Re: [PATCH] IRQ: simplify OMAP2 mask_irq/unmask_irq code

[Philip Balister <philip@balister.org>]

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Paul Walmsley wrote:
> Hello Kyungmin,
> 
> On Wed, 21 May 2008, Kyungmin Park wrote:
> 
>> On Wed, May 21, 2008 at 3:21 AM, Paul Walmsley <paul@pwsan.com> wrote:
>>>  static void omap_mask_irq(unsigned int irq)
>>>  {
>>> -       int offset = (irq >> 5) << 5;
>>> +       int offset = irq & (~(IRQ_BITS_PER_REG - 1));
>>>
>>> -       if (irq >= 64)
>>> -               irq %= 64;
>>> -       else if (irq >= 32)
>>> -               irq %= 32;
>>> +       irq %= IRQ_BITS_PER_REG;
>> Is it the right conversion?
>> If the irq is greater then 32 and less then or equal to  64 it's
>> result is different.
>> E.g, If irq is 63 then original irq is 63, but new code is 31
> 
> Hmm, in that condition, the result looks the same to me: irq % 32, either 
> way?
> 
> More practically, if you look at what it does with that irq variable 
> afterwards, it seems to be a bug if irq is ever greater than 31:
> 
>         intc_bank_write_reg(1 << irq, &irq_banks[0], INTC_MIR_CLEAR0 + 
>                             offset);
> 
> I think the only case where the new code would work differently than the 
> previous code is if irq > 95.  But that would be a bug, since the shift 
> value would then be > 32, for a 32-bit register.
> 
>> And if this code is right, how about to use mask instead of modulo op?
>> irq &= (IRQ_BITS_PER_REG - 1);
> 
> Hehe, very good point, that would probably save even more cycles!  If you 
> agree with the above, perhaps I can convert the code to use that also, 
> and add your Signed-off-by also?

On some code where I used % to detect a counter passing multiples of a 
certain number, oprofile revealed that the % operator burned a lot of 
CPU cycles. I suspect this had to do with the counter increasing to very 
large numbers, but ever since, I've tried to avoid the % operator in 
critical paths.

Philip

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