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

From: Randy Vinson <[email-masked]>
Date: Thu, 07 Jun 2007 12:54:50 -0700

Sergei Shtylyov wrote:
[snippage]
>> 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.

> 
>> 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!
> 
>> 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.