Re: hwo to adjust interrupt?

Ming Zhang
2005-08-12T18:06:48Z

On Fri, 2005-08-12 at 19:51 +0200, Arjan van de Ven wrote:
> 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
a bit off topic, but where u get these information? thx

>
> > 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.
so it is possible that both card raise intr quicker than isr run, then
both isr will do mmio and go ahead, but then what is

device 1 raise intr
device 2 isr run and not belong to it, so quit, but before it clear intr
and quit
device 2 raise intr
device 1 isr run and handle device 1, then clear intr and quit.

then device 2 intr lost?

or i understand this wrong. :)

ming

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