you're on relatively safe ground with the assumption that
> most relevant platforms per se can do it. But if the interrupt line
> happens to be shared with some other device, then level trigger is
> quite popular.


If you think the solution above would be ok, I could try to send a
version which allows the configuration of trigger type and level.

Thanks,
Sascha

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