Re: [Qemu-devel] Re: >2 serial ports?

[Avi Kivity <avi@redhat.com>]

On 03/22/2010 10:35 AM, Michael Tokarev wrote:
> Paul Brook wrote at Wed, 17 Mar 2010 11:18:09 +0000:
>    
>>> Oh, well, yes, I remember.  qemu is more strict on ISA irq sharing now.
>>>    A bit too strict.
>>>
>>> /me goes dig out a old patch which never made it upstream for some
>>> reason I forgot.  Attached.
>>>        
>> This is wrong. Two devices should never be manipulating the same qemu_irq
>> object.  If you want multiple devices connected to the same IRQ then you need
>> an explicit multiplexer. e.g. arm_timer.c:sp804_set_irq.
>>      
> So... what we have to do here?
>
> I've looked at the mentioned routine, here it is:
>
> /* Merge the IRQs from the two component devices.  */
> static void sp804_set_irq(void *opaque, int irq, int level)
> {
>      sp804_state *s = (sp804_state *)opaque;
>
>      s->level[irq] = level;
>      qemu_set_irq(s->irq, s->level[0] || s->level[1]);
> }
>
> But I know nothing about qemu internals, so don't quite
> understand how to do this in case of serial ports.  I
> see it is tracking two timers and raises the irq level
> if at least one half is raised...  That to say - I've
> got the idea, but how to apply it to serial ports?
>    

Two devices have the same s->irq.  Give each on its own qemu_irq, and 
feed it into a multiplexer that ORs them together and sends the result 
to the interrupt controller's qemu_irq:


S1 ----qemu_irq---->+------+

                     |  mux  +---qemu_irq----->  irq controller

S2 ----qemu_irq---->+------+


(the ascii art will come out all wrong, I know it)

-- 

Do not meddle in the internals of kernels, for they are subtle and quick to panic.