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SubjectRe: [PATCH] arm: irq: Allow for specification of no preallocated irqs
On 01/19/2012 04:43 PM, Michael Bohan wrote:
> For cases with SPARSE_IRQ enabled, irqs preallocated with
> arch_probe_nr_irqs() are already marked as allocated in the
> allocated_irqs bitmap. As a consequence, irq chip drivers that
> allocate irqs will feel one of two behaviors:
>
> 1. An allocation will succeed with the starting irq_base one
> more than the preallocated irqs. This will thus waste the
> preceeding interrupt resources that were preallocated, unless a
> legacy chip driver happens to assume ownership of these by some
> platform definition. The GIC driver is a typical primary chip
> driver, and abides to the allocation APIs. So this can be a
> problem in many trivial usecases.
>
> 2. An allocation will fail with < 0. This can also happen in the
> GIC driver, which interprets this value as meaning the irq_descs
> are already preallocated. But in Device Tree configurations, the
> fallback irq_base is -1. This results in an invalid irq_base
> value.
>
> Looking forward, we are moving towards a world where preallocation
> of irqs is no longer necessary. irq_domain is scoped to handle all
> irq_desc allocations in the future. Thus, we should support
> configurations where the platform wants to preallocate no irqs.
>
> One easy way to achieve this is to allow for
> machine_desc->nr_irqs < 0, which indicates not to preallocate any
> interrupts.
>
> Signed-off-by: Michael Bohan <mbohan@codeaurora.org>

I don't know if you saw my recent series on NR_IRQS clean-up (Make
mach/irqs.h optional).

No doubt that arch_probe_nr_irqs is doing the wrong thing on ARM, but no
pre-allocation is not what we want either. We ultimately want
arch_probe_nr_irqs to return NR_IRQS_LEGACY (16) to reserve IRQ0 (aka
NO_IRQ) and legacy ISA IRQs. With my series, NR_IRQS is set to
NR_IRQS_LEGACY for SPARSE_IRQ. You can accomplish the same thing without
that series by setting .nr_irqs to NR_IRQS for non-DT and to
NR_IRQS_LEGACY for DT. For platforms to work in single kernel builds,
they will need to select SPARSE_IRQ.

Rob

> ---
> arch/arm/include/asm/mach/arch.h | 2 +-
> arch/arm/kernel/irq.c | 14 ++++++++++++--
> 2 files changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm/include/asm/mach/arch.h b/arch/arm/include/asm/mach/arch.h
> index d7692ca..cc6506a 100644
> --- a/arch/arm/include/asm/mach/arch.h
> +++ b/arch/arm/include/asm/mach/arch.h
> @@ -22,7 +22,7 @@ struct machine_desc {
> const char *const *dt_compat; /* array of device tree
> * 'compatible' strings */
>
> - unsigned int nr_irqs; /* number of IRQs */
> + int nr_irqs; /* number of IRQs */
>
> #ifdef CONFIG_ZONE_DMA
> unsigned long dma_zone_size; /* size of DMA-able area */
> diff --git a/arch/arm/kernel/irq.c b/arch/arm/kernel/irq.c
> index 3efd82c..f74b173 100644
> --- a/arch/arm/kernel/irq.c
> +++ b/arch/arm/kernel/irq.c
> @@ -129,8 +129,18 @@ void __init init_IRQ(void)
> #ifdef CONFIG_SPARSE_IRQ
> int __init arch_probe_nr_irqs(void)
> {
> - nr_irqs = machine_desc->nr_irqs ? machine_desc->nr_irqs : NR_IRQS;
> - return nr_irqs;
> + /*
> + * machine_desc->nr_irqs < 0 is a special case that
> + * specifies not to preallocate any irq_descs.
> + */
> + if (machine_desc->nr_irqs < 0) {
> + nr_irqs = 0;
> + return nr_irqs;
> + } else {
> + nr_irqs = machine_desc->nr_irqs ?
> + machine_desc->nr_irqs : NR_IRQS;
> + return nr_irqs;
> + }
> }
> #endif
>



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