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

[Gerd Hoffmann]

On 09/14/09 18:55, Blue Swirl wrote:
> 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.

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.

cheers,
  Gerd