For the last few days, I have tried to figure out a good way to share interrupts between RT and non-RT domains. This has included looking through Dmitry's patch, correcting bugs and testing what is possible in my specific case. I'll therefore try to summarize at least a few of my thoughts.

1. When looking through Dmitry's patch I get the impression that the iack handler has very little to do with each interrupt (the test 'prev->iack != intr->iack' is a dead giveaway), but is more of a domain-specific function (or perhaps even just a placeholder for the hijacked Linux ack-function).



2. Somewhat inspired by the figure in "Life with Adeos", I have identified the following cases:

  irq K  | ----------- | ---o    |   // Linux only
  ...
  irq L  | ---o        |         |   // RT-only
  ...
  irq M  | ---o------- | ---o    |   // Shared between domains
  ...
  irq N  | ---o---o--- |         |   // Shared inside single domain
  ...
  irq O  | ---o---o--- | ---o    |   // Shared between and inside single domain

Xenomai currently handles the K & L cases, Dmitrys patch addresses the N case, with edge triggered interrupts the M (and O after Dmitry's patch) case(s) might be handled by returning RT_INTR_CHAINED | RT_INTR_ENABLE from the interrupt handler, for level triggered interrupt the M and O cases can't be handled.

If one looks more closely at the K case (Linux only interrupt), it works by when an interrupt occurs, the call to irq_end is postponed until the Linux interrupt handler has run, i.e. further interrupts are disabled. This can be seen as a lazy version of Philippe's idea of disabling all non-RT interrupts until the RT-domain is idle, i.e. the interrupt is disabled only if it indeed occurs.

If this idea should be generalized to the M (and O) case(s), one can't rely on postponing the irq_end call (since the interrupt is still needed in the RT-domain), but has to rely on some function that disables all non-RT hardware that generates interrupts on that irq-line; such a function naturally has to have intimate knowledge of all hardware that can generate interrupts in order to be able to disable those interrupt sources that are non-RT.

If we then take Jan's observation about the many (Linux-only) interrupts present in an ordinary PC and add it to Philippe's idea of disabling all non-RT interrupts while executing in the RT-domain, I think that the following is a workable (and fairly efficient) way of handling this:

Add hardware dependent enable/disable functions, where the enable is called just before normal execution in a domain starts (i.e. when playing back interrupts, the disable is still in effect), and disable is called when normal domain execution end. This does effectively handle the K case above, with the added benefit that NO non-RT interrupts will occur during RT execution.

In the 8259 case, the disable function could look something like:

  domain_irq_disable(uint irqmask) {
    if (irqmask & 0xff00 != 0xff00) {
      irqmask &= ~0x0004; // Cascaded interrupt is still needed
      outb(irqmask >> 8, PIC_SLAVE_IMR);
    }
    outb(irqmask, PIC_MASTER_IMR);
  }

If we should extend this to handle the M (and O) case(s), the disable function could look like:

  domain_irq_disable(uint irqmask, shared_irq_t *shared[]) {
    int i;

    for (i = 0 ; i < MAX_IRQ ; i++) {
      if (shared[i]) {
        shared_irq_t *next = shared[i];
        irqmask &= ~(1<<i);
        while (next) {
          next->disable();
          next = next->next;
        }
      }
    }
    if (irqmask & 0xff00 != 0xff00) {
      irqmask &= ~0x0004; // Cascaded interrupt is still needed
      outb(irqmask >> 8, PIC_SLAVE_IMR);
    }
    outb(irqmask, PIC_MASTER_IMR);
  }

An obvious optimization of the above scheme, is to never call the disable (or enable) function for the RT-domain, since there all interrupt processing is protected by the hardware.

Comments, anyone?

--

Anders


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