Re: hwo to adjust interrupt?

[Arjan van de Ven <arjan@infradead.org>]

On Fri, 2005-08-12 at 10:35 -0700, Bryan Henderson wrote:
> >>How much extra work does Linux have to do for each interrupt?
> >
> >usually 1 pci mmio read; the rest is negligible.
> 
> I was hoping you would cater better to my ignorance of how PCI interrupt 
> handling works in Linux.
> 
> Is it the case that Linux invokes the registered interrupt handler of each 
> of the drivers for the devices that share the interrupt, and each does an 
> mmio read of its device to find out if it had reason to generate an 
> interrupt?  So the waste is that extra call, and you're saying the CPU 
> instructions involved are negligible compared to the mmio read?

yes. A function call is like half a cycle (a function pointer call is
maybe 40) an mmio read is a lot more

> Are these level-sensitive interrupts, so that if both devices need service 
> at the same time, they generate just one interrupt and neither device 
> driver call is wasted?

ok this is more complex, but if 2 cards raise it quickly after
eachother, before the ISR has run, then you only get the handler called
once afaik.