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SubjectRe: PCI driver in userspace
Pete Zaitcev writes:
> >> I think this is a good example to start with. It has all the
> >> interresting features. I hope it also has interrupt handling
> >> to userspace (by generating SIGIO's).
> >
> > It hasn't, it just continually polls the I/O ports for
> > completition.
> >
> >> Although I dont know if this is a good idea in the first place.
> >
> > I think it depends on the interrupt frequency, and other things.
> >
> > Stelian.
> PCI interrupts must be deactivated in the device before an
> interrupt driver returns and unmasks interrupt in the CPU.
> User processes run with interrupts open. This is why purely
> user level interrupts are IMPOSSIBLE (regardless of interrupt
> frequency).
> Two natural ways to work around this problem are:
> 1. Run user processes with interrupts closed (or at least one
> interrupt source disabled, if your architecture allows that).
> This is what RTOSes often do. I do not think it can be easily
> done on generic Linux.
> 2. Write a small driver that deactivates interrupts and queues
> interrupt events (with SIGIO and some other means). Personally,
> I think that it is dumb in most cases, because once you wrote
> that driver stub, it takes very little work to move the rest
> of the driver into the kernel. The notable exception is XFree86,
> of course.
> We must ask someone to add this to FAQ on

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