Re: [RFC] 85XX: Allow 8259 cascade to share an MPIC interrupt line.

Sergei Shtylyov wrote:
[snippage]

quoted

The threaded interrupt system has threaded routines for the 4 standard
interrupts types used by the generic IRQ system (fasteoi, edge, level
and simple), plus a handler for non-generic interrupts (the ones that
still use __do_IRQ.) When interrupts are being threaded and a
non-cascaded MPIC interrupt occurs, desc->handler normally points to
handle_fasteoi_irq which masks the interrupt source, schedules the
corresponding IRQ thread, issues an EOI and returns to the interrupted
context. At a later time, the scheduler dispatches the IRQ thread which
calls a threaded version of the fasteoi handler. The threaded fasteoi
routine processes the action chain and calls .unmask to re-enable the
interrupt before returning to the calling thread. Once the interrupt has
been unmasked, the entire process is free to repeat as needed.

   Oh, you could have really skipped the basics. ;-)

Maybe not. See below.

quoted

After processing a possible 8259 interrupt, the 8259 cascade handler
calls handle_fasteoi_irq to perform the actions noted above. However,
with the 8259 cascade handler hooked to the interrupt, the threaded IRQ
handler doesn't recognize it as one of the 4 standard generic IRQ types
and calls the threaded version of do_irq instead of the threaded fasteoi
handler.

   Ah, that's it!

quoted

Once the threaded do_irq handler completes the action list, it
uses the .end routine to restart the interrupt flow. Pointing .end to
the standard MPIC unmask routine allows it to balance the interrupt mask
operation performed by handle_fasteoi_irq before it scheduled the IRQ
thread.

   Well, but when MPIC's eoi() method is called then? :-O

It's called from handle_fasteoi_irq as I described in the "basics" ;)

Randy V.