Re: User mode drivers: part 1, interrupt handling (patch for 2.6.11)

From: Zwane Mwaikambo
Date: Sat Mar 12 2005 - 12:15:39 EST

On Sat, 12 Mar 2005, Jon Smirl wrote:

> On Fri, 11 Mar 2005 14:36:10 +1100, Peter Chubb
> <peterc@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > As many of you will be aware, we've been working on infrastructure for
> > user-mode PCI and other drivers. The first step is to be able to
> > handle interrupts from user space. Subsequent patches add
> > infrastructure for setting up DMA for PCI devices.
> I've tried implementing this before and could not get around the
> interrupt problem. Most interrupts on the x86 architecture are shared.
> Disabling the IRQ at the PIC blocks all of the shared IRQs. This works
> (hope your userspace handler is last on the shared handler list) until
> you have a problem in userspace.
> Once you have a problem in userspace there is no way to acknowledge
> the interrupt anymore. I tried to address that by maintaining a timer
> and suspending the hardware through the D0 state to reset it. That had
> some success. Not acknowledging the interrupt results in an interrupt
> loop and reboot.
> The problem can be mitigated by choosing what slot your hardware to
> put your hardware in. This can reduce the number of shared interrupts.
> If you can get exclusive use of the interrupt this method will work.
> If I were designing a new bus I would make interrupt acknowledge part
> of PCI config space in order to allow a single piece of code to
> acknowledge them. Since we can't change the bus the only safe way to
> do this is to build a hardware specific driver for each device to
> acknowledge the interrupt.
> Bottom line is that I could find no reliable solution for handing
> interrupts.

Alan's proposal sounds very plausible and additionally if we find that
we have an irq line screaming we could use the same supplied information
to disable userspace interrupt handled devices first.


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