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

[Blue Swirl <blauwirbel@gmail.com>]

On Tue, Sep 15, 2009 at 10:23 AM, Gerd Hoffmann <kraxel@redhat.com> wrote:
> On 09/14/09 18:55, Blue Swirl wrote:
>>
>> 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.  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.
>
> Hmm, ok.  So scratch this and maybe use a 'sharing the irq is is fine with
> me' flag instead?
>
>> 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?
>
> Well, the fundamental problem is that (1) the drivers must be prepared to
> handle that and (b) the hardware must be designed sanely.  There are cases
> where it works fine (see the list above).  It isn't true in general though.
>  You can't configure two random devices (say sb16 + ne2k) to share the same
> irq and expect everything to work fine.

I wonder if the PREP machine is emulated correctly. I couldn't find
docs about PREP NVRAM or RTC.