Subject: Re: [PATCH 2/2] PowerPC: make 4xx uic use generic edge and level irq handlers

From: Valentine Barshak <[email-masked]>
Date: Wed, 14 Nov 2007 15:31:13 +0300

Benjamin Herrenschmidt wrote:
> On Wed, 2007-11-14 at 13:13 +1100, David Gibson wrote:
>> Hrm.  I *think* I'm convinced this is safe, although acking in a
>> callback which doesn't say it acks is rather yucky.  Essentially this
>> code is trading flow readability (because just reading
>> handle_level_irq will tell you something other than what it does in
>> our case) for smaller code size.  I'm not sure if this is a good trade
>> or not.
>>

It's not just code size. Actually, I was having problems with Ingo's 
real-time patch, that works on all platforms that use generic hard irq 
handlers and doesn't work just on 4xx since we use a custom one here.
And I thought that using generic handlers would be easier to maintain.
I agree that ack'ing in a callback which doesn't say it ack's looks odd, 
but ack'ing level-triggered interrupts is quirky on UIC itself. So, I 
just thought that adding a couple of quirks to mask_ack and unmask 
callbacks was not that bad.

>> There's also one definite problem: according to the discussions I had
>> with Thomas Gleixner when I wrote uic.c, handle_edge_irq is not what
>> we want for edge interrupts.
>>
>> Apparently handle_edge_irq is only for edge interrupts on "broken"
>> PICs which won't latch new interrupts while the irq is masked.  UIC is
>> not in this category, so handle_level_irq is actually what we want,
>> even for an edge irq.
>>
>> Yes, I thought the naming was more than a little confusing, too.
> 
> Hrm... handle_edge_irq works for both and you have a small performance
> benefit in not masking, and thus using handle_edge_irq, so I don't
> totally agree here. Basically, what handle_edge_irq() does is lazy
> masking. Now there -is- an issue here is that if you do lazy masking,
> you need to be able to re-emit in some convenient way.

With the ack quirks added we can use handle_level_irq for edge-triggered 
interrupts. I'll test and resubmit the patch.

> Ben.

Thanks,
Valentine.