Re: [Xen-devel] [PATCH 1/4] x86/hvm: Centralize and simplify the RTC IRQ logic.

[Tim Deegan <tim@xxxxxxx>]

At 13:49 +0000 on 28 Mar (1364478581), Jan Beulich wrote:
> >>> On 28.03.13 at 14:22, Tim Deegan <tim@xxxxxxx> wrote:
> > Signed-off-by: Tim Deegan <tim@xxxxxxx>
> > ---
> >  xen/arch/x86/hvm/rtc.c |   43 ++++++++++++++++++++++---------------------
> >  1 file changed, 22 insertions(+), 21 deletions(-)
> > 
> > diff --git a/xen/arch/x86/hvm/rtc.c b/xen/arch/x86/hvm/rtc.c
> > index c1e09d8..368d3c8 100644
> > --- a/xen/arch/x86/hvm/rtc.c
> > +++ b/xen/arch/x86/hvm/rtc.c
> > @@ -50,14 +50,26 @@ static void rtc_set_time(RTCState *s);
> >  static inline int from_bcd(RTCState *s, int a);
> >  static inline int convert_hour(RTCState *s, int hour);
> >  
> > -static void rtc_toggle_irq(RTCState *s)
> > +static void rtc_update_irq(RTCState *s)
> >  {
> >      struct domain *d = vrtc_domain(s);
> > +    uint8_t irqf;
> >  
> >      ASSERT(spin_is_locked(&s->lock));
> > -    s->hw.cmos_data[RTC_REG_C] |= RTC_IRQF;
> > -    hvm_isa_irq_deassert(d, RTC_IRQ);
> > -    hvm_isa_irq_assert(d, RTC_IRQ);
> > +
> > +    /* IRQ is raised if any of the sources is raised & enabled */
> > +    irqf = (s->hw.cmos_data[RTC_REG_B]
> > +            & s->hw.cmos_data[RTC_REG_C]
> > +            & (RTC_PF|RTC_AF|RTC_UF))
> > +        ? RTC_IRQF : 0;
> > +
> > +    s->hw.cmos_data[RTC_REG_C] &= ~RTC_IRQF;
> > +    s->hw.cmos_data[RTC_REG_C] |= irqf;
> > +
> > +    if ( irqf )
> > +        hvm_isa_irq_assert(d, RTC_IRQ);
> > +    else
> > +        hvm_isa_irq_deassert(d, RTC_IRQ);
> 
> One fundamental question here is - do you or does anyone else
> know why originally the code did this odd looking assert-deassert
> sequence. I'm a little afraid that this change may cause observably
> different behavior. In particular, our ACPI MADT doesn't (and
> shouldn't) declare IRQ8 as level triggered, yet the way you change
> the code would seem to make the interrupt behave as if it was.

Hmm.  I was following the spec for the MC146818A RTC chip, which is
pretty clearly level-triggered.  Looking at the piix4 spec:

  IRQ 8#. IRQ8# is always an active low edge triggered interrupt and can
  not be modified by software.

I'm not sure why the original code always strobes the line.

Tim.

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