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From: Rusty Russell <rusty@rustcorp.com.au>
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Date: Mon, 20 Feb 2012 14:22:00 +1030
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Subject: Re: [Qemu-devel] [Android-virt] [PATCH] arm: make sure that number
	of irqs can be represented in GICD_TYPER.
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To: Christoffer Dall <c.dall@virtualopensystems.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, qemu-devel@nongnu.org, android-virt@lists.cs.columbia.edu, patches@linaro.org

On Sun, 19 Feb 2012 18:40:08 -0500, Christoffer Dall <c.dall@virtualopensys=
tems.com> wrote:
> On Sun, Feb 19, 2012 at 6:07 PM, Rusty Russell <rusty@rustcorp.com.au> wr=
ote:
> > We currently assume that the number of interrupts (ITLinesNumber in
> > the architecture reference manual) is divisible by 32, since we
> > present it to the guest when it reads GICD_TYPER (in gic_dist_readb())
> > as (N - 32) / 1.
> >
> > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> >
> > diff --git a/hw/arm_gic.c b/hw/arm_gic.c
> > index fa6a60a..6446800 100644
> > --- a/hw/arm_gic.c
> > +++ b/hw/arm_gic.c
> > @@ -818,6 +818,15 @@ static void gic_init(gic_state *s, int num_irq)
> > =C2=A0 =C2=A0 =C2=A0 =C2=A0 hw_error("requested %u interrupt lines exce=
eds GIC maximum %d\n",
> > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0num_irq, =
GIC_MAXIRQ);
> > =C2=A0 =C2=A0 }
> > + =C2=A0 =C2=A0/* ITLinesNumber is represented as (N - 32) / 1 (see
> > + =C2=A0 =C2=A0 * gic_dist_readb) so this is an implementation imposed
> > + =C2=A0 =C2=A0 * restriction, not an architectural one:
> > + =C2=A0 =C2=A0 */
>=20
> What is this division by 1? Isn't (N - 32) / 1 =3D=3D (N - 32) and how
> does that explain the division. Was it (N / 32) - 1? Or is this a
> cast? Or am I missing something completely?

That's hilarious; we were concentrating so hard on the wording of the
comment, we didn't notice that I completely screwed the content :)

It is, of course, (N / 32) - 1:

static uint32_t gic_dist_readb(void *opaque, target_phys_addr_t offset)
{
...
        if (offset =3D=3D 4)
            return ((GIC_NIRQ / 32) - 1) | ((NUM_CPU(s) - 1) << 5);

I'll fix it, thanks!
Rusty.