Re: [Qemu-devel] [PATCH v4 5/5] i8259: fix dynamically masking slave IRQs with IMR register

[Jan Kiszka]

On 2012-09-04 19:41, Maciej W. Rozycki wrote:
> On Tue, 4 Sep 2012, Jan Kiszka wrote:
> 
>> What I'm trying to understand and translate from the description is
>> rather "note that for inputs a high-to-low transition cancels the
>> interrupt as in the level-triggered mode." This is surely not what we do
>> right now. OTOH, I'm afraid that switching to this mode in the PIC can
>> cause problems elsewhere, with devices that actually inject short
>> low-high-low signals. Still wrapping my head around it...
> 
>  That won't work reliably with true 8259A hardware -- for an 

Ok, then we have to scan our code base for such device models that won't
survive with real 8259A hardware. That can only be devices attached to
edge-only inputs of the PIC, namely the PIT, the keyboard controller,
the RTC and FPU emulation. They basically need to generate high-low-high
transitions on new events, instead of low-high-low (via qemu_irq_pulse
e.g.). I'm I on the right track?

Thanks,
Jan

> edge-triggered interrupt to propagate up to the CPU first there must be a 
> low-to-high transition and then the high logic state must be maintained up 
> until the start of the second INTA cycle.  If the interrupt request drops 
> before then (e.g. because CPU interrupts have been masked or a 
> higher-priority 8259A has been serviced), then the corresponding IRR bit 
> is cleared and either the interrupt is missed altogether or, if the CPU 
> has already accepted the interrupt and started the first INTA cycle, then 
> the spurious vector is supplied and no ISR bit is set.
> 
>  To put it in different words: the only actual difference between 
> edge-triggered and level-triggered interrupts in the 8259A is that the 
> formers require a leading edge to record another interrupt.  For both 
> trigger modes the high level has to be maintained until the second INTA 
> cycle for the interrupt to be correctly delivered to the CPU and also in 
> both trigger modes a trailing edge cancels the interrupt.
> 
>  This is unlike the traditional edge-triggered mode where the level does 
> not have to be maintained once a leading edge has been correctly recorded 
> (there is usually spike filtering logic implemented on such IRQ inputs so 
> appropriate timings have to be met; because of its unusual interpretation 
> the 8259A obviously does not require such logic).
> 
>  The edge detector logic is also drawn in the 8259A datasheet (that for a 
> change used to be available from one of the Intel sites in the PDF form) 
> and I believe the functionality described can be inferred from that by the 
> curious enough. ;)
> 
>   Maciej
> 

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