Re: [RFC PATCH v3 07/18] xen/arm: ITS: implement hw_irq_controller for LPIs

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2015-07-02 at 18:14 +0530, Vijay Kilari wrote:
> On Thu, Jul 2, 2015 at 6:05 PM, Ian Campbell <ian.campbell@citrix.com> wrote:
> > On Thu, 2015-07-02 at 17:51 +0530, Vijay Kilari wrote:
> >> On Mon, Jun 29, 2015 at 5:29 PM, Ian Campbell <ian.campbell@citrix.com> wrote:
> >> > On Tue, 2015-06-23 at 15:32 +0100, Julien Grall wrote:
> >> > [...]
> >> >> > +{
> >> >> > +    struct its_collection *col;
> >> >> > +    struct its_device *its_dev = get_irq_device(desc);
> >> >> > +    u8 *cfg;
> >> >> > +    u32 virq = irq_to_virq(desc);
> >> >> > +
> >> >> > +    ASSERT(virq < its_dev->nr_lpis);
> >> >> > +
> >> >> > +    cfg = gic_rdists->prop_page + desc->irq - NR_GIC_LPI;
> >> >> > +    if ( enable )
> >> >> > +        *cfg |= LPI_PROP_ENABLED;
> >> >> > +    else
> >> >> > +        *cfg &= ~LPI_PROP_ENABLED;
> >> >> > +
> >> >> > +    /*
> >> >> > +     * Make the above write visible to the redistributors.
> >> >> > +     * And yes, we're flushing exactly: One. Single. Byte.
> >> >> > +     * Humpf...
> >> >> > +     */
> >> >> > +    if ( gic_rdists->flags & RDIST_FLAGS_PROPBASE_NEEDS_FLUSHING )
> >> >> > +        clean_and_invalidate_dcache_va_range(cfg, sizeof(*cfg));
> >> >> > +    else
> >> >> > +        dsb(ishst);
> >> >> > +
> >> >> > +    /* Get collection id for this event id */
> >> >> > +    col = &its_dev->its->collections[virq % num_online_cpus()];
> >> >>
> >> >> This is fragile, you are assuming that num_online_cpus() will never
> >> >> change. Why don't you store the collection in every irq_desc?
> >> >
> >> > The original Linux code upon which this is based doesn't seem to need to
> >> > lookup the collection here, why is flushing needed for us but not Linux?
> >>
> >> We are writing to lpi property table. Even linux code flushes it.
> >
> > Sorry I was referring to the collection look up and inv, not the cache
> > flush, i.e. this bit:
> >
> > +    /* Get collection id for this event id */
> > +    col = &its_dev->its->collections[virq % num_online_cpus()];
> > +    its_send_inv(its_dev, col, virq);
> >
> > Linux doesn't seem to do that INV there.
> 
> Linux does INV.
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/irqchip/irq-gic-v3-its.c?id=refs/tags/v4.1
> 
> line 555

So it does, not sure how I missed that when I first looked.

Linux's approach of saving collection in the its_dev seems preferable to
looking it up like this here though.

Ian.