Date:	Wed, 29 Oct 2014 10:11:45 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Jason Cooper <jason@...edaemon.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/3] genirq: Add support for priority-drop/deactivate
 interrupt controllers

On 28/10/14 20:14, Thomas Gleixner wrote:
> On Tue, 28 Oct 2014, Marc Zyngier wrote:
>> On 28/10/14 15:32, Thomas Gleixner wrote:
>>> Let me make a few assumptions and correct me if I'm wrong as usual.
>>>
>>> 1) The startup/shutdown procedure for such an interrupt is the
>>>    expensive mask/unmask which you want to avoid for the actual
>>>    handling case
>>
>> Indeed.
>>
>>> 2) In case of an actual interrupt the flow (ignoring locking) is:
>>>
>>>    handle_xxx_irq()
>>>
>>> 	mask_irq();	/* chip->irq_mask() maps to EOI */
>>>
>>> 	if (!action || irq_disabled())
>>> 	   return;
>>>
>>> 	handle_actions();
>>>
>>> 	if (irq_threads_active() || irq_disabled())
>>> 	   return;
>>>
>>> 	unmask_irq();	/* chip->irq_unmask() maps to DIR */
>>>
>>>   So that is handle_level_irq() with the chip callbacks being:
>>>
>>>    irq_startup  = gic_unmask
>>>    irq_shutdown = gic_mask
>>>    irq_unmask	= gic_dir
>>>    irq_mask	= gic_eoi
>>
>> So while this works really well for the interrupt handling part, it will
>> break [un]mask_irq(). This is because you can only write to EOI for an
>> interrupt that you have ACKed just before (anything else and the GIC
>> state machine goes crazy). Basically, any use for EOI/DIR outside of the
>> interrupt context itself (hardirq or thread) is really dangerous.
> 
> I really doubt that the DIR invocation is dangerous outside of
> interrupt context. Otherwise your threaded scheme would not work at
> all as the DIR invocation happens in thread context.

There is a small restriction in the use of DIR (quoting the spec):

"If the interrupt identified in the GICC_DIR is not active, and is not a
spurious interrupt, the effect of the register write is UNPREDICTABLE.
This means any GICC_DIR write must identify an interrupt for which there
has been a valid GICC_EOIR or GICC_AEOIR write."

I think that affect the irq_enable you describe below.

> The nice thing about the lazy irq disable code is that the irq_mask(),
> i.e. EOI, invocation actually happens always in hard interrupt
> context. We should never invoke irq_mask() from any other context if
> you supply a startup/shutdown function.
> 
>> If we had a flag like IRQCHIP_UNMASK_IS_STARTUP, we could distinguish
>> this particular case, but that's borderline ugly.
> 
> Indeed. But I don't think it is required. See also below.
> 
>>> 4) In the lazy irq disable case if the interrupt fires mask_irq()
>>>    [EOI] is good enough to silence it.
>>>
>>>    Though in the enable_irq() case you cannot rely on the automatic
>>>    resend of the interrupt when you unmask [DIR]. So we need to make
>>>    sure that even in the level case (dunno whether that's supported in
>>>    that mode) we end up calling the irq_retrigger() callback. But
>>>    that's rather simple to achieve with a new chip flag.
>>
>> I think this one breaks for the same reason as above. And an interrupt
>> masked with EOI cannot easily be restarted without clearing the ACTIVE
>> bit (and everything becomes even more of a complete madness).
> 
> So we already established that irq_mask()/EOI will only be called from
> the actual interrupt context and irq_unmask()/DIR must be safe to be
> called from any context in order to make the EOI/DIR based threaded
> optimization work.
> 
> So the only interesting code path is enable_irq() which invokes
> irq_enable() and then the resend/retrigger machinery. 
> 
> irq_enable() calls chip->irq_unmask(), i.e. DIR. So that clears the
> ACTIVE bit and then the IRQ either gets resent by hardware (in case of
> level as the device interrupt is still active) or retriggered by the
> irq_retrigger() callback.

The problem I see here is for an interrupt that has been flagged as
disabled with irq_disabled(), but that hasn't fired. We'd end up doing a
DIR on something that hasn't had an EOI first. I think that's the only
wrinkle in this scheme.

I'll implement something today, that will help me thinking.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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