Re: [PATCH] x86/apic: Update virtual irq base for DT/OF based system as well

[Andy Shevchenko]

On Wed, Aug 21, 2019 at 01:20:53PM +0200, Thomas Gleixner wrote:
> On Wed, 21 Aug 2019, Tanwar, Rahul wrote:
> > On 21/8/2019 4:34 PM, Thomas Gleixner wrote:
> > 
> > > Secondly, this link is irrelevant. ioapic_dynirq_base has nothing to do
> > > with virtual IRQ number 0. It's a boundary for the dynamic allocation of
> > > virtual interrupt numbers so that the core allocator does not pick
> > > interrupts out of the IOAPIC's fixed interrupt number space.
> > > 
> > > This can be legitimately 0 when IOAPIC is not enabled at all.
> > > 
> > > Can you please explain what kind of problem you were seing and what this
> > > really fixes?
> >
> > The problem is that device tree infrastructure considers 0 IRQ value as
> > invalid/error value whereas for ACPI, 0 is a valid value.
> 
> Sure.
> 
> > Without this change, the problem that we see is that the first driver
> > using of_irq_get_xx() or its variants fails because of 0 IRQ number. With
> > this change, allocated IRQ number is never 0 so it works ok.
> 
> Well, this still is not a proper explanation. Just because it works does
> not make it correct in the first place.
> 
> ioapic_dynirq_base is pretty much irrelevant for a DT machine. The reason
> why it exists is that for regular BIOS the interrupt numbers are hard
> mapped to the IOAPIC pins. ioapic_dynirq_base is used to protect this hard
> mapped interrupt number space. The core allocator does not allocate from
> that space unless it is explicitely told to do so, which is the case for
> IOAPIC_DOMAIN_STRICT where the allocation tells the core to allocate the
> associated GSI number.
> 
> On DT the interrupt number is irrelevant as DT describes the irq controller
> and the pin to which a device is connected and does not make assumptions
> about the interrupt number. So the core can freely allocate any available
> interrupt number except 0. That's already prevented in the core code.
> 
> But x86 implements arch_dynirq_lower_bound() which overrides the core limit
> and because ioapic_dynirq_base is zero in the DT case it allows VIRQ 0 to
> be allocated which then causes of_irq*() to fail.
> 
> So your change prevents that by excluding the 'GSI' range from allocation,
> which means that the first irq number which is handed out is 24, assumed
> you have one IOAPIC with 24 pins.

I have tested this on the ACPI-based system where we have 55 lines of IOAPIC,
no PIC, and some GPIO lines. Overall I see that nr_irqs is 512 and shifting
by 55 freezes 10% of the space for nothing. Luckily we have SPARSE_IRQS
selected for any X86, so, it wouldn't waste memory.

I think we may do slightly better if we just limit the change to the certain
cases.

-- 
With Best Regards,
Andy Shevchenko