Re: [Qemu-devel] [PATCH 8/9] isa: refine irq reservations

[Markus Armbruster]

Blue Swirl <address@hidden> writes:

> On Mon, Sep 14, 2009 at 6:49 PM, Gerd Hoffmann <address@hidden> wrote:
>> There are a few cases where IRQ sharing on the ISA bus is used and
>> possible.  In general only devices of the same kind can do that.
>> A few use cases:
>>
>>  * serial lines 1+3 share irq 4
>>  * serial lines 2+4 share irq 3
>>  * parallel ports share irq 7
>>  * ppc/prep: ide ports share irq 13
>
> There is another ppc/prep case where RTC and m48t59 share irq 8. The
> attached patch converts m48t59 to ISA and makes this more visible.
>
> Does sharing of IRQs really have to be fatal? Wasn't ISA bus edge
> triggered and so IRQ sharing was a bit unreliable (but not completely
> broken) also on real HW?

If I remember correctly (and that's a considerable if), sufficiently
well-behaved hardware with sufficiently smart drivers could share IRQs.