[Milkymist-devel] IRQ handling code

[Sébastien Bourdeauducq]

Hi,

On Wednesday 17 March 2010 00:15:48 Michael Walle wrote:
> Am Tuesday 16 March 2010 15:53:41 schrieb Sébastien Bourdeauducq:
> > Therefore, it gets ALL interrupts (masked or not). We should AND register
> > IP with register IM.
> 
> yep. imho that should be done within the asm code.

Yes, but this requires extra asm instructions that no longer fit between two 
LM32 exception vectors and therefore a jump is needed. I wanted to avoid it, 
that's why I did it this way. But since it is inconsistent with other Linux 
ports, I will change it.

> other linux ports pass only one single irq number to do_IRQ().
> 
> What about the following steps:
>   1. interrupt vector gets called
>   2. save regs
>   3. r2 <- IM & IP
>   4. find position of first bit set in r2, store it in r1
>  (4a. r2 <- pt_regs)
>   5. mask irq
>   6. call do_IRQ (which only takes one irq number)
>   7. ack irq
>   8. unmask irq
>   9. restore regs
>  10. eret
> 
> Processing only one interrupt at a time should avoid the second problem,
> mentioned above.

Sounds good. It takes more CPU cycles to achieve the same thing when 
interrupts come at the same time, but since I guess this rarely happens, the 
small performance improvement is not worth the cost of doing differently than 
other Linux ports.

> do_IRQ() just calls irq_enter/exit generic_handle_irq.
>
> > Do you see a particular reason for using the lm32_current_irq_mask
> > "cache", or is it just Theobroma's programmers being stupid?
>
> Nope, maybe it had something to do with that IPIPE thing. Imho it should be
> removed.

Ok. Let's do that then.

Thanks,
Sébastien