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

[Thomas Gleixner]

On Wed, 21 Aug 2019, Andy Shevchenko wrote:
> On Wed, Aug 21, 2019 at 01:20:53PM +0200, Thomas Gleixner wrote:
> > 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.

For DT we can actually avoid that completely. See below.

For ACPI not unfortunately as the stupid GSI mapping is hard coded.

Thanks,

	tglx

8<-------------
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -2438,7 +2438,13 @@ unsigned int arch_dynirq_lower_bound(uns
 	 * dmar_alloc_hwirq() may be called before setup_IO_APIC(), so use
 	 * gsi_top if ioapic_dynirq_base hasn't been initialized yet.
 	 */
-	return ioapic_initialized ? ioapic_dynirq_base : gsi_top;
+	if (!ioapic_initialized)
+		return gsi_top;
+	/*
+	 * For DT enabled machines ioapic_dynirq_base is irrelevant and not
+	 * updated. So simply return @from if ioapic_dynirq_base == 0.
+	 */
+	return ioapic_dynirq_base ? : from;
 }
 
 #ifdef CONFIG_X86_32