linux-kernel - Re: [PATCH -rt] powerpc update

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Date:	Sat, 28 Oct 2006 19:05:16 +0400
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Daniel Walker <dwalker@...sta.com>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, tglx@...utronix.de, mgreer@...sta.com
Subject: Re: [PATCH -rt] powerpc update

Hello.

Ingo Molnar wrote:
> * Daniel Walker <dwalker@...sta.com> wrote:

>>Pay close attention to the fasteoi interrupt threading. I added usage 
>>of mask/unmask instead of using level handling, which worked well on 
>>PPC.

> this is wrong - it should be doing mask+ack.

    It's what it was doing effectively. And what was wrong was calling ack() 
which OpenPIC driver didn't (and was not obliged to) support.

> also note that you changed:

>>-		goto out_unlock;

> to:

>>+		goto out;

> and you even tried to hide your tracks:
> 
> 
>> out:
>> 	desc->chip->eoi(irq);
>>-out_unlock:
>> 	spin_unlock(&desc->lock);

> :-)

> really, the ->eoi() op should only be called for true fasteoi cases. 

    Why is that? eoi() is effectively the same as ack() in this case. I must 
note that what's the "standard" 8259 driver is doing in mask_ack() seems 
misleading since it actually masks IRQ and sends EOI there.

> What we want here is to turn the fasteoi handler into a handler that 
> does mask+ack and then unmask. Not 'mask+eoi ... unmask' as your patch 
> does.

    That's effectively the same for OpenPIC. Maybe that implemetation just 
didn't look graceful but it was *correct*. And the current one is at least 
incomplete.

    I can see 3 ways to get out of this situation now:

1. Revert this change and use mask() + eoi() approach suggested by Daniel.

2. Add the ack() handler to OpenPIC driver -- and point it to mpic_eoi().

3. Do the same as x86 APIC driver does and use level/egde flows instead of 
fasteoi for the case when IRQs are threaded -- that ensues doing (2) as well.

    Note that all three aproaches lead to the effectively the same behavior 
WRT OpenPIC (except for the edge-triggered IRQs in 3rd case). Opinions?

> 	Ingo

WBR, Sergei
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