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Subject: Re: [Qemu-devel] [PATCH 8/9] isa: refine irq reservations
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From: Markus Armbruster <armbru@redhat.com>
Date: Mon, 14 Sep 2009 22:09:35 +0200
In-Reply-To: <f43fc5580909140955t4f914f26n5c7f7a247b9b6e64@mail.gmail.com>
	(Blue Swirl's message of "Mon\, 14 Sep 2009 19\:55\:23 +0300")
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To: Blue Swirl <blauwirbel@gmail.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>, qemu-devel@nongnu.org

Blue Swirl <blauwirbel@gmail.com> writes:

> On Mon, Sep 14, 2009 at 6:49 PM, Gerd Hoffmann <kraxel@redhat.com> wrote:
>> There are a few cases where IRQ sharing on the ISA bus is used and
>> possible. =C2=A0In general only devices of the same kind can do that.
>> A few use cases:
>>
>> =C2=A0* serial lines 1+3 share irq 4
>> =C2=A0* serial lines 2+4 share irq 3
>> =C2=A0* parallel ports share irq 7
>> =C2=A0* 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.